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 <div class="section" id="programming-faq">
<h1><a class="toc-backref" href="#id1">Programming FAQ</a><a class="headerlink" href="#programming-faq" title="Permalink to this headline">¶</a></h1>
<div class="contents topic" id="contents">
Contents
<ul class="simple">
<li><a class="reference internal" href="#programming-faq" id="id1">Programming FAQ</a><ul>
<li><a class="reference internal" href="#general-questions" id="id2">General Questions</a><ul>
<li><a class="reference internal" href="#is-there-a-source-code-level-debugger-with-breakpoints-single-stepping-etc" id="id3">Is there a source code level debugger with breakpoints, single-stepping, etc.?</a></li>
<li><a class="reference internal" href="#is-there-a-tool-to-help-find-bugs-or-perform-static-analysis" id="id4">Is there a tool to help find bugs or perform static analysis?</a></li>
<li><a class="reference internal" href="#how-can-i-create-a-stand-alone-binary-from-a-python-script" id="id5">How can I create a stand-alone binary from a Python script?</a></li>
<li><a class="reference internal" href="#are-there-coding-standards-or-a-style-guide-for-python-programs" id="id6">Are there coding standards or a style guide for Python programs?</a></li>
<li><a class="reference internal" href="#my-program-is-too-slow-how-do-i-speed-it-up" id="id7">My program is too slow. How do I speed it up?</a></li>
</ul>
</li>
<li><a class="reference internal" href="#core-language" id="id8">Core Language</a><ul>
<li><a class="reference internal" href="#why-am-i-getting-an-unboundlocalerror-when-the-variable-has-a-value" id="id9">Why am I getting an UnboundLocalError when the variable has a value?</a></li>
<li><a class="reference internal" href="#what-are-the-rules-for-local-and-global-variables-in-python" id="id10">What are the rules for local and global variables in Python?</a></li>
<li><a class="reference internal" href="#why-do-lambdas-defined-in-a-loop-with-different-values-all-return-the-same-result" id="id11">Why do lambdas defined in a loop with different values all return the same result?</a></li>
<li><a class="reference internal" href="#how-do-i-share-global-variables-across-modules" id="id12">How do I share global variables across modules?</a></li>
<li><a class="reference internal" href="#what-are-the-best-practices-for-using-import-in-a-module" id="id13">What are the “best practices” for using import in a module?</a></li>
<li><a class="reference internal" href="#why-are-default-values-shared-between-objects" id="id14">Why are default values shared between objects?</a></li>
<li><a class="reference internal" href="#how-can-i-pass-optional-or-keyword-parameters-from-one-function-to-another" id="id15">How can I pass optional or keyword parameters from one function to another?</a></li>
<li><a class="reference internal" href="#what-is-the-difference-between-arguments-and-parameters" id="id16">What is the difference between arguments and parameters?</a></li>
<li><a class="reference internal" href="#why-did-changing-list-y-also-change-list-x" id="id17">Why did changing list ‘y’ also change list ‘x’?</a></li>
<li><a class="reference internal" href="#how-do-i-write-a-function-with-output-parameters-call-by-reference" id="id18">How do I write a function with output parameters (call by reference)?</a></li>
<li><a class="reference internal" href="#how-do-you-make-a-higher-order-function-in-python" id="id19">How do you make a higher order function in Python?</a></li>
<li><a class="reference internal" href="#how-do-i-copy-an-object-in-python" id="id20">How do I copy an object in Python?</a></li>
<li><a class="reference internal" href="#how-can-i-find-the-methods-or-attributes-of-an-object" id="id21">How can I find the methods or attributes of an object?</a></li>
<li><a class="reference internal" href="#how-can-my-code-discover-the-name-of-an-object" id="id22">How can my code discover the name of an object?</a></li>
<li><a class="reference internal" href="#what-s-up-with-the-comma-operator-s-precedence" id="id23">What’s up with the comma operator’s precedence?</a></li>
<li><a class="reference internal" href="#is-there-an-equivalent-of-c-s-ternary-operator" id="id24">Is there an equivalent of C’s “?:” ternary operator?</a></li>
<li><a class="reference internal" href="#is-it-possible-to-write-obfuscated-one-liners-in-python" id="id25">Is it possible to write obfuscated one-liners in Python?</a></li>
</ul>
</li>
<li><a class="reference internal" href="#numbers-and-strings" id="id26">Numbers and strings</a><ul>
<li><a class="reference internal" href="#how-do-i-specify-hexadecimal-and-octal-integers" id="id27">How do I specify hexadecimal and octal integers?</a></li>
<li><a class="reference internal" href="#why-does-22-10-return-3" id="id28">Why does -22 // 10 return -3?</a></li>
<li><a class="reference internal" href="#how-do-i-convert-a-string-to-a-number" id="id29">How do I convert a string to a number?</a></li>
<li><a class="reference internal" href="#how-do-i-convert-a-number-to-a-string" id="id30">How do I convert a number to a string?</a></li>
<li><a class="reference internal" href="#how-do-i-modify-a-string-in-place" id="id31">How do I modify a string in place?</a></li>
<li><a class="reference internal" href="#how-do-i-use-strings-to-call-functions-methods" id="id32">How do I use strings to call functions/methods?</a></li>
<li><a class="reference internal" href="#is-there-an-equivalent-to-perl-s-chomp-for-removing-trailing-newlines-from-strings" id="id33">Is there an equivalent to Perl’s chomp() for removing trailing newlines from strings?</a></li>
<li><a class="reference internal" href="#is-there-a-scanf-or-sscanf-equivalent" id="id34">Is there a scanf() or sscanf() equivalent?</a></li>
<li><a class="reference internal" href="#what-does-unicodeerror-ascii-decoding-encoding-error-ordinal-not-in-range-128-mean" id="id35">What does ‘UnicodeError: ASCII [decoding,encoding] error: ordinal not in range(128)’ mean?</a></li>
</ul>
</li>
<li><a class="reference internal" href="#sequences-tuples-lists" id="id36">Sequences (Tuples/Lists)</a><ul>
<li><a class="reference internal" href="#how-do-i-convert-between-tuples-and-lists" id="id37">How do I convert between tuples and lists?</a></li>
<li><a class="reference internal" href="#what-s-a-negative-index" id="id38">What’s a negative index?</a></li>
<li><a class="reference internal" href="#how-do-i-iterate-over-a-sequence-in-reverse-order" id="id39">How do I iterate over a sequence in reverse order?</a></li>
<li><a class="reference internal" href="#how-do-you-remove-duplicates-from-a-list" id="id40">How do you remove duplicates from a list?</a></li>
<li><a class="reference internal" href="#how-do-you-make-an-array-in-python" id="id41">How do you make an array in Python?</a></li>
<li><a class="reference internal" href="#how-do-i-create-a-multidimensional-list" id="id42">How do I create a multidimensional list?</a></li>
<li><a class="reference internal" href="#how-do-i-apply-a-method-to-a-sequence-of-objects" id="id43">How do I apply a method to a sequence of objects?</a></li>
<li><a class="reference internal" href="#why-does-a-tuple-i-item-raise-an-exception-when-the-addition-works" id="id44">Why does a_tuple[i] += [‘item’] raise an exception when the addition works?</a></li>
</ul>
</li>
<li><a class="reference internal" href="#dictionaries" id="id45">Dictionaries</a><ul>
<li><a class="reference internal" href="#how-can-i-get-a-dictionary-to-display-its-keys-in-a-consistent-order" id="id46">How can I get a dictionary to display its keys in a consistent order?</a></li>
<li><a class="reference internal" href="#i-want-to-do-a-complicated-sort-can-you-do-a-schwartzian-transform-in-python" id="id47">I want to do a complicated sort: can you do a Schwartzian Transform in Python?</a></li>
<li><a class="reference internal" href="#how-can-i-sort-one-list-by-values-from-another-list" id="id48">How can I sort one list by values from another list?</a></li>
</ul>
</li>
<li><a class="reference internal" href="#objects" id="id49">Objects</a><ul>
<li><a class="reference internal" href="#what-is-a-class" id="id50">What is a class?</a></li>
<li><a class="reference internal" href="#what-is-a-method" id="id51">What is a method?</a></li>
<li><a class="reference internal" href="#what-is-self" id="id52">What is self?</a></li>
<li><a class="reference internal" href="#how-do-i-check-if-an-object-is-an-instance-of-a-given-class-or-of-a-subclass-of-it" id="id53">How do I check if an object is an instance of a given class or of a subclass of it?</a></li>
<li><a class="reference internal" href="#what-is-delegation" id="id54">What is delegation?</a></li>
<li><a class="reference internal" href="#how-do-i-call-a-method-defined-in-a-base-class-from-a-derived-class-that-overrides-it" id="id55">How do I call a method defined in a base class from a derived class that overrides it?</a></li>
<li><a class="reference internal" href="#how-can-i-organize-my-code-to-make-it-easier-to-change-the-base-class" id="id56">How can I organize my code to make it easier to change the base class?</a></li>
<li><a class="reference internal" href="#how-do-i-create-static-class-data-and-static-class-methods" id="id57">How do I create static class data and static class methods?</a></li>
<li><a class="reference internal" href="#how-can-i-overload-constructors-or-methods-in-python" id="id58">How can I overload constructors (or methods) in Python?</a></li>
<li><a class="reference internal" href="#i-try-to-use-spam-and-i-get-an-error-about-someclassname-spam" id="id59">I try to use __spam and I get an error about _SomeClassName__spam.</a></li>
<li><a class="reference internal" href="#my-class-defines-del-but-it-is-not-called-when-i-delete-the-object" id="id60">My class defines __del__ but it is not called when I delete the object.</a></li>
<li><a class="reference internal" href="#how-do-i-get-a-list-of-all-instances-of-a-given-class" id="id61">How do I get a list of all instances of a given class?</a></li>
<li><a class="reference internal" href="#why-does-the-result-of-id-appear-to-be-not-unique" id="id62">Why does the result of <code class="docutils literal notranslate">id()</code> appear to be not unique?</a></li>
</ul>
</li>
<li><a class="reference internal" href="#modules" id="id63">Modules</a><ul>
<li><a class="reference internal" href="#how-do-i-create-a-pyc-file" id="id64">How do I create a .pyc file?</a></li>
<li><a class="reference internal" href="#how-do-i-find-the-current-module-name" id="id65">How do I find the current module name?</a></li>
<li><a class="reference internal" href="#how-can-i-have-modules-that-mutually-import-each-other" id="id66">How can I have modules that mutually import each other?</a></li>
<li><a class="reference internal" href="#import-x-y-z-returns-module-x-how-do-i-get-z" id="id67">__import__(‘x.y.z’) returns &lt;module ‘x’&gt;; how do I get z?</a></li>
<li><a class="reference internal" href="#when-i-edit-an-imported-module-and-reimport-it-the-changes-don-t-show-up-why-does-this-happen" id="id68">When I edit an imported module and reimport it, the changes don’t show up. Why does this happen?</a></li>
</ul>
</li>
</ul>
</li>
</ul>
</div>
<div class="section" id="general-questions">
<h2><a class="toc-backref" href="#id2">General Questions</a><a class="headerlink" href="#general-questions" title="Permalink to this headline">¶</a></h2>
<div class="section" id="is-there-a-source-code-level-debugger-with-breakpoints-single-stepping-etc">
<h3><a class="toc-backref" href="#id3">Is there a source code level debugger with breakpoints, single-stepping, etc.?</a><a class="headerlink" href="#is-there-a-source-code-level-debugger-with-breakpoints-single-stepping-etc" title="Permalink to this headline">¶</a></h3>
Yes.
The pdb module is a simple but adequate console-mode debugger for Python. It is
part of the standard Python library, and is <a class="reference internal" href="../library/pdb.html#module-pdb" title="pdb: The Python debugger for interactive interpreters."><code class="xref py py-mod docutils literal notranslate">documented in the Library
Reference Manual</code></a>. You can also write your own debugger by using the code
for pdb as an example.
The IDLE interactive development environment, which is part of the standard
Python distribution (normally available as Tools/scripts/idle), includes a
graphical debugger.
PythonWin is a Python IDE that includes a GUI debugger based on pdb. The
Pythonwin debugger colors breakpoints and has quite a few cool features such as
debugging non-Pythonwin programs. Pythonwin is available as part of the <a class="reference external" href="https://sourceforge.net/projects/pywin32/">Python
for Windows Extensions</a> project and
as a part of the ActivePython distribution (see
<a class="reference external" href="https://www.activestate.com/activepython">https://www.activestate.com/activepython</a>).
<a class="reference external" href="http://boa-constructor.sourceforge.net/">Boa Constructor</a> is an IDE and GUI
builder that uses wxWidgets. It offers visual frame creation and manipulation,
an object inspector, many views on the source like object browsers, inheritance
hierarchies, doc string generated html documentation, an advanced debugger,
integrated help, and Zope support.
<a class="reference external" href="http://eric-ide.python-projects.org/">Eric</a> is an IDE built on PyQt
and the Scintilla editing component.
Pydb is a version of the standard Python debugger pdb, modified for use with DDD
(Data Display Debugger), a popular graphical debugger front end. Pydb can be
found at <a class="reference external" href="http://bashdb.sourceforge.net/pydb/">http://bashdb.sourceforge.net/pydb/</a> and DDD can be found at
<a class="reference external" href="https://www.gnu.org/software/ddd">https://www.gnu.org/software/ddd</a>.
There are a number of commercial Python IDEs that include graphical debuggers.
They include:
<ul class="simple">
<li>Wing IDE (<a class="reference external" href="https://wingware.com/">https://wingware.com/</a>)</li>
<li>Komodo IDE (<a class="reference external" href="https://komodoide.com/">https://komodoide.com/</a>)</li>
<li>PyCharm (<a class="reference external" href="https://www.jetbrains.com/pycharm/">https://www.jetbrains.com/pycharm/</a>)</li>
</ul>
</div>
<div class="section" id="is-there-a-tool-to-help-find-bugs-or-perform-static-analysis">
<h3><a class="toc-backref" href="#id4">Is there a tool to help find bugs or perform static analysis?</a><a class="headerlink" href="#is-there-a-tool-to-help-find-bugs-or-perform-static-analysis" title="Permalink to this headline">¶</a></h3>
Yes.
PyChecker is a static analysis tool that finds bugs in Python source code and
warns about code complexity and style. You can get PyChecker from
<a class="reference external" href="http://pychecker.sourceforge.net/">http://pychecker.sourceforge.net/</a>.
<a class="reference external" href="https://www.pylint.org/">Pylint</a> is another tool that checks
if a module satisfies a coding standard, and also makes it possible to write
plug-ins to add a custom feature. In addition to the bug checking that
PyChecker performs, Pylint offers some additional features such as checking line
length, whether variable names are well-formed according to your coding
standard, whether declared interfaces are fully implemented, and more.
<a class="reference external" href="https://docs.pylint.org/">https://docs.pylint.org/</a> provides a full list of Pylint’s features.
</div>
<div class="section" id="how-can-i-create-a-stand-alone-binary-from-a-python-script">
<h3><a class="toc-backref" href="#id5">How can I create a stand-alone binary from a Python script?</a><a class="headerlink" href="#how-can-i-create-a-stand-alone-binary-from-a-python-script" title="Permalink to this headline">¶</a></h3>
You don’t need the ability to compile Python to C code if all you want is a
stand-alone program that users can download and run without having to install
the Python distribution first. There are a number of tools that determine the
set of modules required by a program and bind these modules together with a
Python binary to produce a single executable.
One is to use the freeze tool, which is included in the Python source tree as
<code class="docutils literal notranslate">Tools/freeze</code>. It converts Python byte code to C arrays; a C compiler you can
embed all your modules into a new program, which is then linked with the
standard Python modules.
It works by scanning your source recursively for import statements (in both
forms) and looking for the modules in the standard Python path as well as in the
source directory (for built-in modules). It then turns the bytecode for modules
written in Python into C code (array initializers that can be turned into code
objects using the marshal module) and creates a custom-made config file that
only contains those built-in modules which are actually used in the program. It
then compiles the generated C code and links it with the rest of the Python
interpreter to form a self-contained binary which acts exactly like your script.
Obviously, freeze requires a C compiler. There are several other utilities
which don’t. One is Thomas Heller’s py2exe (Windows only) at
<blockquote>
<div><a class="reference external" href="http://www.py2exe.org/">http://www.py2exe.org/</a></div></blockquote>
Another tool is Anthony Tuininga’s <a class="reference external" href="http://cx-freeze.sourceforge.net/">cx_Freeze</a>.
</div>
<div class="section" id="are-there-coding-standards-or-a-style-guide-for-python-programs">
<h3><a class="toc-backref" href="#id6">Are there coding standards or a style guide for Python programs?</a><a class="headerlink" href="#are-there-coding-standards-or-a-style-guide-for-python-programs" title="Permalink to this headline">¶</a></h3>
Yes. The coding style required for standard library modules is documented as
<a class="pep reference external" href="https://www.python.org/dev/peps/pep-0008">PEP 8</a>.
</div>
<div class="section" id="my-program-is-too-slow-how-do-i-speed-it-up">
<h3><a class="toc-backref" href="#id7">My program is too slow. How do I speed it up?</a><a class="headerlink" href="#my-program-is-too-slow-how-do-i-speed-it-up" title="Permalink to this headline">¶</a></h3>
That’s a tough one, in general. There are many tricks to speed up Python code;
consider rewriting parts in C as a last resort.
In some cases it’s possible to automatically translate Python to C or x86
assembly language, meaning that you don’t have to modify your code to gain
increased speed.
<a class="reference external" href="http://www.cosc.canterbury.ac.nz/~greg/python/Pyrex/">Pyrex</a> can compile a
slightly modified version of Python code into a C extension, and can be used on
many different platforms.
<a class="reference external" href="http://psyco.sourceforge.net">Psyco</a> is a just-in-time compiler that
translates Python code into x86 assembly language. If you can use it, Psyco can
provide dramatic speedups for critical functions.
The rest of this answer will discuss various tricks for squeezing a bit more
speed out of Python code. Never apply any optimization tricks unless you know
you need them, after profiling has indicated that a particular function is the
heavily executed hot spot in the code. Optimizations almost always make the
code less clear, and you shouldn’t pay the costs of reduced clarity (increased
development time, greater likelihood of bugs) unless the resulting performance
benefit is worth it.
There is a page on the wiki devoted to <a class="reference external" href="https://wiki.python.org/moin/PythonSpeed/PerformanceTips">performance tips</a>.
Guido van Rossum has written up an anecdote related to optimization at
<a class="reference external" href="https://www.python.org/doc/essays/list2str">https://www.python.org/doc/essays/list2str</a>.
One thing to notice is that function and (especially) method calls are rather
expensive; if you have designed a purely OO interface with lots of tiny
functions that don’t do much more than get or set an instance variable or call
another method, you might consider using a more direct way such as directly
accessing instance variables. Also see the standard module <a class="reference internal" href="../library/profile.html#module-profile" title="profile: Python source profiler."><code class="xref py py-mod docutils literal notranslate">profile</code></a> which
makes it possible to find out where your program is spending most of its time
(if you have some patience – the profiling itself can slow your program down by
an order of magnitude).
Remember that many standard optimization heuristics you may know from other
programming experience may well apply to Python. For example it may be faster
to send output to output devices using larger writes rather than smaller ones in
order to reduce the overhead of kernel system calls. Thus CGI scripts that
write all output in “one shot” may be faster than those that write lots of small
pieces of output.
Also, be sure to use Python’s core features where appropriate. For example,
slicing allows programs to chop up lists and other sequence objects in a single
tick of the interpreter’s mainloop using highly optimized C implementations.
Thus to get the same effect as:
<div class="highlight-default notranslate"><div class="highlight"><pre>L2 = []
for i in range(3):
 L2.append(L1[i])
</pre></div>
</div>
it is much shorter and far faster to use
<div class="highlight-default notranslate"><div class="highlight"><pre>L2 = list(L1[:3]) # &quot;list&quot; is redundant if L1 is a list.
</pre></div>
</div>
Note that the functionally-oriented built-in functions such as <a class="reference internal" href="../library/functions.html#map" title="map"><code class="xref py py-func docutils literal notranslate">map()</code></a>,
<a class="reference internal" href="../library/functions.html#zip" title="zip"><code class="xref py py-func docutils literal notranslate">zip()</code></a>, and friends can be a convenient accelerator for loops that
perform a single task. For example to pair the elements of two lists
together:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; zip([1, 2, 3], [4, 5, 6])
[(1, 4), (2, 5), (3, 6)]
</pre></div>
</div>
or to compute a number of sines:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; map(math.sin, (1, 2, 3, 4))
[0.841470984808, 0.909297426826, 0.14112000806, -0.756802495308]
</pre></div>
</div>
The operation completes very quickly in such cases.
Other examples include the <code class="docutils literal notranslate">join()</code> and <code class="docutils literal notranslate">split()</code> <a class="reference internal" href="../library/stdtypes.html#string-methods">methods
of string objects</a>.
For example if s1..s7 are large (10K+) strings then
<code class="docutils literal notranslate">&quot;&quot;.join([s1,s2,s3,s4,s5,s6,s7])</code> may be far faster than the more obvious
<code class="docutils literal notranslate">s1+s2+s3+s4+s5+s6+s7</code>, since the “summation” will compute many
subexpressions, whereas <code class="docutils literal notranslate">join()</code> does all the copying in one pass. For
manipulating strings, use the <code class="docutils literal notranslate">replace()</code> and the <code class="docutils literal notranslate">format()</code> <a class="reference internal" href="../library/stdtypes.html#string-methods">methods
on string objects</a>. Use regular expressions only when you’re
not dealing with constant string patterns. You may still use <a class="reference internal" href="../library/stdtypes.html#string-formatting">the old %
operations</a> <code class="docutils literal notranslate">string % tuple</code> and <code class="docutils literal notranslate">string % dictionary</code>.
Be sure to use the <code class="xref py py-meth docutils literal notranslate">list.sort()</code> built-in method to do sorting, and see the
<a class="reference external" href="https://wiki.python.org/moin/HowTo/Sorting">sorting mini-HOWTO</a> for examples
of moderately advanced usage. <code class="xref py py-meth docutils literal notranslate">list.sort()</code> beats other techniques for
sorting in all but the most extreme circumstances.
Another common trick is to “push loops into functions or methods.” For example
suppose you have a program that runs slowly and you use the profiler to
determine that a Python function <code class="docutils literal notranslate">ff()</code> is being called lots of times. If you
notice that <code class="docutils literal notranslate">ff()</code>:
<div class="highlight-default notranslate"><div class="highlight"><pre>def ff(x):
 ... # do something with x computing result...
 return result
</pre></div>
</div>
tends to be called in loops like:
<div class="highlight-default notranslate"><div class="highlight"><pre>list = map(ff, oldlist)
</pre></div>
</div>
or:
<div class="highlight-default notranslate"><div class="highlight"><pre>for x in sequence:
 value = ff(x)
 ... # do something with value...
</pre></div>
</div>
then you can often eliminate function call overhead by rewriting <code class="docutils literal notranslate">ff()</code> to:
<div class="highlight-default notranslate"><div class="highlight"><pre>def ffseq(seq):
 resultseq = []
 for x in seq:
 ... # do something with x computing result...
 resultseq.append(result)
 return resultseq
</pre></div>
</div>
and rewrite the two examples to <code class="docutils literal notranslate">list = ffseq(oldlist)</code> and to:
<div class="highlight-default notranslate"><div class="highlight"><pre>for value in ffseq(sequence):
 ... # do something with value...
</pre></div>
</div>
Single calls to <code class="docutils literal notranslate">ff(x)</code> translate to <code class="docutils literal notranslate">ffseq([x])[0]</code> with little penalty.
Of course this technique is not always appropriate and there are other variants
which you can figure out.
You can gain some performance by explicitly storing the results of a function or
method lookup into a local variable. A loop like:
<div class="highlight-default notranslate"><div class="highlight"><pre>for key in token:
 dict[key] = dict.get(key, 0) + 1
</pre></div>
</div>
resolves <code class="docutils literal notranslate">dict.get</code> every iteration. If the method isn’t going to change, a
slightly faster implementation is:
<div class="highlight-default notranslate"><div class="highlight"><pre>dict_get = dict.get # look up the method once
for key in token:
 dict[key] = dict_get(key, 0) + 1
</pre></div>
</div>
Default arguments can be used to determine values once, at compile time instead
of at run time. This can only be done for functions or objects which will not
be changed during program execution, such as replacing
<div class="highlight-default notranslate"><div class="highlight"><pre>def degree_sin(deg):
 return math.sin(deg * math.pi / 180.0)
</pre></div>
</div>
with
<div class="highlight-default notranslate"><div class="highlight"><pre>def degree_sin(deg, factor=math.pi/180.0, sin=math.sin):
 return sin(deg * factor)
</pre></div>
</div>
Because this trick uses default arguments for terms which should not be changed,
it should only be used when you are not concerned with presenting a possibly
confusing API to your users.
</div>
</div>
<div class="section" id="core-language">
<h2><a class="toc-backref" href="#id8">Core Language</a><a class="headerlink" href="#core-language" title="Permalink to this headline">¶</a></h2>
<div class="section" id="why-am-i-getting-an-unboundlocalerror-when-the-variable-has-a-value">
<h3><a class="toc-backref" href="#id9">Why am I getting an UnboundLocalError when the variable has a value?</a><a class="headerlink" href="#why-am-i-getting-an-unboundlocalerror-when-the-variable-has-a-value" title="Permalink to this headline">¶</a></h3>
It can be a surprise to get the UnboundLocalError in previously working
code when it is modified by adding an assignment statement somewhere in
the body of a function.
This code:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; x = 10
&gt;&gt;&gt; def bar():
... print x
&gt;&gt;&gt; bar()
10
</pre></div>
</div>
works, but this code:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; x = 10
&gt;&gt;&gt; def foo():
... print x
... x += 1
</pre></div>
</div>
results in an UnboundLocalError:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; foo()
Traceback (most recent call last):
 ...
UnboundLocalError: local variable &#39;x&#39; referenced before assignment
</pre></div>
</div>
This is because when you make an assignment to a variable in a scope, that
variable becomes local to that scope and shadows any similarly named variable
in the outer scope. Since the last statement in foo assigns a new value to
<code class="docutils literal notranslate">x</code>, the compiler recognizes it as a local variable. Consequently when the
earlier <code class="docutils literal notranslate">print x</code> attempts to print the uninitialized local variable and
an error results.
In the example above you can access the outer scope variable by declaring it
global:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; x = 10
&gt;&gt;&gt; def foobar():
... global x
... print x
... x += 1
&gt;&gt;&gt; foobar()
10
</pre></div>
</div>
This explicit declaration is required in order to remind you that (unlike the
superficially analogous situation with class and instance variables) you are
actually modifying the value of the variable in the outer scope:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; print x
11
</pre></div>
</div>
</div>
<div class="section" id="what-are-the-rules-for-local-and-global-variables-in-python">
<h3><a class="toc-backref" href="#id10">What are the rules for local and global variables in Python?</a><a class="headerlink" href="#what-are-the-rules-for-local-and-global-variables-in-python" title="Permalink to this headline">¶</a></h3>
In Python, variables that are only referenced inside a function are implicitly
global. If a variable is assigned a value anywhere within the function’s body,
it’s assumed to be a local unless explicitly declared as global.
Though a bit surprising at first, a moment’s consideration explains this. On
one hand, requiring <a class="reference internal" href="../reference/simple_stmts.html#global"><code class="xref std std-keyword docutils literal notranslate">global</code></a> for assigned variables provides a bar
against unintended side-effects. On the other hand, if <code class="docutils literal notranslate">global</code> was required
for all global references, you’d be using <code class="docutils literal notranslate">global</code> all the time. You’d have
to declare as global every reference to a built-in function or to a component of
an imported module. This clutter would defeat the usefulness of the <code class="docutils literal notranslate">global</code>
declaration for identifying side-effects.
</div>
<div class="section" id="why-do-lambdas-defined-in-a-loop-with-different-values-all-return-the-same-result">
<h3><a class="toc-backref" href="#id11">Why do lambdas defined in a loop with different values all return the same result?</a><a class="headerlink" href="#why-do-lambdas-defined-in-a-loop-with-different-values-all-return-the-same-result" title="Permalink to this headline">¶</a></h3>
Assume you use a for loop to define a few different lambdas (or even plain
functions), e.g.:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; squares = []
&gt;&gt;&gt; for x in range(5):
... squares.append(lambda: x**2)
</pre></div>
</div>
This gives you a list that contains 5 lambdas that calculate <code class="docutils literal notranslate">x**2</code>. You
might expect that, when called, they would return, respectively, <code class="docutils literal notranslate">0</code>, <code class="docutils literal notranslate">1</code>,
<code class="docutils literal notranslate">4</code>, <code class="docutils literal notranslate">9</code>, and <code class="docutils literal notranslate">16</code>. However, when you actually try you will see that
they all return <code class="docutils literal notranslate">16</code>:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; squares[2]()
16
&gt;&gt;&gt; squares[4]()
16
</pre></div>
</div>
This happens because <code class="docutils literal notranslate">x</code> is not local to the lambdas, but is defined in
the outer scope, and it is accessed when the lambda is called — not when it
is defined. At the end of the loop, the value of <code class="docutils literal notranslate">x</code> is <code class="docutils literal notranslate">4</code>, so all the
functions now return <code class="docutils literal notranslate">4**2</code>, i.e. <code class="docutils literal notranslate">16</code>. You can also verify this by
changing the value of <code class="docutils literal notranslate">x</code> and see how the results of the lambdas change:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; x = 8
&gt;&gt;&gt; squares[2]()
64
</pre></div>
</div>
In order to avoid this, you need to save the values in variables local to the
lambdas, so that they don’t rely on the value of the global <code class="docutils literal notranslate">x</code>:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; squares = []
&gt;&gt;&gt; for x in range(5):
... squares.append(lambda n=x: n**2)
</pre></div>
</div>
Here, <code class="docutils literal notranslate">n=x</code> creates a new variable <code class="docutils literal notranslate">n</code> local to the lambda and computed
when the lambda is defined so that it has the same value that <code class="docutils literal notranslate">x</code> had at
that point in the loop. This means that the value of <code class="docutils literal notranslate">n</code> will be <code class="docutils literal notranslate">0</code>
in the first lambda, <code class="docutils literal notranslate">1</code> in the second, <code class="docutils literal notranslate">2</code> in the third, and so on.
Therefore each lambda will now return the correct result:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; squares[2]()
4
&gt;&gt;&gt; squares[4]()
16
</pre></div>
</div>
Note that this behaviour is not peculiar to lambdas, but applies to regular
functions too.
</div>
<div class="section" id="how-do-i-share-global-variables-across-modules">
<h3><a class="toc-backref" href="#id12">How do I share global variables across modules?</a><a class="headerlink" href="#how-do-i-share-global-variables-across-modules" title="Permalink to this headline">¶</a></h3>
The canonical way to share information across modules within a single program is
to create a special module (often called config or cfg). Just import the config
module in all modules of your application; the module then becomes available as
a global name. Because there is only one instance of each module, any changes
made to the module object get reflected everywhere. For example:
config.py:
<div class="highlight-default notranslate"><div class="highlight"><pre>x = 0 # Default value of the &#39;x&#39; configuration setting
</pre></div>
</div>
mod.py:
<div class="highlight-default notranslate"><div class="highlight"><pre>import config
config.x = 1
</pre></div>
</div>
main.py:
<div class="highlight-default notranslate"><div class="highlight"><pre>import config
import mod
print config.x
</pre></div>
</div>
Note that using a module is also the basis for implementing the Singleton design
pattern, for the same reason.
</div>
<div class="section" id="what-are-the-best-practices-for-using-import-in-a-module">
<h3><a class="toc-backref" href="#id13">What are the “best practices” for using import in a module?</a><a class="headerlink" href="#what-are-the-best-practices-for-using-import-in-a-module" title="Permalink to this headline">¶</a></h3>
In general, don’t use <code class="docutils literal notranslate">from modulename import *</code>. Doing so clutters the
importer’s namespace, and makes it much harder for linters to detect undefined
names.
Import modules at the top of a file. Doing so makes it clear what other modules
your code requires and avoids questions of whether the module name is in scope.
Using one import per line makes it easy to add and delete module imports, but
using multiple imports per line uses less screen space.
It’s good practice if you import modules in the following order:
<ol class="arabic simple">
<li>standard library modules – e.g. <code class="docutils literal notranslate">sys</code>, <code class="docutils literal notranslate">os</code>, <code class="docutils literal notranslate">getopt</code>, <code class="docutils literal notranslate">re</code></li>
<li>third-party library modules (anything installed in Python’s site-packages
directory) – e.g. mx.DateTime, ZODB, PIL.Image, etc.</li>
<li>locally-developed modules</li>
</ol>
Only use explicit relative package imports. If you’re writing code that’s in
the <code class="docutils literal notranslate">package.sub.m1</code> module and want to import <code class="docutils literal notranslate">package.sub.m2</code>, do not just
write <code class="docutils literal notranslate">import m2</code>, even though it’s legal. Write <code class="docutils literal notranslate">from package.sub import
m2</code> or <code class="docutils literal notranslate">from . import m2</code> instead.
It is sometimes necessary to move imports to a function or class to avoid
problems with circular imports. Gordon McMillan says:
<blockquote>
<div>Circular imports are fine where both modules use the “import &lt;module&gt;” form
of import. They fail when the 2nd module wants to grab a name out of the
first (“from module import name”) and the import is at the top level. That’s
because names in the 1st are not yet available, because the first module is
busy importing the 2nd.</div></blockquote>
In this case, if the second module is only used in one function, then the import
can easily be moved into that function. By the time the import is called, the
first module will have finished initializing, and the second module can do its
import.
It may also be necessary to move imports out of the top level of code if some of
the modules are platform-specific. In that case, it may not even be possible to
import all of the modules at the top of the file. In this case, importing the
correct modules in the corresponding platform-specific code is a good option.
Only move imports into a local scope, such as inside a function definition, if
it’s necessary to solve a problem such as avoiding a circular import or are
trying to reduce the initialization time of a module. This technique is
especially helpful if many of the imports are unnecessary depending on how the
program executes. You may also want to move imports into a function if the
modules are only ever used in that function. Note that loading a module the
first time may be expensive because of the one time initialization of the
module, but loading a module multiple times is virtually free, costing only a
couple of dictionary lookups. Even if the module name has gone out of scope,
the module is probably available in <a class="reference internal" href="../library/sys.html#sys.modules" title="sys.modules"><code class="xref py py-data docutils literal notranslate">sys.modules</code></a>.
</div>
<div class="section" id="why-are-default-values-shared-between-objects">
<h3><a class="toc-backref" href="#id14">Why are default values shared between objects?</a><a class="headerlink" href="#why-are-default-values-shared-between-objects" title="Permalink to this headline">¶</a></h3>
This type of bug commonly bites neophyte programmers. Consider this function:
<div class="highlight-default notranslate"><div class="highlight"><pre>def foo(mydict={}): # Danger: shared reference to one dict for all calls
 ... compute something ...
 mydict[key] = value
 return mydict
</pre></div>
</div>
The first time you call this function, <code class="docutils literal notranslate">mydict</code> contains a single item. The
second time, <code class="docutils literal notranslate">mydict</code> contains two items because when <code class="docutils literal notranslate">foo()</code> begins
executing, <code class="docutils literal notranslate">mydict</code> starts out with an item already in it.
It is often expected that a function call creates new objects for default
values. This is not what happens. Default values are created exactly once, when
the function is defined. If that object is changed, like the dictionary in this
example, subsequent calls to the function will refer to this changed object.
By definition, immutable objects such as numbers, strings, tuples, and <code class="docutils literal notranslate">None</code>,
are safe from change. Changes to mutable objects such as dictionaries, lists,
and class instances can lead to confusion.
Because of this feature, it is good programming practice to not use mutable
objects as default values. Instead, use <code class="docutils literal notranslate">None</code> as the default value and
inside the function, check if the parameter is <code class="docutils literal notranslate">None</code> and create a new
list/dictionary/whatever if it is. For example, don’t write:
<div class="highlight-default notranslate"><div class="highlight"><pre>def foo(mydict={}):
 ...
</pre></div>
</div>
but:
<div class="highlight-default notranslate"><div class="highlight"><pre>def foo(mydict=None):
 if mydict is None:
 mydict = {} # create a new dict for local namespace
</pre></div>
</div>
This feature can be useful. When you have a function that’s time-consuming to
compute, a common technique is to cache the parameters and the resulting value
of each call to the function, and return the cached value if the same value is
requested again. This is called “memoizing”, and can be implemented like this:
<div class="highlight-default notranslate"><div class="highlight"><pre># Callers will never provide a third parameter for this function.
def expensive(arg1, arg2, _cache={}):
 if (arg1, arg2) in _cache:
 return _cache[(arg1, arg2)]

# Calculate the value
 result = ... expensive computation ...
 _cache[(arg1, arg2)] = result # Store result in the cache
 return result
</pre></div>
</div>
You could use a global variable containing a dictionary instead of the default
value; it’s a matter of taste.
</div>
<div class="section" id="how-can-i-pass-optional-or-keyword-parameters-from-one-function-to-another">
<h3><a class="toc-backref" href="#id15">How can I pass optional or keyword parameters from one function to another?</a><a class="headerlink" href="#how-can-i-pass-optional-or-keyword-parameters-from-one-function-to-another" title="Permalink to this headline">¶</a></h3>
Collect the arguments using the <code class="docutils literal notranslate">*</code> and <code class="docutils literal notranslate">**</code> specifiers in the function’s
parameter list; this gives you the positional arguments as a tuple and the
keyword arguments as a dictionary. You can then pass these arguments when
calling another function by using <code class="docutils literal notranslate">*</code> and <code class="docutils literal notranslate">**</code>:
<div class="highlight-default notranslate"><div class="highlight"><pre>def f(x, *args, **kwargs):
 ...
 kwargs[&#39;width&#39;] = &#39;14.3c&#39;
 ...
 g(x, *args, **kwargs)
</pre></div>
</div>
In the unlikely case that you care about Python versions older than 2.0, use
<a class="reference internal" href="../library/functions.html#apply" title="apply"><code class="xref py py-func docutils literal notranslate">apply()</code></a>:
<div class="highlight-default notranslate"><div class="highlight"><pre>def f(x, *args, **kwargs):
 ...
 kwargs[&#39;width&#39;] = &#39;14.3c&#39;
 ...
 apply(g, (x,)+args, kwargs)
</pre></div>
</div>
</div>
<div class="section" id="what-is-the-difference-between-arguments-and-parameters">
<h3><a class="toc-backref" href="#id16">What is the difference between arguments and parameters?</a><a class="headerlink" href="#what-is-the-difference-between-arguments-and-parameters" title="Permalink to this headline">¶</a></h3>
<a class="reference internal" href="../glossary.html#term-parameter">Parameters</a> are defined by the names that appear in a
function definition, whereas <a class="reference internal" href="../glossary.html#term-argument">arguments</a> are the values
actually passed to a function when calling it. Parameters define what types of
arguments a function can accept. For example, given the function definition:
<div class="highlight-default notranslate"><div class="highlight"><pre>def func(foo, bar=None, **kwargs):
 pass
</pre></div>
</div>
foo, bar and kwargs are parameters of <code class="docutils literal notranslate">func</code>. However, when calling
<code class="docutils literal notranslate">func</code>, for example:
<div class="highlight-default notranslate"><div class="highlight"><pre>func(42, bar=314, extra=somevar)
</pre></div>
</div>
the values <code class="docutils literal notranslate">42</code>, <code class="docutils literal notranslate">314</code>, and <code class="docutils literal notranslate">somevar</code> are arguments.
</div>
<div class="section" id="why-did-changing-list-y-also-change-list-x">
<h3><a class="toc-backref" href="#id17">Why did changing list ‘y’ also change list ‘x’?</a><a class="headerlink" href="#why-did-changing-list-y-also-change-list-x" title="Permalink to this headline">¶</a></h3>
If you wrote code like:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; x = []
&gt;&gt;&gt; y = x
&gt;&gt;&gt; y.append(10)
&gt;&gt;&gt; y
[10]
&gt;&gt;&gt; x
[10]
</pre></div>
</div>
you might be wondering why appending an element to <code class="docutils literal notranslate">y</code> changed <code class="docutils literal notranslate">x</code> too.
There are two factors that produce this result:
<ol class="arabic simple">
<li>Variables are simply names that refer to objects. Doing <code class="docutils literal notranslate">y = x</code> doesn’t
create a copy of the list – it creates a new variable <code class="docutils literal notranslate">y</code> that refers to
the same object <code class="docutils literal notranslate">x</code> refers to. This means that there is only one object
(the list), and both <code class="docutils literal notranslate">x</code> and <code class="docutils literal notranslate">y</code> refer to it.</li>
<li>Lists are <a class="reference internal" href="../glossary.html#term-mutable">mutable</a>, which means that you can change their content.</li>
</ol>
After the call to <code class="xref py py-meth docutils literal notranslate">append()</code>, the content of the mutable object has
changed from <code class="docutils literal notranslate">[]</code> to <code class="docutils literal notranslate">[10]</code>. Since both the variables refer to the same
object, using either name accesses the modified value <code class="docutils literal notranslate">[10]</code>.
If we instead assign an immutable object to <code class="docutils literal notranslate">x</code>:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; x = 5 # ints are immutable
&gt;&gt;&gt; y = x
&gt;&gt;&gt; x = x + 1 # 5 can&#39;t be mutated, we are creating a new object here
&gt;&gt;&gt; x
6
&gt;&gt;&gt; y
5
</pre></div>
</div>
we can see that in this case <code class="docutils literal notranslate">x</code> and <code class="docutils literal notranslate">y</code> are not equal anymore. This is
because integers are <a class="reference internal" href="../glossary.html#term-immutable">immutable</a>, and when we do <code class="docutils literal notranslate">x = x + 1</code> we are not
mutating the int <code class="docutils literal notranslate">5</code> by incrementing its value; instead, we are creating a
new object (the int <code class="docutils literal notranslate">6</code>) and assigning it to <code class="docutils literal notranslate">x</code> (that is, changing which
object <code class="docutils literal notranslate">x</code> refers to). After this assignment we have two objects (the ints
<code class="docutils literal notranslate">6</code> and <code class="docutils literal notranslate">5</code>) and two variables that refer to them (<code class="docutils literal notranslate">x</code> now refers to
<code class="docutils literal notranslate">6</code> but <code class="docutils literal notranslate">y</code> still refers to <code class="docutils literal notranslate">5</code>).
Some operations (for example <code class="docutils literal notranslate">y.append(10)</code> and <code class="docutils literal notranslate">y.sort()</code>) mutate the
object, whereas superficially similar operations (for example <code class="docutils literal notranslate">y = y + [10]</code>
and <code class="docutils literal notranslate">sorted(y)</code>) create a new object. In general in Python (and in all cases
in the standard library) a method that mutates an object will return <code class="docutils literal notranslate">None</code>
to help avoid getting the two types of operations confused. So if you
mistakenly write <code class="docutils literal notranslate">y.sort()</code> thinking it will give you a sorted copy of <code class="docutils literal notranslate">y</code>,
you’ll instead end up with <code class="docutils literal notranslate">None</code>, which will likely cause your program to
generate an easily diagnosed error.
However, there is one class of operations where the same operation sometimes
has different behaviors with different types: the augmented assignment
operators. For example, <code class="docutils literal notranslate">+=</code> mutates lists but not tuples or ints (<code class="docutils literal notranslate">a_list
+= [1, 2, 3]</code> is equivalent to <code class="docutils literal notranslate">a_list.extend([1, 2, 3])</code> and mutates
<code class="docutils literal notranslate">a_list</code>, whereas <code class="docutils literal notranslate">some_tuple += (1, 2, 3)</code> and <code class="docutils literal notranslate">some_int += 1</code> create
new objects).
In other words:
<ul class="simple">
<li>If we have a mutable object (<code class="xref py py-class docutils literal notranslate">list</code>, <a class="reference internal" href="../library/stdtypes.html#dict" title="dict"><code class="xref py py-class docutils literal notranslate">dict</code></a>, <a class="reference internal" href="../library/stdtypes.html#set" title="set"><code class="xref py py-class docutils literal notranslate">set</code></a>,
etc.), we can use some specific operations to mutate it and all the variables
that refer to it will see the change.</li>
<li>If we have an immutable object (<a class="reference internal" href="../library/functions.html#str" title="str"><code class="xref py py-class docutils literal notranslate">str</code></a>, <a class="reference internal" href="../library/functions.html#int" title="int"><code class="xref py py-class docutils literal notranslate">int</code></a>, <a class="reference internal" href="../library/functions.html#tuple" title="tuple"><code class="xref py py-class docutils literal notranslate">tuple</code></a>,
etc.), all the variables that refer to it will always see the same value,
but operations that transform that value into a new value always return a new
object.</li>
</ul>
If you want to know if two variables refer to the same object or not, you can
use the <a class="reference internal" href="../reference/expressions.html#is"><code class="xref std std-keyword docutils literal notranslate">is</code></a> operator, or the built-in function <a class="reference internal" href="../library/functions.html#id" title="id"><code class="xref py py-func docutils literal notranslate">id()</code></a>.
</div>
<div class="section" id="how-do-i-write-a-function-with-output-parameters-call-by-reference">
<h3><a class="toc-backref" href="#id18">How do I write a function with output parameters (call by reference)?</a><a class="headerlink" href="#how-do-i-write-a-function-with-output-parameters-call-by-reference" title="Permalink to this headline">¶</a></h3>
Remember that arguments are passed by assignment in Python. Since assignment
just creates references to objects, there’s no alias between an argument name in
the caller and callee, and so no call-by-reference per se. You can achieve the
desired effect in a number of ways.
<ol class="arabic">
<li>By returning a tuple of the results:
<div class="highlight-default notranslate"><div class="highlight"><pre>def func2(a, b):
 a = &#39;new-value&#39; # a and b are local names
 b = b + 1 # assigned to new objects
 return a, b # return new values

x, y = &#39;old-value&#39;, 99
x, y = func2(x, y)
print x, y # output: new-value 100
</pre></div>
</div>
This is almost always the clearest solution.
</li>
<li>By using global variables. This isn’t thread-safe, and is not recommended.
</li>
<li>By passing a mutable (changeable in-place) object:
<div class="highlight-default notranslate"><div class="highlight"><pre>def func1(a):
 a[0] = &#39;new-value&#39; # &#39;a&#39; references a mutable list
 a[1] = a[1] + 1 # changes a shared object

args = [&#39;old-value&#39;, 99]
func1(args)
print args[0], args[1] # output: new-value 100
</pre></div>
</div>
</li>
<li>By passing in a dictionary that gets mutated:
<div class="highlight-default notranslate"><div class="highlight"><pre>def func3(args):
 args[&#39;a&#39;] = &#39;new-value&#39; # args is a mutable dictionary
 args[&#39;b&#39;] = args[&#39;b&#39;] + 1 # change it in-place

args = {&#39;a&#39;: &#39;old-value&#39;, &#39;b&#39;: 99}
func3(args)
print args[&#39;a&#39;], args[&#39;b&#39;]
</pre></div>
</div>
</li>
<li>Or bundle up values in a class instance:
<div class="highlight-default notranslate"><div class="highlight"><pre>class callByRef:
 def __init__(self, **args):
 for (key, value) in args.items():
 setattr(self, key, value)

def func4(args):
 args.a = &#39;new-value&#39; # args is a mutable callByRef
 args.b = args.b + 1 # change object in-place

args = callByRef(a=&#39;old-value&#39;, b=99)
func4(args)
print args.a, args.b
</pre></div>
</div>
There’s almost never a good reason to get this complicated.
</li>
</ol>
Your best choice is to return a tuple containing the multiple results.
</div>
<div class="section" id="how-do-you-make-a-higher-order-function-in-python">
<h3><a class="toc-backref" href="#id19">How do you make a higher order function in Python?</a><a class="headerlink" href="#how-do-you-make-a-higher-order-function-in-python" title="Permalink to this headline">¶</a></h3>
You have two choices: you can use nested scopes or you can use callable objects.
For example, suppose you wanted to define <code class="docutils literal notranslate">linear(a,b)</code> which returns a
function <code class="docutils literal notranslate">f(x)</code> that computes the value <code class="docutils literal notranslate">a*x+b</code>. Using nested scopes:
<div class="highlight-default notranslate"><div class="highlight"><pre>def linear(a, b):
 def result(x):
 return a * x + b
 return result
</pre></div>
</div>
Or using a callable object:
<div class="highlight-default notranslate"><div class="highlight"><pre>class linear:

def __init__(self, a, b):
 self.a, self.b = a, b

def __call__(self, x):
 return self.a * x + self.b
</pre></div>
</div>
In both cases,
<div class="highlight-default notranslate"><div class="highlight"><pre>taxes = linear(0.3, 2)
</pre></div>
</div>
gives a callable object where <code class="docutils literal notranslate">taxes(10e6) == 0.3 * 10e6 + 2</code>.
The callable object approach has the disadvantage that it is a bit slower and
results in slightly longer code. However, note that a collection of callables
can share their signature via inheritance:
<div class="highlight-default notranslate"><div class="highlight"><pre>class exponential(linear):
 # __init__ inherited
 def __call__(self, x):
 return self.a * (x ** self.b)
</pre></div>
</div>
Object can encapsulate state for several methods:
<div class="highlight-default notranslate"><div class="highlight"><pre>class counter:

value = 0

def set(self, x):
 self.value = x

def up(self):
 self.value = self.value + 1

def down(self):
 self.value = self.value - 1

count = counter()
inc, dec, reset = count.up, count.down, count.set
</pre></div>
</div>
Here <code class="docutils literal notranslate">inc()</code>, <code class="docutils literal notranslate">dec()</code> and <code class="docutils literal notranslate">reset()</code> act like functions which share the
same counting variable.
</div>
<div class="section" id="how-do-i-copy-an-object-in-python">
<h3><a class="toc-backref" href="#id20">How do I copy an object in Python?</a><a class="headerlink" href="#how-do-i-copy-an-object-in-python" title="Permalink to this headline">¶</a></h3>
In general, try <a class="reference internal" href="../library/copy.html#copy.copy" title="copy.copy"><code class="xref py py-func docutils literal notranslate">copy.copy()</code></a> or <a class="reference internal" href="../library/copy.html#copy.deepcopy" title="copy.deepcopy"><code class="xref py py-func docutils literal notranslate">copy.deepcopy()</code></a> for the general case.
Not all objects can be copied, but most can.
Some objects can be copied more easily. Dictionaries have a <a class="reference internal" href="../library/stdtypes.html#dict.copy" title="dict.copy"><code class="xref py py-meth docutils literal notranslate">copy()</code></a>
method:
<div class="highlight-default notranslate"><div class="highlight"><pre>newdict = olddict.copy()
</pre></div>
</div>
Sequences can be copied by slicing:
<div class="highlight-default notranslate"><div class="highlight"><pre>new_l = l[:]
</pre></div>
</div>
</div>
<div class="section" id="how-can-i-find-the-methods-or-attributes-of-an-object">
<h3><a class="toc-backref" href="#id21">How can I find the methods or attributes of an object?</a><a class="headerlink" href="#how-can-i-find-the-methods-or-attributes-of-an-object" title="Permalink to this headline">¶</a></h3>
For an instance x of a user-defined class, <code class="docutils literal notranslate">dir(x)</code> returns an alphabetized
list of the names containing the instance attributes and methods and attributes
defined by its class.
</div>
<div class="section" id="how-can-my-code-discover-the-name-of-an-object">
<h3><a class="toc-backref" href="#id22">How can my code discover the name of an object?</a><a class="headerlink" href="#how-can-my-code-discover-the-name-of-an-object" title="Permalink to this headline">¶</a></h3>
Generally speaking, it can’t, because objects don’t really have names.
Essentially, assignment always binds a name to a value; The same is true of
<code class="docutils literal notranslate">def</code> and <code class="docutils literal notranslate">class</code> statements, but in that case the value is a
callable. Consider the following code:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; class A:
... pass
...
&gt;&gt;&gt; B = A
&gt;&gt;&gt; a = B()
&gt;&gt;&gt; b = a
&gt;&gt;&gt; print b
&lt;__main__.A instance at 0x16D07CC&gt;
&gt;&gt;&gt; print a
&lt;__main__.A instance at 0x16D07CC&gt;
</pre></div>
</div>
Arguably the class has a name: even though it is bound to two names and invoked
through the name B the created instance is still reported as an instance of
class A. However, it is impossible to say whether the instance’s name is a or
b, since both names are bound to the same value.
Generally speaking it should not be necessary for your code to “know the names”
of particular values. Unless you are deliberately writing introspective
programs, this is usually an indication that a change of approach might be
beneficial.
In comp.lang.python, Fredrik Lundh once gave an excellent analogy in answer to
this question:
<blockquote>
<div>The same way as you get the name of that cat you found on your porch: the cat
(object) itself cannot tell you its name, and it doesn’t really care – so
the only way to find out what it’s called is to ask all your neighbours
(namespaces) if it’s their cat (object)…
….and don’t be surprised if you’ll find that it’s known by many names, or
no name at all!
</div></blockquote>
</div>
<div class="section" id="what-s-up-with-the-comma-operator-s-precedence">
<h3><a class="toc-backref" href="#id23">What’s up with the comma operator’s precedence?</a><a class="headerlink" href="#what-s-up-with-the-comma-operator-s-precedence" title="Permalink to this headline">¶</a></h3>
Comma is not an operator in Python. Consider this session:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; &quot;a&quot; in &quot;b&quot;, &quot;a&quot;
(False, &#39;a&#39;)
</pre></div>
</div>
Since the comma is not an operator, but a separator between expressions the
above is evaluated as if you had entered:
<div class="highlight-default notranslate"><div class="highlight"><pre>(&quot;a&quot; in &quot;b&quot;), &quot;a&quot;
</pre></div>
</div>
not:
<div class="highlight-default notranslate"><div class="highlight"><pre>&quot;a&quot; in (&quot;b&quot;, &quot;a&quot;)
</pre></div>
</div>
The same is true of the various assignment operators (<code class="docutils literal notranslate">=</code>, <code class="docutils literal notranslate">+=</code> etc). They
are not truly operators but syntactic delimiters in assignment statements.
</div>
<div class="section" id="is-there-an-equivalent-of-c-s-ternary-operator">
<h3><a class="toc-backref" href="#id24">Is there an equivalent of C’s “?:” ternary operator?</a><a class="headerlink" href="#is-there-an-equivalent-of-c-s-ternary-operator" title="Permalink to this headline">¶</a></h3>
Yes, this feature was added in Python 2.5. The syntax would be as follows:
<div class="highlight-default notranslate"><div class="highlight"><pre>[on_true] if [expression] else [on_false]

x, y = 50, 25

small = x if x &lt; y else y
</pre></div>
</div>
For versions previous to 2.5 the answer would be ‘No’.
</div>
<div class="section" id="is-it-possible-to-write-obfuscated-one-liners-in-python">
<h3><a class="toc-backref" href="#id25">Is it possible to write obfuscated one-liners in Python?</a><a class="headerlink" href="#is-it-possible-to-write-obfuscated-one-liners-in-python" title="Permalink to this headline">¶</a></h3>
Yes. Usually this is done by nesting <a class="reference internal" href="../reference/expressions.html#lambda"><code class="xref std std-keyword docutils literal notranslate">lambda</code></a> within
<a class="reference internal" href="../reference/expressions.html#lambda"><code class="xref std std-keyword docutils literal notranslate">lambda</code></a>. See the following three examples, due to Ulf Bartelt:
<div class="highlight-default notranslate"><div class="highlight"><pre># Primes &lt; 1000
print filter(None,map(lambda y:y*reduce(lambda x,y:x*y!=0,
map(lambda x,y=y:y%x,range(2,int(pow(y,0.5)+1))),1),range(2,1000)))

# First 10 Fibonacci numbers
print map(lambda x,f=lambda x,f:(f(x-1,f)+f(x-2,f)) if x&gt;1 else 1: f(x,f),
range(10))

# Mandelbrot set
print (lambda Ru,Ro,Iu,Io,IM,Sx,Sy:reduce(lambda x,y:x+y,map(lambda y,
Iu=Iu,Io=Io,Ru=Ru,Ro=Ro,Sy=Sy,L=lambda yc,Iu=Iu,Io=Io,Ru=Ru,Ro=Ro,i=IM,
Sx=Sx,Sy=Sy:reduce(lambda x,y:x+y,map(lambda x,xc=Ru,yc=yc,Ru=Ru,Ro=Ro,
i=i,Sx=Sx,F=lambda xc,yc,x,y,k,f=lambda xc,yc,x,y,k,f:(k&lt;=0)or (x*x+y*y
&gt;=4.0) or 1+f(xc,yc,x*x-y*y+xc,2.0*x*y+yc,k-1,f):f(xc,yc,x,y,k,f):chr(
64+F(Ru+x*(Ro-Ru)/Sx,yc,0,0,i)),range(Sx))):L(Iu+y*(Io-Iu)/Sy),range(Sy
))))(-2.1, 0.7, -1.2, 1.2, 30, 80, 24)
# \___ ___/ \___ ___/ | | |__ lines on screen
# V V | |______ columns on screen
# | | |__________ maximum of &quot;iterations&quot;
# | |_________________ range on y axis
# |____________________________ range on x axis
</pre></div>
</div>
Don’t try this at home, kids!
</div>
</div>
<div class="section" id="numbers-and-strings">
<h2><a class="toc-backref" href="#id26">Numbers and strings</a><a class="headerlink" href="#numbers-and-strings" title="Permalink to this headline">¶</a></h2>
<div class="section" id="how-do-i-specify-hexadecimal-and-octal-integers">
<h3><a class="toc-backref" href="#id27">How do I specify hexadecimal and octal integers?</a><a class="headerlink" href="#how-do-i-specify-hexadecimal-and-octal-integers" title="Permalink to this headline">¶</a></h3>
To specify an octal digit, precede the octal value with a zero, and then a lower
or uppercase “o”. For example, to set the variable “a” to the octal value “10”
(8 in decimal), type:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; a = 0o10
&gt;&gt;&gt; a
8
</pre></div>
</div>
Hexadecimal is just as easy. Simply precede the hexadecimal number with a zero,
and then a lower or uppercase “x”. Hexadecimal digits can be specified in lower
or uppercase. For example, in the Python interpreter:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; a = 0xa5
&gt;&gt;&gt; a
165
&gt;&gt;&gt; b = 0XB2
&gt;&gt;&gt; b
178
</pre></div>
</div>
</div>
<div class="section" id="why-does-22-10-return-3">
<h3><a class="toc-backref" href="#id28">Why does -22 // 10 return -3?</a><a class="headerlink" href="#why-does-22-10-return-3" title="Permalink to this headline">¶</a></h3>
It’s primarily driven by the desire that <code class="docutils literal notranslate">i % j</code> have the same sign as <code class="docutils literal notranslate">j</code>.
If you want that, and also want:
<div class="highlight-default notranslate"><div class="highlight"><pre>i == (i // j) * j + (i % j)
</pre></div>
</div>
then integer division has to return the floor. C also requires that identity to
hold, and then compilers that truncate <code class="docutils literal notranslate">i // j</code> need to make <code class="docutils literal notranslate">i % j</code> have
the same sign as <code class="docutils literal notranslate">i</code>.
There are few real use cases for <code class="docutils literal notranslate">i % j</code> when <code class="docutils literal notranslate">j</code> is negative. When <code class="docutils literal notranslate">j</code>
is positive, there are many, and in virtually all of them it’s more useful for
<code class="docutils literal notranslate">i % j</code> to be <code class="docutils literal notranslate">&gt;= 0</code>. If the clock says 10 now, what did it say 200 hours
ago? <code class="docutils literal notranslate">-190 % 12 == 2</code> is useful; <code class="docutils literal notranslate">-190 % 12 == -10</code> is a bug waiting to
bite.
<div class="admonition note">
Note
On Python 2, <code class="docutils literal notranslate">a / b</code> returns the same as <code class="docutils literal notranslate">a // b</code> if
<code class="docutils literal notranslate">__future__.division</code> is not in effect. This is also known as “classic”
division.
</div>
</div>
<div class="section" id="how-do-i-convert-a-string-to-a-number">
<h3><a class="toc-backref" href="#id29">How do I convert a string to a number?</a><a class="headerlink" href="#how-do-i-convert-a-string-to-a-number" title="Permalink to this headline">¶</a></h3>
For integers, use the built-in <a class="reference internal" href="../library/functions.html#int" title="int"><code class="xref py py-func docutils literal notranslate">int()</code></a> type constructor, e.g. <code class="docutils literal notranslate">int('144')
== 144</code>. Similarly, <a class="reference internal" href="../library/functions.html#float" title="float"><code class="xref py py-func docutils literal notranslate">float()</code></a> converts to floating-point,
e.g. <code class="docutils literal notranslate">float('144') == 144.0</code>.
By default, these interpret the number as decimal, so that <code class="docutils literal notranslate">int('0144') ==
144</code> and <code class="docutils literal notranslate">int('0x144')</code> raises <a class="reference internal" href="../library/exceptions.html#exceptions.ValueError" title="exceptions.ValueError"><code class="xref py py-exc docutils literal notranslate">ValueError</code></a>. <code class="docutils literal notranslate">int(string, base)</code> takes
the base to convert from as a second optional argument, so <code class="docutils literal notranslate">int('0x144', 16) ==
324</code>. If the base is specified as 0, the number is interpreted using Python’s
rules: a leading ‘0’ indicates octal, and ‘0x’ indicates a hex number.
Do not use the built-in function <a class="reference internal" href="../library/functions.html#eval" title="eval"><code class="xref py py-func docutils literal notranslate">eval()</code></a> if all you need is to convert
strings to numbers. <a class="reference internal" href="../library/functions.html#eval" title="eval"><code class="xref py py-func docutils literal notranslate">eval()</code></a> will be significantly slower and it presents a
security risk: someone could pass you a Python expression that might have
unwanted side effects. For example, someone could pass
<code class="docutils literal notranslate">__import__('os').system(&quot;rm -rf $HOME&quot;)</code> which would erase your home
directory.
<a class="reference internal" href="../library/functions.html#eval" title="eval"><code class="xref py py-func docutils literal notranslate">eval()</code></a> also has the effect of interpreting numbers as Python expressions,
so that e.g. <code class="docutils literal notranslate">eval('09')</code> gives a syntax error because Python regards numbers
starting with ‘0’ as octal (base 8).
</div>
<div class="section" id="how-do-i-convert-a-number-to-a-string">
<h3><a class="toc-backref" href="#id30">How do I convert a number to a string?</a><a class="headerlink" href="#how-do-i-convert-a-number-to-a-string" title="Permalink to this headline">¶</a></h3>
To convert, e.g., the number 144 to the string ‘144’, use the built-in type
constructor <a class="reference internal" href="../library/functions.html#str" title="str"><code class="xref py py-func docutils literal notranslate">str()</code></a>. If you want a hexadecimal or octal representation, use
the built-in functions <a class="reference internal" href="../library/functions.html#hex" title="hex"><code class="xref py py-func docutils literal notranslate">hex()</code></a> or <a class="reference internal" href="../library/functions.html#oct" title="oct"><code class="xref py py-func docutils literal notranslate">oct()</code></a>. For fancy formatting, see
the <a class="reference internal" href="../library/string.html#formatstrings">Format String Syntax</a> section, e.g. <code class="docutils literal notranslate">&quot;{:04d}&quot;.format(144)</code> yields
<code class="docutils literal notranslate">'0144'</code> and <code class="docutils literal notranslate">&quot;{:.3f}&quot;.format(1/3)</code> yields <code class="docutils literal notranslate">'0.333'</code>. You may also use
<a class="reference internal" href="../library/stdtypes.html#string-formatting">the % operator</a> on strings. See the library reference
manual for details.
</div>
<div class="section" id="how-do-i-modify-a-string-in-place">
<h3><a class="toc-backref" href="#id31">How do I modify a string in place?</a><a class="headerlink" href="#how-do-i-modify-a-string-in-place" title="Permalink to this headline">¶</a></h3>
You can’t, because strings are immutable. If you need an object with this
ability, try converting the string to a list or use the array module:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; import io
&gt;&gt;&gt; s = &quot;Hello, world&quot;
&gt;&gt;&gt; a = list(s)
&gt;&gt;&gt; print a
[&#39;H&#39;, &#39;e&#39;, &#39;l&#39;, &#39;l&#39;, &#39;o&#39;, &#39;,&#39;, &#39; &#39;, &#39;w&#39;, &#39;o&#39;, &#39;r&#39;, &#39;l&#39;, &#39;d&#39;]
&gt;&gt;&gt; a[7:] = list(&quot;there!&quot;)
&gt;&gt;&gt; &#39;&#39;.join(a)
&#39;Hello, there!&#39;

&gt;&gt;&gt; import array
&gt;&gt;&gt; a = array.array(&#39;c&#39;, s)
&gt;&gt;&gt; print a
array(&#39;c&#39;, &#39;Hello, world&#39;)
&gt;&gt;&gt; a[0] = &#39;y&#39;; print a
array(&#39;c&#39;, &#39;yello, world&#39;)
&gt;&gt;&gt; a.tostring()
&#39;yello, world&#39;
</pre></div>
</div>
</div>
<div class="section" id="how-do-i-use-strings-to-call-functions-methods">
<h3><a class="toc-backref" href="#id32">How do I use strings to call functions/methods?</a><a class="headerlink" href="#how-do-i-use-strings-to-call-functions-methods" title="Permalink to this headline">¶</a></h3>
There are various techniques.
<ul>
<li>The best is to use a dictionary that maps strings to functions. The primary
advantage of this technique is that the strings do not need to match the names
of the functions. This is also the primary technique used to emulate a case
construct:
<div class="highlight-default notranslate"><div class="highlight"><pre>def a():
 pass

def b():
 pass

dispatch = {&#39;go&#39;: a, &#39;stop&#39;: b} # Note lack of parens for funcs

dispatch[get_input()]() # Note trailing parens to call function
</pre></div>
</div>
</li>
<li>Use the built-in function <a class="reference internal" href="../library/functions.html#getattr" title="getattr"><code class="xref py py-func docutils literal notranslate">getattr()</code></a>:
<div class="highlight-default notranslate"><div class="highlight"><pre>import foo
getattr(foo, &#39;bar&#39;)()
</pre></div>
</div>
Note that <a class="reference internal" href="../library/functions.html#getattr" title="getattr"><code class="xref py py-func docutils literal notranslate">getattr()</code></a> works on any object, including classes, class
instances, modules, and so on.
This is used in several places in the standard library, like this:
<div class="highlight-default notranslate"><div class="highlight"><pre>class Foo:
 def do_foo(self):
 ...

def do_bar(self):
 ...

f = getattr(foo_instance, &#39;do_&#39; + opname)
f()
</pre></div>
</div>
</li>
<li>Use <a class="reference internal" href="../library/functions.html#locals" title="locals"><code class="xref py py-func docutils literal notranslate">locals()</code></a> or <a class="reference internal" href="../library/functions.html#eval" title="eval"><code class="xref py py-func docutils literal notranslate">eval()</code></a> to resolve the function name:
<div class="highlight-default notranslate"><div class="highlight"><pre>def myFunc():
 print &quot;hello&quot;

fname = &quot;myFunc&quot;

f = locals()[fname]
f()

f = eval(fname)
f()
</pre></div>
</div>
Note: Using <a class="reference internal" href="../library/functions.html#eval" title="eval"><code class="xref py py-func docutils literal notranslate">eval()</code></a> is slow and dangerous. If you don’t have absolute
control over the contents of the string, someone could pass a string that
resulted in an arbitrary function being executed.
</li>
</ul>
</div>
<div class="section" id="is-there-an-equivalent-to-perl-s-chomp-for-removing-trailing-newlines-from-strings">
<h3><a class="toc-backref" href="#id33">Is there an equivalent to Perl’s chomp() for removing trailing newlines from strings?</a><a class="headerlink" href="#is-there-an-equivalent-to-perl-s-chomp-for-removing-trailing-newlines-from-strings" title="Permalink to this headline">¶</a></h3>
Starting with Python 2.2, you can use <code class="docutils literal notranslate">S.rstrip(&quot;\r\n&quot;)</code> to remove all
occurrences of any line terminator from the end of the string <code class="docutils literal notranslate">S</code> without
removing other trailing whitespace. If the string <code class="docutils literal notranslate">S</code> represents more than
one line, with several empty lines at the end, the line terminators for all the
blank lines will be removed:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; lines = (&quot;line 1 \r\n&quot;
... &quot;\r\n&quot;
... &quot;\r\n&quot;)
&gt;&gt;&gt; lines.rstrip(&quot;\n\r&quot;)
&#39;line 1 &#39;
</pre></div>
</div>
Since this is typically only desired when reading text one line at a time, using
<code class="docutils literal notranslate">S.rstrip()</code> this way works well.
For older versions of Python, there are two partial substitutes:
<ul class="simple">
<li>If you want to remove all trailing whitespace, use the <code class="docutils literal notranslate">rstrip()</code> method of
string objects. This removes all trailing whitespace, not just a single
newline.</li>
<li>Otherwise, if there is only one line in the string <code class="docutils literal notranslate">S</code>, use
<code class="docutils literal notranslate">S.splitlines()[0]</code>.</li>
</ul>
</div>
<div class="section" id="is-there-a-scanf-or-sscanf-equivalent">
<h3><a class="toc-backref" href="#id34">Is there a scanf() or sscanf() equivalent?</a><a class="headerlink" href="#is-there-a-scanf-or-sscanf-equivalent" title="Permalink to this headline">¶</a></h3>
Not as such.
For simple input parsing, the easiest approach is usually to split the line into
whitespace-delimited words using the <a class="reference internal" href="../library/stdtypes.html#str.split" title="str.split"><code class="xref py py-meth docutils literal notranslate">split()</code></a> method of string objects
and then convert decimal strings to numeric values using <a class="reference internal" href="../library/functions.html#int" title="int"><code class="xref py py-func docutils literal notranslate">int()</code></a> or
<a class="reference internal" href="../library/functions.html#float" title="float"><code class="xref py py-func docutils literal notranslate">float()</code></a>. <code class="docutils literal notranslate">split()</code> supports an optional “sep” parameter which is useful
if the line uses something other than whitespace as a separator.
For more complicated input parsing, regular expressions are more powerful
than C’s <code class="xref c c-func docutils literal notranslate">sscanf()</code> and better suited for the task.
</div>
<div class="section" id="what-does-unicodeerror-ascii-decoding-encoding-error-ordinal-not-in-range-128-mean">
<h3><a class="toc-backref" href="#id35">What does ‘UnicodeError: ASCII [decoding,encoding] error: ordinal not in range(128)’ mean?</a><a class="headerlink" href="#what-does-unicodeerror-ascii-decoding-encoding-error-ordinal-not-in-range-128-mean" title="Permalink to this headline">¶</a></h3>
This error indicates that your Python installation can handle only 7-bit ASCII
strings. There are a couple ways to fix or work around the problem.
If your programs must handle data in arbitrary character set encodings, the
environment the application runs in will generally identify the encoding of the
data it is handing you. You need to convert the input to Unicode data using
that encoding. For example, a program that handles email or web input will
typically find character set encoding information in Content-Type headers. This
can then be used to properly convert input data to Unicode. Assuming the string
referred to by <code class="docutils literal notranslate">value</code> is encoded as UTF-8:
<div class="highlight-default notranslate"><div class="highlight"><pre>value = unicode(value, &quot;utf-8&quot;)
</pre></div>
</div>
will return a Unicode object. If the data is not correctly encoded as UTF-8,
the above call will raise a <a class="reference internal" href="../library/exceptions.html#exceptions.UnicodeError" title="exceptions.UnicodeError"><code class="xref py py-exc docutils literal notranslate">UnicodeError</code></a> exception.
If you only want strings converted to Unicode which have non-ASCII data, you can
try converting them first assuming an ASCII encoding, and then generate Unicode
objects if that fails:
<div class="highlight-default notranslate"><div class="highlight"><pre>try:
 x = unicode(value, &quot;ascii&quot;)
except UnicodeError:
 value = unicode(value, &quot;utf-8&quot;)
else:
 # value was valid ASCII data
 pass
</pre></div>
</div>
It’s possible to set a default encoding in a file called <code class="docutils literal notranslate">sitecustomize.py</code>
that’s part of the Python library. However, this isn’t recommended because
changing the Python-wide default encoding may cause third-party extension
modules to fail.
Note that on Windows, there is an encoding known as “mbcs”, which uses an
encoding specific to your current locale. In many cases, and particularly when
working with COM, this may be an appropriate default encoding to use.
</div>
</div>
<div class="section" id="sequences-tuples-lists">
<h2><a class="toc-backref" href="#id36">Sequences (Tuples/Lists)</a><a class="headerlink" href="#sequences-tuples-lists" title="Permalink to this headline">¶</a></h2>
<div class="section" id="how-do-i-convert-between-tuples-and-lists">
<h3><a class="toc-backref" href="#id37">How do I convert between tuples and lists?</a><a class="headerlink" href="#how-do-i-convert-between-tuples-and-lists" title="Permalink to this headline">¶</a></h3>
The type constructor <code class="docutils literal notranslate">tuple(seq)</code> converts any sequence (actually, any
iterable) into a tuple with the same items in the same order.
For example, <code class="docutils literal notranslate">tuple([1, 2, 3])</code> yields <code class="docutils literal notranslate">(1, 2, 3)</code> and <code class="docutils literal notranslate">tuple('abc')</code>
yields <code class="docutils literal notranslate">('a', 'b', 'c')</code>. If the argument is a tuple, it does not make a copy
but returns the same object, so it is cheap to call <a class="reference internal" href="../library/functions.html#tuple" title="tuple"><code class="xref py py-func docutils literal notranslate">tuple()</code></a> when you
aren’t sure that an object is already a tuple.
The type constructor <code class="docutils literal notranslate">list(seq)</code> converts any sequence or iterable into a list
with the same items in the same order. For example, <code class="docutils literal notranslate">list((1, 2, 3))</code> yields
<code class="docutils literal notranslate">[1, 2, 3]</code> and <code class="docutils literal notranslate">list('abc')</code> yields <code class="docutils literal notranslate">['a', 'b', 'c']</code>. If the argument
is a list, it makes a copy just like <code class="docutils literal notranslate">seq[:]</code> would.
</div>
<div class="section" id="what-s-a-negative-index">
<h3><a class="toc-backref" href="#id38">What’s a negative index?</a><a class="headerlink" href="#what-s-a-negative-index" title="Permalink to this headline">¶</a></h3>
Python sequences are indexed with positive numbers and negative numbers. For
positive numbers 0 is the first index 1 is the second index and so forth. For
negative indices -1 is the last index and -2 is the penultimate (next to last)
index and so forth. Think of <code class="docutils literal notranslate">seq[-n]</code> as the same as <code class="docutils literal notranslate">seq[len(seq)-n]</code>.
Using negative indices can be very convenient. For example <code class="docutils literal notranslate">S[:-1]</code> is all of
the string except for its last character, which is useful for removing the
trailing newline from a string.
</div>
<div class="section" id="how-do-i-iterate-over-a-sequence-in-reverse-order">
<h3><a class="toc-backref" href="#id39">How do I iterate over a sequence in reverse order?</a><a class="headerlink" href="#how-do-i-iterate-over-a-sequence-in-reverse-order" title="Permalink to this headline">¶</a></h3>
Use the <a class="reference internal" href="../library/functions.html#reversed" title="reversed"><code class="xref py py-func docutils literal notranslate">reversed()</code></a> built-in function, which is new in Python 2.4:
<div class="highlight-default notranslate"><div class="highlight"><pre>for x in reversed(sequence):
 ... # do something with x ...
</pre></div>
</div>
This won’t touch your original sequence, but build a new copy with reversed
order to iterate over.
With Python 2.3, you can use an extended slice syntax:
<div class="highlight-default notranslate"><div class="highlight"><pre>for x in sequence[::-1]:
 ... # do something with x ...
</pre></div>
</div>
</div>
<div class="section" id="how-do-you-remove-duplicates-from-a-list">
<h3><a class="toc-backref" href="#id40">How do you remove duplicates from a list?</a><a class="headerlink" href="#how-do-you-remove-duplicates-from-a-list" title="Permalink to this headline">¶</a></h3>
See the Python Cookbook for a long discussion of many ways to do this:
<blockquote>
<div><a class="reference external" href="https://code.activestate.com/recipes/52560/">https://code.activestate.com/recipes/52560/</a></div></blockquote>
If you don’t mind reordering the list, sort it and then scan from the end of the
list, deleting duplicates as you go:
<div class="highlight-default notranslate"><div class="highlight"><pre>if mylist:
 mylist.sort()
 last = mylist[-1]
 for i in range(len(mylist)-2, -1, -1):
 if last == mylist[i]:
 del mylist[i]
 else:
 last = mylist[i]
</pre></div>
</div>
If all elements of the list may be used as dictionary keys (i.e. they are all
hashable) this is often faster
<div class="highlight-default notranslate"><div class="highlight"><pre>d = {}
for x in mylist:
 d[x] = 1
mylist = list(d.keys())
</pre></div>
</div>
In Python 2.5 and later, the following is possible instead:
<div class="highlight-default notranslate"><div class="highlight"><pre>mylist = list(set(mylist))
</pre></div>
</div>
This converts the list into a set, thereby removing duplicates, and then back
into a list.
</div>
<div class="section" id="how-do-you-make-an-array-in-python">
<h3><a class="toc-backref" href="#id41">How do you make an array in Python?</a><a class="headerlink" href="#how-do-you-make-an-array-in-python" title="Permalink to this headline">¶</a></h3>
Use a list:
<div class="highlight-default notranslate"><div class="highlight"><pre>[&quot;this&quot;, 1, &quot;is&quot;, &quot;an&quot;, &quot;array&quot;]
</pre></div>
</div>
Lists are equivalent to C or Pascal arrays in their time complexity; the primary
difference is that a Python list can contain objects of many different types.
The <code class="docutils literal notranslate">array</code> module also provides methods for creating arrays of fixed types
with compact representations, but they are slower to index than lists. Also
note that the Numeric extensions and others define array-like structures with
various characteristics as well.
To get Lisp-style linked lists, you can emulate cons cells using tuples:
<div class="highlight-default notranslate"><div class="highlight"><pre>lisp_list = (&quot;like&quot;, (&quot;this&quot;, (&quot;example&quot;, None) ) )
</pre></div>
</div>
If mutability is desired, you could use lists instead of tuples. Here the
analogue of lisp car is <code class="docutils literal notranslate">lisp_list[0]</code> and the analogue of cdr is
<code class="docutils literal notranslate">lisp_list[1]</code>. Only do this if you’re sure you really need to, because it’s
usually a lot slower than using Python lists.
</div>
<div class="section" id="how-do-i-create-a-multidimensional-list">
<h3><a class="toc-backref" href="#id42">How do I create a multidimensional list?</a><a class="headerlink" href="#how-do-i-create-a-multidimensional-list" title="Permalink to this headline">¶</a></h3>
You probably tried to make a multidimensional array like this:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; A = [[None] * 2] * 3
</pre></div>
</div>
This looks correct if you print it:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; A
[[None, None], [None, None], [None, None]]
</pre></div>
</div>
But when you assign a value, it shows up in multiple places:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; A[0][0] = 5
&gt;&gt;&gt; A
[[5, None], [5, None], [5, None]]
</pre></div>
</div>
The reason is that replicating a list with <code class="docutils literal notranslate">*</code> doesn’t create copies, it only
creates references to the existing objects. The <code class="docutils literal notranslate">*3</code> creates a list
containing 3 references to the same list of length two. Changes to one row will
show in all rows, which is almost certainly not what you want.
The suggested approach is to create a list of the desired length first and then
fill in each element with a newly created list:
<div class="highlight-default notranslate"><div class="highlight"><pre>A = [None] * 3
for i in range(3):
 A[i] = [None] * 2
</pre></div>
</div>
This generates a list containing 3 different lists of length two. You can also
use a list comprehension:
<div class="highlight-default notranslate"><div class="highlight"><pre>w, h = 2, 3
A = [[None] * w for i in range(h)]
</pre></div>
</div>
Or, you can use an extension that provides a matrix datatype; <a class="reference external" href="http://www.numpy.org/">NumPy</a> is the best known.
</div>
<div class="section" id="how-do-i-apply-a-method-to-a-sequence-of-objects">
<h3><a class="toc-backref" href="#id43">How do I apply a method to a sequence of objects?</a><a class="headerlink" href="#how-do-i-apply-a-method-to-a-sequence-of-objects" title="Permalink to this headline">¶</a></h3>
Use a list comprehension:
<div class="highlight-default notranslate"><div class="highlight"><pre>result = [obj.method() for obj in mylist]
</pre></div>
</div>
More generically, you can try the following function:
<div class="highlight-default notranslate"><div class="highlight"><pre>def method_map(objects, method, arguments):
 &quot;&quot;&quot;method_map([a,b], &quot;meth&quot;, (1,2)) gives [a.meth(1,2), b.meth(1,2)]&quot;&quot;&quot;
 nobjects = len(objects)
 methods = map(getattr, objects, [method]*nobjects)
 return map(apply, methods, [arguments]*nobjects)
</pre></div>
</div>
</div>
<div class="section" id="why-does-a-tuple-i-item-raise-an-exception-when-the-addition-works">
<h3><a class="toc-backref" href="#id44">Why does a_tuple[i] += [‘item’] raise an exception when the addition works?</a><a class="headerlink" href="#why-does-a-tuple-i-item-raise-an-exception-when-the-addition-works" title="Permalink to this headline">¶</a></h3>
This is because of a combination of the fact that augmented assignment
operators are assignment operators, and the difference between mutable and
immutable objects in Python.
This discussion applies in general when augmented assignment operators are
applied to elements of a tuple that point to mutable objects, but we’ll use
a <code class="docutils literal notranslate">list</code> and <code class="docutils literal notranslate">+=</code> as our exemplar.
If you wrote:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; a_tuple = (1, 2)
&gt;&gt;&gt; a_tuple[0] += 1
Traceback (most recent call last):
 ...
TypeError: &#39;tuple&#39; object does not support item assignment
</pre></div>
</div>
The reason for the exception should be immediately clear: <code class="docutils literal notranslate">1</code> is added to the
object <code class="docutils literal notranslate">a_tuple[0]</code> points to (<code class="docutils literal notranslate">1</code>), producing the result object, <code class="docutils literal notranslate">2</code>,
but when we attempt to assign the result of the computation, <code class="docutils literal notranslate">2</code>, to element
<code class="docutils literal notranslate">0</code> of the tuple, we get an error because we can’t change what an element of
a tuple points to.
Under the covers, what this augmented assignment statement is doing is
approximately this:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; result = a_tuple[0] + 1
&gt;&gt;&gt; a_tuple[0] = result
Traceback (most recent call last):
 ...
TypeError: &#39;tuple&#39; object does not support item assignment
</pre></div>
</div>
It is the assignment part of the operation that produces the error, since a
tuple is immutable.
When you write something like:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; a_tuple = ([&#39;foo&#39;], &#39;bar&#39;)
&gt;&gt;&gt; a_tuple[0] += [&#39;item&#39;]
Traceback (most recent call last):
 ...
TypeError: &#39;tuple&#39; object does not support item assignment
</pre></div>
</div>
The exception is a bit more surprising, and even more surprising is the fact
that even though there was an error, the append worked:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; a_tuple[0]
[&#39;foo&#39;, &#39;item&#39;]
</pre></div>
</div>
To see why this happens, you need to know that (a) if an object implements an
<code class="docutils literal notranslate">__iadd__</code> magic method, it gets called when the <code class="docutils literal notranslate">+=</code> augmented assignment
is executed, and its return value is what gets used in the assignment statement;
and (b) for lists, <code class="docutils literal notranslate">__iadd__</code> is equivalent to calling <code class="docutils literal notranslate">extend</code> on the list
and returning the list. That’s why we say that for lists, <code class="docutils literal notranslate">+=</code> is a
“shorthand” for <code class="docutils literal notranslate">list.extend</code>:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; a_list = []
&gt;&gt;&gt; a_list += [1]
&gt;&gt;&gt; a_list
[1]
</pre></div>
</div>
This is equivalent to:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; result = a_list.__iadd__([1])
&gt;&gt;&gt; a_list = result
</pre></div>
</div>
The object pointed to by a_list has been mutated, and the pointer to the
mutated object is assigned back to <code class="docutils literal notranslate">a_list</code>. The end result of the
assignment is a no-op, since it is a pointer to the same object that <code class="docutils literal notranslate">a_list</code>
was previously pointing to, but the assignment still happens.
Thus, in our tuple example what is happening is equivalent to:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; result = a_tuple[0].__iadd__([&#39;item&#39;])
&gt;&gt;&gt; a_tuple[0] = result
Traceback (most recent call last):
 ...
TypeError: &#39;tuple&#39; object does not support item assignment
</pre></div>
</div>
The <code class="docutils literal notranslate">__iadd__</code> succeeds, and thus the list is extended, but even though
<code class="docutils literal notranslate">result</code> points to the same object that <code class="docutils literal notranslate">a_tuple[0]</code> already points to,
that final assignment still results in an error, because tuples are immutable.
</div>
</div>
<div class="section" id="dictionaries">
<h2><a class="toc-backref" href="#id45">Dictionaries</a><a class="headerlink" href="#dictionaries" title="Permalink to this headline">¶</a></h2>
<div class="section" id="how-can-i-get-a-dictionary-to-display-its-keys-in-a-consistent-order">
<h3><a class="toc-backref" href="#id46">How can I get a dictionary to display its keys in a consistent order?</a><a class="headerlink" href="#how-can-i-get-a-dictionary-to-display-its-keys-in-a-consistent-order" title="Permalink to this headline">¶</a></h3>
You can’t. Dictionaries store their keys in an unpredictable order, so the
display order of a dictionary’s elements will be similarly unpredictable.
This can be frustrating if you want to save a printable version to a file, make
some changes and then compare it with some other printed dictionary. In this
case, use the <code class="docutils literal notranslate">pprint</code> module to pretty-print the dictionary; the items will
be presented in order sorted by the key.
A more complicated solution is to subclass <code class="docutils literal notranslate">dict</code> to create a
<code class="docutils literal notranslate">SortedDict</code> class that prints itself in a predictable order. Here’s one
simpleminded implementation of such a class:
<div class="highlight-default notranslate"><div class="highlight"><pre>class SortedDict(dict):
 def __repr__(self):
 keys = sorted(self.keys())
 result = (&quot;{!r}: {!r}&quot;.format(k, self[k]) for k in keys)
 return &quot;{{{}}}&quot;.format(&quot;, &quot;.join(result))

__str__ = __repr__
</pre></div>
</div>
This will work for many common situations you might encounter, though it’s far
from a perfect solution. The largest flaw is that if some values in the
dictionary are also dictionaries, their values won’t be presented in any
particular order.
</div>
<div class="section" id="i-want-to-do-a-complicated-sort-can-you-do-a-schwartzian-transform-in-python">
<h3><a class="toc-backref" href="#id47">I want to do a complicated sort: can you do a Schwartzian Transform in Python?</a><a class="headerlink" href="#i-want-to-do-a-complicated-sort-can-you-do-a-schwartzian-transform-in-python" title="Permalink to this headline">¶</a></h3>
The technique, attributed to Randal Schwartz of the Perl community, sorts the
elements of a list by a metric which maps each element to its “sort value”. In
Python, use the <code class="docutils literal notranslate">key</code> argument for the <code class="xref py py-func docutils literal notranslate">sort()</code> function:
<div class="highlight-default notranslate"><div class="highlight"><pre>Isorted = L[:]
Isorted.sort(key=lambda s: int(s[10:15]))
</pre></div>
</div>
</div>
<div class="section" id="how-can-i-sort-one-list-by-values-from-another-list">
<h3><a class="toc-backref" href="#id48">How can I sort one list by values from another list?</a><a class="headerlink" href="#how-can-i-sort-one-list-by-values-from-another-list" title="Permalink to this headline">¶</a></h3>
Merge them into a single list of tuples, sort the resulting list, and then pick
out the element you want.
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; list1 = [&quot;what&quot;, &quot;I&#39;m&quot;, &quot;sorting&quot;, &quot;by&quot;]
&gt;&gt;&gt; list2 = [&quot;something&quot;, &quot;else&quot;, &quot;to&quot;, &quot;sort&quot;]
&gt;&gt;&gt; pairs = zip(list1, list2)
&gt;&gt;&gt; pairs
[(&#39;what&#39;, &#39;something&#39;), (&quot;I&#39;m&quot;, &#39;else&#39;), (&#39;sorting&#39;, &#39;to&#39;), (&#39;by&#39;, &#39;sort&#39;)]
&gt;&gt;&gt; pairs.sort()
&gt;&gt;&gt; result = [ x[1] for x in pairs ]
&gt;&gt;&gt; result
[&#39;else&#39;, &#39;sort&#39;, &#39;to&#39;, &#39;something&#39;]
</pre></div>
</div>
An alternative for the last step is:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; result = []
&gt;&gt;&gt; for p in pairs: result.append(p[1])
</pre></div>
</div>
If you find this more legible, you might prefer to use this instead of the final
list comprehension. However, it is almost twice as slow for long lists. Why?
First, the <code class="docutils literal notranslate">append()</code> operation has to reallocate memory, and while it uses
some tricks to avoid doing that each time, it still has to do it occasionally,
and that costs quite a bit. Second, the expression “result.append” requires an
extra attribute lookup, and third, there’s a speed reduction from having to make
all those function calls.
</div>
</div>
<div class="section" id="objects">
<h2><a class="toc-backref" href="#id49">Objects</a><a class="headerlink" href="#objects" title="Permalink to this headline">¶</a></h2>
<div class="section" id="what-is-a-class">
<h3><a class="toc-backref" href="#id50">What is a class?</a><a class="headerlink" href="#what-is-a-class" title="Permalink to this headline">¶</a></h3>
A class is the particular object type created by executing a class statement.
Class objects are used as templates to create instance objects, which embody
both the data (attributes) and code (methods) specific to a datatype.
A class can be based on one or more other classes, called its base class(es). It
then inherits the attributes and methods of its base classes. This allows an
object model to be successively refined by inheritance. You might have a
generic <code class="docutils literal notranslate">Mailbox</code> class that provides basic accessor methods for a mailbox,
and subclasses such as <code class="docutils literal notranslate">MboxMailbox</code>, <code class="docutils literal notranslate">MaildirMailbox</code>, <code class="docutils literal notranslate">OutlookMailbox</code>
that handle various specific mailbox formats.
</div>
<div class="section" id="what-is-a-method">
<h3><a class="toc-backref" href="#id51">What is a method?</a><a class="headerlink" href="#what-is-a-method" title="Permalink to this headline">¶</a></h3>
A method is a function on some object <code class="docutils literal notranslate">x</code> that you normally call as
<code class="docutils literal notranslate">x.name(arguments...)</code>. Methods are defined as functions inside the class
definition:
<div class="highlight-default notranslate"><div class="highlight"><pre>class C:
 def meth(self, arg):
 return arg * 2 + self.attribute
</pre></div>
</div>
</div>
<div class="section" id="what-is-self">
<h3><a class="toc-backref" href="#id52">What is self?</a><a class="headerlink" href="#what-is-self" title="Permalink to this headline">¶</a></h3>
Self is merely a conventional name for the first argument of a method. A method
defined as <code class="docutils literal notranslate">meth(self, a, b, c)</code> should be called as <code class="docutils literal notranslate">x.meth(a, b, c)</code> for
some instance <code class="docutils literal notranslate">x</code> of the class in which the definition occurs; the called
method will think it is called as <code class="docutils literal notranslate">meth(x, a, b, c)</code>.
See also <a class="reference internal" href="design.html#why-self">Why must ‘self’ be used explicitly in method definitions and calls?</a>.
</div>
<div class="section" id="how-do-i-check-if-an-object-is-an-instance-of-a-given-class-or-of-a-subclass-of-it">
<h3><a class="toc-backref" href="#id53">How do I check if an object is an instance of a given class or of a subclass of it?</a><a class="headerlink" href="#how-do-i-check-if-an-object-is-an-instance-of-a-given-class-or-of-a-subclass-of-it" title="Permalink to this headline">¶</a></h3>
Use the built-in function <code class="docutils literal notranslate">isinstance(obj, cls)</code>. You can check if an object
is an instance of any of a number of classes by providing a tuple instead of a
single class, e.g. <code class="docutils literal notranslate">isinstance(obj, (class1, class2, ...))</code>, and can also
check whether an object is one of Python’s built-in types, e.g.
<code class="docutils literal notranslate">isinstance(obj, str)</code> or <code class="docutils literal notranslate">isinstance(obj, (int, long, float, complex))</code>.
Note that most programs do not use <a class="reference internal" href="../library/functions.html#isinstance" title="isinstance"><code class="xref py py-func docutils literal notranslate">isinstance()</code></a> on user-defined classes
very often. If you are developing the classes yourself, a more proper
object-oriented style is to define methods on the classes that encapsulate a
particular behaviour, instead of checking the object’s class and doing a
different thing based on what class it is. For example, if you have a function
that does something:
<div class="highlight-default notranslate"><div class="highlight"><pre>def search(obj):
 if isinstance(obj, Mailbox):
 ... # code to search a mailbox
 elif isinstance(obj, Document):
 ... # code to search a document
 elif ...
</pre></div>
</div>
A better approach is to define a <code class="docutils literal notranslate">search()</code> method on all the classes and just
call it:
<div class="highlight-default notranslate"><div class="highlight"><pre>class Mailbox:
 def search(self):
 ... # code to search a mailbox

class Document:
 def search(self):
 ... # code to search a document

obj.search()
</pre></div>
</div>
</div>
<div class="section" id="what-is-delegation">
<h3><a class="toc-backref" href="#id54">What is delegation?</a><a class="headerlink" href="#what-is-delegation" title="Permalink to this headline">¶</a></h3>
Delegation is an object oriented technique (also called a design pattern).
Let’s say you have an object <code class="docutils literal notranslate">x</code> and want to change the behaviour of just one
of its methods. You can create a new class that provides a new implementation
of the method you’re interested in changing and delegates all other methods to
the corresponding method of <code class="docutils literal notranslate">x</code>.
Python programmers can easily implement delegation. For example, the following
class implements a class that behaves like a file but converts all written data
to uppercase:
<div class="highlight-default notranslate"><div class="highlight"><pre>class UpperOut:

def __init__(self, outfile):
 self._outfile = outfile

def write(self, s):
 self._outfile.write(s.upper())

def __getattr__(self, name):
 return getattr(self._outfile, name)
</pre></div>
</div>
Here the <code class="docutils literal notranslate">UpperOut</code> class redefines the <code class="docutils literal notranslate">write()</code> method to convert the
argument string to uppercase before calling the underlying
<code class="docutils literal notranslate">self.__outfile.write()</code> method. All other methods are delegated to the
underlying <code class="docutils literal notranslate">self.__outfile</code> object. The delegation is accomplished via the
<code class="docutils literal notranslate">__getattr__</code> method; consult <a class="reference internal" href="../reference/datamodel.html#attribute-access">the language reference</a>
for more information about controlling attribute access.
Note that for more general cases delegation can get trickier. When attributes
must be set as well as retrieved, the class must define a <a class="reference internal" href="../reference/datamodel.html#object.__setattr__" title="object.__setattr__"><code class="xref py py-meth docutils literal notranslate">__setattr__()</code></a>
method too, and it must do so carefully. The basic implementation of
<a class="reference internal" href="../reference/datamodel.html#object.__setattr__" title="object.__setattr__"><code class="xref py py-meth docutils literal notranslate">__setattr__()</code></a> is roughly equivalent to the following:
<div class="highlight-default notranslate"><div class="highlight"><pre>class X:
 ...
 def __setattr__(self, name, value):
 self.__dict__[name] = value
 ...
</pre></div>
</div>
Most <a class="reference internal" href="../reference/datamodel.html#object.__setattr__" title="object.__setattr__"><code class="xref py py-meth docutils literal notranslate">__setattr__()</code></a> implementations must modify <code class="docutils literal notranslate">self.__dict__</code> to store
local state for self without causing an infinite recursion.
</div>
<div class="section" id="how-do-i-call-a-method-defined-in-a-base-class-from-a-derived-class-that-overrides-it">
<h3><a class="toc-backref" href="#id55">How do I call a method defined in a base class from a derived class that overrides it?</a><a class="headerlink" href="#how-do-i-call-a-method-defined-in-a-base-class-from-a-derived-class-that-overrides-it" title="Permalink to this headline">¶</a></h3>
If you’re using new-style classes, use the built-in <a class="reference internal" href="../library/functions.html#super" title="super"><code class="xref py py-func docutils literal notranslate">super()</code></a> function:
<div class="highlight-default notranslate"><div class="highlight"><pre>class Derived(Base):
 def meth(self):
 super(Derived, self).meth()
</pre></div>
</div>
If you’re using classic classes: For a class definition such as <code class="docutils literal notranslate">class
Derived(Base): ...</code> you can call method <code class="docutils literal notranslate">meth()</code> defined in <code class="docutils literal notranslate">Base</code> (or one
of <code class="docutils literal notranslate">Base</code>’s base classes) as <code class="docutils literal notranslate">Base.meth(self, arguments...)</code>. Here,
<code class="docutils literal notranslate">Base.meth</code> is an unbound method, so you need to provide the <code class="docutils literal notranslate">self</code>
argument.
</div>
<div class="section" id="how-can-i-organize-my-code-to-make-it-easier-to-change-the-base-class">
<h3><a class="toc-backref" href="#id56">How can I organize my code to make it easier to change the base class?</a><a class="headerlink" href="#how-can-i-organize-my-code-to-make-it-easier-to-change-the-base-class" title="Permalink to this headline">¶</a></h3>
You could define an alias for the base class, assign the real base class to it
before your class definition, and use the alias throughout your class. Then all
you have to change is the value assigned to the alias. Incidentally, this trick
is also handy if you want to decide dynamically (e.g. depending on availability
of resources) which base class to use. Example:
<div class="highlight-default notranslate"><div class="highlight"><pre>BaseAlias = &lt;real base class&gt;

class Derived(BaseAlias):
 def meth(self):
 BaseAlias.meth(self)
 ...
</pre></div>
</div>
</div>
<div class="section" id="how-do-i-create-static-class-data-and-static-class-methods">
<h3><a class="toc-backref" href="#id57">How do I create static class data and static class methods?</a><a class="headerlink" href="#how-do-i-create-static-class-data-and-static-class-methods" title="Permalink to this headline">¶</a></h3>
Both static data and static methods (in the sense of C++ or Java) are supported
in Python.
For static data, simply define a class attribute. To assign a new value to the
attribute, you have to explicitly use the class name in the assignment:
<div class="highlight-default notranslate"><div class="highlight"><pre>class C:
 count = 0 # number of times C.__init__ called

def __init__(self):
 C.count = C.count + 1

def getcount(self):
 return C.count # or return self.count
</pre></div>
</div>
<code class="docutils literal notranslate">c.count</code> also refers to <code class="docutils literal notranslate">C.count</code> for any <code class="docutils literal notranslate">c</code> such that <code class="docutils literal notranslate">isinstance(c,
C)</code> holds, unless overridden by <code class="docutils literal notranslate">c</code> itself or by some class on the base-class
search path from <code class="docutils literal notranslate">c.__class__</code> back to <code class="docutils literal notranslate">C</code>.
Caution: within a method of C, an assignment like <code class="docutils literal notranslate">self.count = 42</code> creates a
new and unrelated instance named “count” in <code class="docutils literal notranslate">self</code>’s own dict. Rebinding of a
class-static data name must always specify the class whether inside a method or
not:
<div class="highlight-default notranslate"><div class="highlight"><pre>C.count = 314
</pre></div>
</div>
Static methods are possible since Python 2.2:
<div class="highlight-default notranslate"><div class="highlight"><pre>class C:
 def static(arg1, arg2, arg3):
 # No &#39;self&#39; parameter!
 ...
 static = staticmethod(static)
</pre></div>
</div>
With Python 2.4’s decorators, this can also be written as
<div class="highlight-default notranslate"><div class="highlight"><pre>class C:
 @staticmethod
 def static(arg1, arg2, arg3):
 # No &#39;self&#39; parameter!
 ...
</pre></div>
</div>
However, a far more straightforward way to get the effect of a static method is
via a simple module-level function:
<div class="highlight-default notranslate"><div class="highlight"><pre>def getcount():
 return C.count
</pre></div>
</div>
If your code is structured so as to define one class (or tightly related class
hierarchy) per module, this supplies the desired encapsulation.
</div>
<div class="section" id="how-can-i-overload-constructors-or-methods-in-python">
<h3><a class="toc-backref" href="#id58">How can I overload constructors (or methods) in Python?</a><a class="headerlink" href="#how-can-i-overload-constructors-or-methods-in-python" title="Permalink to this headline">¶</a></h3>
This answer actually applies to all methods, but the question usually comes up
first in the context of constructors.
In C++ you’d write
<div class="highlight-c notranslate"><div class="highlight"><pre>class C {
 C() { cout &lt;&lt; &quot;No arguments\n&quot;; }
 C(int i) { cout &lt;&lt; &quot;Argument is &quot; &lt;&lt; i &lt;&lt; &quot;\n&quot;; }
}
</pre></div>
</div>
In Python you have to write a single constructor that catches all cases using
default arguments. For example:
<div class="highlight-default notranslate"><div class="highlight"><pre>class C:
 def __init__(self, i=None):
 if i is None:
 print &quot;No arguments&quot;
 else:
 print &quot;Argument is&quot;, i
</pre></div>
</div>
This is not entirely equivalent, but close enough in practice.
You could also try a variable-length argument list, e.g.
<div class="highlight-default notranslate"><div class="highlight"><pre>def __init__(self, *args):
 ...
</pre></div>
</div>
The same approach works for all method definitions.
</div>
<div class="section" id="i-try-to-use-spam-and-i-get-an-error-about-someclassname-spam">
<h3><a class="toc-backref" href="#id59">I try to use __spam and I get an error about _SomeClassName__spam.</a><a class="headerlink" href="#i-try-to-use-spam-and-i-get-an-error-about-someclassname-spam" title="Permalink to this headline">¶</a></h3>
Variable names with double leading underscores are “mangled” to provide a simple
but effective way to define class private variables. Any identifier of the form
<code class="docutils literal notranslate">__spam</code> (at least two leading underscores, at most one trailing underscore)
is textually replaced with <code class="docutils literal notranslate">_classname__spam</code>, where <code class="docutils literal notranslate">classname</code> is the
current class name with any leading underscores stripped.
This doesn’t guarantee privacy: an outside user can still deliberately access
the “_classname__spam” attribute, and private values are visible in the object’s
<code class="docutils literal notranslate">__dict__</code>. Many Python programmers never bother to use private variable
names at all.
</div>
<div class="section" id="my-class-defines-del-but-it-is-not-called-when-i-delete-the-object">
<h3><a class="toc-backref" href="#id60">My class defines __del__ but it is not called when I delete the object.</a><a class="headerlink" href="#my-class-defines-del-but-it-is-not-called-when-i-delete-the-object" title="Permalink to this headline">¶</a></h3>
There are several possible reasons for this.
The del statement does not necessarily call <a class="reference internal" href="../reference/datamodel.html#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate">__del__()</code></a> – it simply
decrements the object’s reference count, and if this reaches zero
<a class="reference internal" href="../reference/datamodel.html#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate">__del__()</code></a> is called.
If your data structures contain circular links (e.g. a tree where each child has
a parent reference and each parent has a list of children) the reference counts
will never go back to zero. Once in a while Python runs an algorithm to detect
such cycles, but the garbage collector might run some time after the last
reference to your data structure vanishes, so your <a class="reference internal" href="../reference/datamodel.html#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate">__del__()</code></a> method may be
called at an inconvenient and random time. This is inconvenient if you’re trying
to reproduce a problem. Worse, the order in which object’s <a class="reference internal" href="../reference/datamodel.html#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate">__del__()</code></a>
methods are executed is arbitrary. You can run <a class="reference internal" href="../library/gc.html#gc.collect" title="gc.collect"><code class="xref py py-func docutils literal notranslate">gc.collect()</code></a> to force a
collection, but there are pathological cases where objects will never be
collected.
Despite the cycle collector, it’s still a good idea to define an explicit
<code class="docutils literal notranslate">close()</code> method on objects to be called whenever you’re done with them. The
<code class="docutils literal notranslate">close()</code> method can then remove attributes that refer to subobjecs. Don’t
call <a class="reference internal" href="../reference/datamodel.html#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate">__del__()</code></a> directly – <a class="reference internal" href="../reference/datamodel.html#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate">__del__()</code></a> should call <code class="docutils literal notranslate">close()</code> and
<code class="docutils literal notranslate">close()</code> should make sure that it can be called more than once for the same
object.
Another way to avoid cyclical references is to use the <a class="reference internal" href="../library/weakref.html#module-weakref" title="weakref: Support for weak references and weak dictionaries."><code class="xref py py-mod docutils literal notranslate">weakref</code></a> module,
which allows you to point to objects without incrementing their reference count.
Tree data structures, for instance, should use weak references for their parent
and sibling references (if they need them!).
If the object has ever been a local variable in a function that caught an
expression in an except clause, chances are that a reference to the object still
exists in that function’s stack frame as contained in the stack trace.
Normally, calling <a class="reference internal" href="../library/sys.html#sys.exc_clear" title="sys.exc_clear"><code class="xref py py-func docutils literal notranslate">sys.exc_clear()</code></a> will take care of this by clearing the
last recorded exception.
Finally, if your <a class="reference internal" href="../reference/datamodel.html#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate">__del__()</code></a> method raises an exception, a warning message
is printed to <a class="reference internal" href="../library/sys.html#sys.stderr" title="sys.stderr"><code class="xref py py-data docutils literal notranslate">sys.stderr</code></a>.
</div>
<div class="section" id="how-do-i-get-a-list-of-all-instances-of-a-given-class">
<h3><a class="toc-backref" href="#id61">How do I get a list of all instances of a given class?</a><a class="headerlink" href="#how-do-i-get-a-list-of-all-instances-of-a-given-class" title="Permalink to this headline">¶</a></h3>
Python does not keep track of all instances of a class (or of a built-in type).
You can program the class’s constructor to keep track of all instances by
keeping a list of weak references to each instance.
</div>
<div class="section" id="why-does-the-result-of-id-appear-to-be-not-unique">
<h3><a class="toc-backref" href="#id62">Why does the result of <code class="docutils literal notranslate">id()</code> appear to be not unique?</a><a class="headerlink" href="#why-does-the-result-of-id-appear-to-be-not-unique" title="Permalink to this headline">¶</a></h3>
The <a class="reference internal" href="../library/functions.html#id" title="id"><code class="xref py py-func docutils literal notranslate">id()</code></a> builtin returns an integer that is guaranteed to be unique during
the lifetime of the object. Since in CPython, this is the object’s memory
address, it happens frequently that after an object is deleted from memory, the
next freshly created object is allocated at the same position in memory. This
is illustrated by this example:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; id(1000)
13901272
&gt;&gt;&gt; id(2000)
13901272
</pre></div>
</div>
The two ids belong to different integer objects that are created before, and
deleted immediately after execution of the <code class="docutils literal notranslate">id()</code> call. To be sure that
objects whose id you want to examine are still alive, create another reference
to the object:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; a = 1000; b = 2000
&gt;&gt;&gt; id(a)
13901272
&gt;&gt;&gt; id(b)
13891296
</pre></div>
</div>
</div>
</div>
<div class="section" id="modules">
<h2><a class="toc-backref" href="#id63">Modules</a><a class="headerlink" href="#modules" title="Permalink to this headline">¶</a></h2>
<div class="section" id="how-do-i-create-a-pyc-file">
<h3><a class="toc-backref" href="#id64">How do I create a .pyc file?</a><a class="headerlink" href="#how-do-i-create-a-pyc-file" title="Permalink to this headline">¶</a></h3>
When a module is imported for the first time (or when the source is more recent
than the current compiled file) a <code class="docutils literal notranslate">.pyc</code> file containing the compiled code
should be created in the same directory as the <code class="docutils literal notranslate">.py</code> file.
One reason that a <code class="docutils literal notranslate">.pyc</code> file may not be created is permissions problems with
the directory. This can happen, for example, if you develop as one user but run
as another, such as if you are testing with a web server. Creation of a .pyc
file is automatic if you’re importing a module and Python has the ability
(permissions, free space, etc…) to write the compiled module back to the
directory.
Running Python on a top level script is not considered an import and no
<code class="docutils literal notranslate">.pyc</code> will be created. For example, if you have a top-level module
<code class="docutils literal notranslate">foo.py</code> that imports another module <code class="docutils literal notranslate">xyz.py</code>, when you run <code class="docutils literal notranslate">foo</code>,
<code class="docutils literal notranslate">xyz.pyc</code> will be created since <code class="docutils literal notranslate">xyz</code> is imported, but no <code class="docutils literal notranslate">foo.pyc</code> file
will be created since <code class="docutils literal notranslate">foo.py</code> isn’t being imported.
If you need to create <code class="docutils literal notranslate">foo.pyc</code> – that is, to create a <code class="docutils literal notranslate">.pyc</code> file for a module
that is not imported – you can, using the <a class="reference internal" href="../library/py_compile.html#module-py_compile" title="py_compile: Generate byte-code files from Python source files."><code class="xref py py-mod docutils literal notranslate">py_compile</code></a> and
<a class="reference internal" href="../library/compileall.html#module-compileall" title="compileall: Tools for byte-compiling all Python source files in a directory tree."><code class="xref py py-mod docutils literal notranslate">compileall</code></a> modules.
The <a class="reference internal" href="../library/py_compile.html#module-py_compile" title="py_compile: Generate byte-code files from Python source files."><code class="xref py py-mod docutils literal notranslate">py_compile</code></a> module can manually compile any module. One way is to use
the <code class="docutils literal notranslate">compile()</code> function in that module interactively:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; import py_compile
&gt;&gt;&gt; py_compile.compile(&#39;foo.py&#39;) 
</pre></div>
</div>
This will write the <code class="docutils literal notranslate">.pyc</code> to the same location as <code class="docutils literal notranslate">foo.py</code> (or you can
override that with the optional parameter <code class="docutils literal notranslate">cfile</code>).
You can also automatically compile all files in a directory or directories using
the <a class="reference internal" href="../library/compileall.html#module-compileall" title="compileall: Tools for byte-compiling all Python source files in a directory tree."><code class="xref py py-mod docutils literal notranslate">compileall</code></a> module. You can do it from the shell prompt by running
<code class="docutils literal notranslate">compileall.py</code> and providing the path of a directory containing Python files
to compile:
<div class="highlight-default notranslate"><div class="highlight"><pre>python -m compileall .
</pre></div>
</div>
</div>
<div class="section" id="how-do-i-find-the-current-module-name">
<h3><a class="toc-backref" href="#id65">How do I find the current module name?</a><a class="headerlink" href="#how-do-i-find-the-current-module-name" title="Permalink to this headline">¶</a></h3>
A module can find out its own module name by looking at the predefined global
variable <code class="docutils literal notranslate">__name__</code>. If this has the value <code class="docutils literal notranslate">'__main__'</code>, the program is
running as a script. Many modules that are usually used by importing them also
provide a command-line interface or a self-test, and only execute this code
after checking <code class="docutils literal notranslate">__name__</code>:
<div class="highlight-default notranslate"><div class="highlight"><pre>def main():
 print &#39;Running test...&#39;
 ...

if __name__ == &#39;__main__&#39;:
 main()
</pre></div>
</div>
</div>
<div class="section" id="how-can-i-have-modules-that-mutually-import-each-other">
<h3><a class="toc-backref" href="#id66">How can I have modules that mutually import each other?</a><a class="headerlink" href="#how-can-i-have-modules-that-mutually-import-each-other" title="Permalink to this headline">¶</a></h3>
Suppose you have the following modules:
foo.py:
<div class="highlight-default notranslate"><div class="highlight"><pre>from bar import bar_var
foo_var = 1
</pre></div>
</div>
bar.py:
<div class="highlight-default notranslate"><div class="highlight"><pre>from foo import foo_var
bar_var = 2
</pre></div>
</div>
The problem is that the interpreter will perform the following steps:
<ul class="simple">
<li>main imports foo</li>
<li>Empty globals for foo are created</li>
<li>foo is compiled and starts executing</li>
<li>foo imports bar</li>
<li>Empty globals for bar are created</li>
<li>bar is compiled and starts executing</li>
<li>bar imports foo (which is a no-op since there already is a module named foo)</li>
<li>bar.foo_var = foo.foo_var</li>
</ul>
The last step fails, because Python isn’t done with interpreting <code class="docutils literal notranslate">foo</code> yet and
the global symbol dictionary for <code class="docutils literal notranslate">foo</code> is still empty.
The same thing happens when you use <code class="docutils literal notranslate">import foo</code>, and then try to access
<code class="docutils literal notranslate">foo.foo_var</code> in global code.
There are (at least) three possible workarounds for this problem.
Guido van Rossum recommends avoiding all uses of <code class="docutils literal notranslate">from &lt;module&gt; import ...</code>,
and placing all code inside functions. Initializations of global variables and
class variables should use constants or built-in functions only. This means
everything from an imported module is referenced as <code class="docutils literal notranslate">&lt;module&gt;.&lt;name&gt;</code>.
Jim Roskind suggests performing steps in the following order in each module:
<ul class="simple">
<li>exports (globals, functions, and classes that don’t need imported base
classes)</li>
<li><code class="docutils literal notranslate">import</code> statements</li>
<li>active code (including globals that are initialized from imported values).</li>
</ul>
van Rossum doesn’t like this approach much because the imports appear in a
strange place, but it does work.
Matthias Urlichs recommends restructuring your code so that the recursive import
is not necessary in the first place.
These solutions are not mutually exclusive.
</div>
<div class="section" id="import-x-y-z-returns-module-x-how-do-i-get-z">
<h3><a class="toc-backref" href="#id67">__import__(‘x.y.z’) returns &lt;module ‘x’&gt;; how do I get z?</a><a class="headerlink" href="#import-x-y-z-returns-module-x-how-do-i-get-z" title="Permalink to this headline">¶</a></h3>
Consider using the convenience function <a class="reference internal" href="../library/importlib.html#importlib.import_module" title="importlib.import_module"><code class="xref py py-func docutils literal notranslate">import_module()</code></a> from
<a class="reference internal" href="../library/importlib.html#module-importlib" title="importlib: Convenience wrappers for __import__"><code class="xref py py-mod docutils literal notranslate">importlib</code></a> instead:
<div class="highlight-default notranslate"><div class="highlight"><pre>z = importlib.import_module(&#39;x.y.z&#39;)
</pre></div>
</div>
</div>
<div class="section" id="when-i-edit-an-imported-module-and-reimport-it-the-changes-don-t-show-up-why-does-this-happen">
<h3><a class="toc-backref" href="#id68">When I edit an imported module and reimport it, the changes don’t show up. Why does this happen?</a><a class="headerlink" href="#when-i-edit-an-imported-module-and-reimport-it-the-changes-don-t-show-up-why-does-this-happen" title="Permalink to this headline">¶</a></h3>
For reasons of efficiency as well as consistency, Python only reads the module
file on the first time a module is imported. If it didn’t, in a program
consisting of many modules where each one imports the same basic module, the
basic module would be parsed and re-parsed many times. To force rereading of a
changed module, do this:
<div class="highlight-default notranslate"><div class="highlight"><pre>import modname
reload(modname)
</pre></div>
</div>
Warning: this technique is not 100% fool-proof. In particular, modules
containing statements like
<div class="highlight-default notranslate"><div class="highlight"><pre>from modname import some_objects
</pre></div>
</div>
will continue to work with the old version of the imported objects. If the
module contains class definitions, existing class instances will not be
updated to use the new class definition. This can result in the following
paradoxical behaviour:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; import cls
&gt;&gt;&gt; c = cls.C() # Create an instance of C
&gt;&gt;&gt; reload(cls)
&lt;module &#39;cls&#39; from &#39;cls.pyc&#39;&gt;
&gt;&gt;&gt; isinstance(c, cls.C) # isinstance is false?!?
False
</pre></div>
</div>
The nature of the problem is made clear if you print out the class objects:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; c.__class__
&lt;class cls.C at 0x7352a0&gt;
&gt;&gt;&gt; cls.C
&lt;class cls.C at 0x4198d0&gt;
</pre></div>
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