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<HEAD>
<TITLE>Google Python Style Guide</TITLE>
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<BODY>
<H1>Google Python Style Guide</H1>
<p align="right">
Revision 2.15
</p>
<address>
Amit Patel<br>
Antoine Picard<br>
Eugene Jhong<br>
Jeremy Hylton<br>
Matt Smart<br>
Mike Shields<br>
</address>
<DIV style="margin-left: 50%; font-size: 75%;">
<P>
Each style point has a summary for which additional information is available
by toggling the accompanying arrow button that looks this way:
<SPAN class="showhide_button" style="margin-left: 0; float: none">▶</SPAN>.
You may toggle all summaries with the big arrow button:
</P>
<DIV style=" font-size: larger; margin-left: +2em;">
<SPAN class="showhide_button" style="font-size: 180%; float: none" onclick="javascript:ShowHideAll()" name="show_hide_all_button" id="show_hide_all_button">▶</SPAN>
Toggle all summaries
</DIV>
</DIV>
<DIV class="toc">
<DIV class="toc_title">Table of Contents</DIV>
<TABLE>
<TR valign="top" class="">
<TD><DIV class="toc_category"><A href="#Python_Language_Rules">Python Language Rules</A></DIV></TD>
<TD><DIV class="toc_stylepoint">
<SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#pychecker">pychecker</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Imports">Imports</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Packages">Packages</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Exceptions">Exceptions</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Global_variables">Global variables</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Nested/Local/Inner_Classes_and_Functions">Nested/Local/Inner Classes and Functions</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#List_Comprehensions">List Comprehensions</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Default_Iterators_and_Operators">Default Iterators and Operators</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Generators">Generators</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Lambda_Functions">Lambda Functions</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Default_Argument_Values">Default Argument Values</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Properties">Properties</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#True/False_evaluations">True/False evaluations</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Deprecated_Language_Features">Deprecated Language Features</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Lexical_Scoping">Lexical Scoping</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Function_and_Method_Decorators">Function and Method Decorators</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Threading">Threading</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Power_Features">Power Features</A></SPAN> </DIV></TD>
</TR>
<TR valign="top" class="">
<TD><DIV class="toc_category"><A href="#Python_Style_Rules">Python Style Rules</A></DIV></TD>
<TD><DIV class="toc_stylepoint">
<SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Semicolons">Semicolons</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Line_length">Line length</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Parentheses">Parentheses</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Indentation">Indentation</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Blank_Lines">Blank Lines</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Whitespace">Whitespace</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Python_Interpreter">Python Interpreter</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Comments">Comments</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Classes">Classes</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Strings">Strings</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#TODO_Comments">TODO Comments</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Imports_formatting">Imports formatting</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Statements">Statements</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Access_Control">Access Control</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Naming">Naming</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Main">Main</A></SPAN> </DIV></TD>
</TR>
</TABLE>
</DIV>
<DIV class="">
<H2 name="Important_Note" id="Important_Note">Important Note</H2>
<DIV class="">
<H3><A name="Displaying_Hidden_Details_in_this_Guide" id="Displaying_Hidden_Details_in_this_Guide">Displaying Hidden Details in this Guide</A></H3>
<SPAN class="link_button" id="link-Displaying_Hidden_Details_in_this_Guide__button" name="link-Displaying_Hidden_Details_in_this_Guide__button"><A href="?showone=Displaying_Hidden_Details_in_this_Guide#Displaying_Hidden_Details_in_this_Guide">
link
</A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Displaying_Hidden_Details_in_this_Guide__body','Displaying_Hidden_Details_in_this_Guide__button')" name="Displaying_Hidden_Details_in_this_Guide__button" id="Displaying_Hidden_Details_in_this_Guide__button">▶</SPAN>
<DIV style="display:inline;" class="">
This style guide contains many details that are initially
hidden from view. They are marked by the triangle icon, which you
see here on your left. Click it now.
You should see "Hooray" appear below.
</DIV>
<DIV class=""><DIV class="stylepoint_body" name="Displaying_Hidden_Details_in_this_Guide__body" id="Displaying_Hidden_Details_in_this_Guide__body" style="display: none">
<p>
Hooray! Now you know you can expand points to get more
details. Alternatively, there's a "toggle all" at the
top of this document.
</p>
</DIV></DIV>
</DIV>
</DIV>
<DIV class="">
<H2 name="Background" id="Background">Background</H2>
<p>
Python is the main scripting language used at Google. This
style guide is a list of <em>do</em>s and <em>don't</em>s for Python
programs.
</p>
</DIV>
<DIV class="">
<H2 name="Python_Language_Rules" id="Python_Language_Rules">Python Language Rules</H2>
<DIV class="">
<H3><A name="pychecker" id="pychecker">pychecker</A></H3>
<SPAN class="link_button" id="link-pychecker__button" name="link-pychecker__button"><A href="?showone=pychecker#pychecker">
link
</A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('pychecker__body','pychecker__button')" name="pychecker__button" id="pychecker__button">▶</SPAN>
<DIV style="display:inline;" class="">
Run <code>pychecker</code> over your code.
</DIV>
<DIV class=""><DIV class="stylepoint_body" name="pychecker__body" id="pychecker__body" style="display: none">
<P class="">
<SPAN class="stylepoint_section">Definition: </SPAN>
PyChecker is a tool for finding bugs in Python source code. It finds
problems that are typically caught by a compiler for less dynamic
languages like C and C++. It is similar to lint. Because of the
dynamic nature of Python, some warnings may be incorrect; however,
spurious warnings should be fairly infrequent.
</P>
<P class="">
<SPAN class="stylepoint_section">Pros: </SPAN>
Catches easy-to-miss errors like typos, use-vars-before-assignment, etc.
</P>
<P class="">
<SPAN class="stylepoint_section">Cons: </SPAN>
<code>pychecker</code> isn't perfect. To take
advantage of it, we'll need to sometimes: a) Write around it b)
Suppress its warnings c) Improve it or d) Ignore it.
</P>
<P class="">
<SPAN class="stylepoint_section">Decision: </SPAN>
Make sure you run <code>pychecker</code> on your code.
</P>
<p>
For information on how to run <code>pychecker</code>, see the
<a HREF="http://pychecker.sourceforge.net">pychecker
homepage</a>
</p>
<p>
To suppress warnings, you can set a module-level variable named
<code>__pychecker__</code> to suppress appropriate warnings.
For example:
</p>
<DIV class=""><PRE>
<span class="external"></span>__pychecker__ = 'no-callinit no-classattr'</PRE></DIV>
<p>
Suppressing in this way has the advantage that we can easily search
for suppressions and revisit them.
</p>
<p>
You can get a list of pychecker warnings by doing
<code>pychecker --help</code>.
</p>
<p>
Unused argument warnings can be suppressed by using `_' as the
identifier for the unused argument or prefixing the argument name with
`unused_'. In situations where changing the argument names is
infeasible, you can mention them at the beginning of the function.
For example:
</p>
<DIV class=""><PRE>
<span class="external"></span>def foo(a, unused_b, unused_c, d=None, e=None):
<span class="external"> </span>(d, e) = (d, e) # Silence pychecker
<span class="external"> </span>return a
<span class="external"></span>
</PRE></DIV>
<p>
Ideally, pychecker would be extended to ensure that such `unused
declarations' were true.
</p>
</DIV></DIV>
</DIV>
<DIV class="">
<H3><A name="Imports" id="Imports">Imports</A></H3>
<SPAN class="link_button" id="link-Imports__button" name="link-Imports__button"><A href="?showone=Imports#Imports">
link
</A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Imports__body','Imports__button')" name="Imports__button" id="Imports__button">▶</SPAN>
<DIV style="display:inline;" class="">
Use <code>import</code>s for packages and modules only.
</DIV>
<DIV class=""><DIV class="stylepoint_body" name="Imports__body" id="Imports__body" style="display: none">
<P class="">
<SPAN class="stylepoint_section">Definition: </SPAN>
Reusability mechanism for sharing code from one module to another.
</P>
<P class="">
<SPAN class="stylepoint_section">Pros: </SPAN>
Simplest and most commonly used way of sharing things.
</P>
<P class="">
<SPAN class="stylepoint_section">Cons: </SPAN> <code>from foo import *</code> or
<code>from foo import Bar</code> is
very nasty and can lead to serious maintenance issues because
it makes it hard to find module dependencies.
</P>
<P class="">
<SPAN class="stylepoint_section">Decision: </SPAN>
Use <code>import x</code> for importing packages and modules.
Use <code>from x import y</code> only when <code>x</code> is a
package and <code>y</code> is a module. This allows the
importer to refer to the module without specifying the full
package prefix. For example the module
<code>sound.effects.echo</code> may be imported as follows:
</P>
<DIV class=""><PRE>
<span class="external"></span>from sound.effects import echo
<span class="external"></span>...
<span class="external"></span>echo.echofilter(input, output, delay=0.7, atten=4)
<span class="external"></span>
</PRE></DIV>
<p>
Even if the module is in the same package, do not directly import
the module without the full package name. This might cause the
package to be imported twice (with unintended side effects) when the
"main" module that is used to start an application lives inside a
package (and uses modules from that same package).
</p>
</DIV></DIV>
</DIV>
<DIV class="">
<H3><A name="Packages" id="Packages">Packages</A></H3>
<SPAN class="link_button" id="link-Packages__button" name="link-Packages__button"><A href="?showone=Packages#Packages">
link
</A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Packages__body','Packages__button')" name="Packages__button" id="Packages__button">▶</SPAN>
<DIV style="display:inline;" class="">
Import and refer to each module using the full pathname location of
that module.
</DIV>
<DIV class=""><DIV class="stylepoint_body" name="Packages__body" id="Packages__body" style="display: none">
<P class="">
<SPAN class="stylepoint_section">Pros: </SPAN>
Avoids conflicts in module names. Makes it easier to find modules.
</P>
<P class="">
<SPAN class="stylepoint_section">Cons: </SPAN>
Makes it harder to deploy code because you have to replicate the
package hierarchy.
</P>
<P class="">
<SPAN class="stylepoint_section">Decision: </SPAN>
All new code should refer to modules based on their package
name.
</P>
<p>
Imports should be as follows:
</p>
<DIV class=""><PRE># Reference in code with complete name.
import sound.effects.echo
# Reference in code with just module name.
from sound.effects import echo
</PRE></DIV>
</DIV></DIV>
</DIV>
<DIV class="">
<H3><A name="Exceptions" id="Exceptions">Exceptions</A></H3>
<SPAN class="link_button" id="link-Exceptions__button" name="link-Exceptions__button"><A href="?showone=Exceptions#Exceptions">
link
</A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Exceptions__body','Exceptions__button')" name="Exceptions__button" id="Exceptions__button">▶</SPAN>
<DIV style="display:inline;" class="">
Exceptions are allowed but must be used carefully.
</DIV>
<DIV class=""><DIV class="stylepoint_body" name="Exceptions__body" id="Exceptions__body" style="display: none">
<P class="">
<SPAN class="stylepoint_section">Definition: </SPAN>
Exceptions are a means of breaking out of the normal flow of control
of a code block to handle errors or other exceptional conditions.
</P>
<P class="">
<SPAN class="stylepoint_section">Pros: </SPAN>
The control flow of normal operation code is not cluttered by
error-handling code. It also allows the control flow to skip multiple
frames when a certain condition occurs, e.g., returning from N
nested functions in one step instead of having to carry-through
error codes.
</P>
<P class="">
<SPAN class="stylepoint_section">Cons: </SPAN>
May cause the control flow to be confusing. Easy to miss error
cases when making library calls.
</P>
<P class="">
<SPAN class="stylepoint_section">Decision: </SPAN>
Exceptions must follow certain conditions:
<ul>
<li>Raise exceptions like this: <code>raise MyException("Error
message")</code> or <code>raise MyException</code>. Do not
use the two-argument form (<code>raise MyException, "Error
message"</code>) or deprecated string-based exceptions
(<code>raise "Error message"</code>).</li>
<li>Modules or packages should define their own domain-specific
base exception class, which should inherit from the built-in
Exception class. The base exception for a module should be called
<code>Error</code>.
<DIV class=""><PRE>
<span class="external"></span>class Error(Exception):
<span class="external"> </span>pass</PRE></DIV>
</li>
<li>Never use catch-all <code>except:</code> statements, or
catch <code>Exception</code> or <code>StandardError</code>,
unless you are re-raising the exception or in the outermost
block in your thread (and printing an error message). Python
is very tolerant in this regard and <code>except:</code> will
really catch everything including Python syntax errors. It is
easy to hide real bugs using <code>except:</code>.</li>
<li>Minimize the amount of code in a
<code>try</code>/<code>except</code> block. The larger the
body of the <code>try</code>, the more likely that an
exception will be raised by a line of code that you didn't
expect to raise an exception. In those cases,
the <code>try</code>/<code>except</code> block hides a real
error.</li>
<li>Use the <code>finally</code> clause to execute code whether
or not an exception is raised in the <code>try</code> block.
This is often useful for cleanup, i.e., closing a file.</li>
</ul>
</P>
</DIV></DIV>
</DIV>
<DIV class="">
<H3><A name="Global_variables" id="Global_variables">Global variables</A></H3>
<SPAN class="link_button" id="link-Global_variables__button" name="link-Global_variables__button"><A href="?showone=Global_variables#Global_variables">
link
</A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Global_variables__body','Global_variables__button')" name="Global_variables__button" id="Global_variables__button">▶</SPAN>
<DIV style="display:inline;" class="">
Avoid global variables.
</DIV>
<DIV class=""><DIV class="stylepoint_body" name="Global_variables__body" id="Global_variables__body" style="display: none">
<P class="">
<SPAN class="stylepoint_section">Definition: </SPAN>
Variables that are declared at the module level.
</P>
<P class="">
<SPAN class="stylepoint_section">Pros: </SPAN>
Occasionally useful.
</P>
<P class="">
<SPAN class="stylepoint_section">Cons: </SPAN>
Has the potential to change module behavior during the import,
because assignments to module-level variables are done when the
module is imported.
</P>
<P class="">
<SPAN class="stylepoint_section">Decision: </SPAN>
Avoid global variables in favor of class variables. Some
exceptions are:
<ul>
<li>Default options for scripts.</li>
<li>Module-level constants. For example: <code>PI = 3.14159</code>.
Constants should be named using all caps with underscores;
see <a HREF="#Naming">Naming</a> below.</li>
<li>It is sometimes useful for globals to cache values needed
or returned by functions.</li>
<li>If needed, globals should be made internal to the module
and accessed through public module level functions;
see <a HREF="#Naming">Naming</a> below.</li>
</ul>
</P>
</DIV></DIV>
</DIV>
<DIV class="">
<H3><A name="Nested/Local/Inner_Classes_and_Functions" id="Nested/Local/Inner_Classes_and_Functions">Nested/Local/Inner Classes and Functions</A></H3>
<SPAN class="link_button" id="link-Nested/Local/Inner_Classes_and_Functions__button" name="link-Nested/Local/Inner_Classes_and_Functions__button"><A href="?showone=Nested/Local/Inner_Classes_and_Functions#Nested/Local/Inner_Classes_and_Functions">
link
</A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Nested/Local/Inner_Classes_and_Functions__body','Nested/Local/Inner_Classes_and_Functions__button')" name="Nested/Local/Inner_Classes_and_Functions__button" id="Nested/Local/Inner_Classes_and_Functions__button">▶</SPAN>
<DIV style="display:inline;" class="">
Nested/local/inner classes and functions are fine.
</DIV>
<DIV class=""><DIV class="stylepoint_body" name="Nested/Local/Inner_Classes_and_Functions__body" id="Nested/Local/Inner_Classes_and_Functions__body" style="display: none">
<P class="">
<SPAN class="stylepoint_section">Definition: </SPAN>
A class can be defined inside of a method, function, or class. A
function can be defined inside a method or function. Nested functions
have read-only access to variables defined in enclosing scopes.
</P>
<P class="">
<SPAN class="stylepoint_section">Pros: </SPAN>
Allows definition of utility classes and functions that are only
used inside of a very limited scope. Very <a HREF="http://en.wikipedia.org/wiki/Abstract_data_type">ADT</a>-y.
</P>
<P class="">
<SPAN class="stylepoint_section">Cons: </SPAN>
Instances of nested or local classes cannot be pickled.
</P>
<P class="">
<SPAN class="stylepoint_section">Decision: </SPAN>
They are fine.
</P>
</DIV></DIV>
</DIV>
<DIV class="">
<H3><A name="List_Comprehensions" id="List_Comprehensions">List Comprehensions</A></H3>
<SPAN class="link_button" id="link-List_Comprehensions__button" name="link-List_Comprehensions__button"><A href="?showone=List_Comprehensions#List_Comprehensions">
link
</A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('List_Comprehensions__body','List_Comprehensions__button')" name="List_Comprehensions__button" id="List_Comprehensions__button">▶</SPAN>
<DIV style="display:inline;" class="">
Okay to use for simple cases.
</DIV>
<DIV class=""><DIV class="stylepoint_body" name="List_Comprehensions__body" id="List_Comprehensions__body" style="display: none">
<P class="">
<SPAN class="stylepoint_section">Definition: </SPAN>
List comprehensions and generator expressions provide a concise
and efficient way to create lists and iterators without
resorting to the use of <code>map()</code>,
<code>filter()</code>, or <code>lambda</code>.
</P>
<P class="">
<SPAN class="stylepoint_section">Pros: </SPAN>
Simple list comprehensions can be clearer and simpler than
other list creation techniques. Generator expressions can be
very efficient, since they avoid the creation of a list
entirely.
</P>
<P class="">
<SPAN class="stylepoint_section">Cons: </SPAN>
Complicated list comprehensions or generator expressions can be
hard to read.
</P>
<P class="">
<SPAN class="stylepoint_section">Decision: </SPAN>
Okay to use for simple cases. Each portion must fit on one line:
mapping expression, <code>for</code> clause, filter expression.
Multiple <code>for</code> clauses or filter expressions are not
permitted. Use loops instead when things get more complicated.
</P>
<DIV class=""><PRE class="badcode">No<span class="external"></span>:
<span class="external"></span>result = [(x, y) for x in range(10) for y in range(5) if x * y > 10]
<span class="external"></span>return ((x, y, z)
<span class="external"></span> for x in xrange(5)
<span class="external"></span> for y in xrange(5)
<span class="external"></span> if x != y
<span class="external"></span> for z in xrange(5)
<span class="external"></span> if y != z)</PRE></DIV>
<DIV class=""><PRE>Ye<span class="external"></span>s:
<span class="external"></span>result = []
<span class="external"></span>for x in range(10):
<span class="external"> </span>for y in range(5):
<span class="external"> </span>if x * y > 10:
<span class="external"> </span>result.append((x, y))
<span class="external"></span>for x in xrange(5):
<span class="external"> </span>for y in xrange(5):
<span class="external"> </span>if x != y:
<span class="external"> </span>for z in xrange(5):
<span class="external"> </span>if y != z:
<span class="external"> </span>yield (x, y, z)
<span class="external"></span>return ((x, complicated_transform(x))
<span class="external"></span> for x in long_generator_function(parameter)
<span class="external"></span> if x is not None)
<span class="external"></span>squares = [x * x for x in range(10)]
<span class="external"></span>eat(jelly_bean for jelly_bean in jelly_beans
<span class="external"></span> if jelly_bean.color == 'black')</PRE></DIV>
</DIV></DIV>
</DIV>
<DIV class="">
<H3><A name="Default_Iterators_and_Operators" id="Default_Iterators_and_Operators">Default Iterators and Operators</A></H3>
<SPAN class="link_button" id="link-Default_Iterators_and_Operators__button" name="link-Default_Iterators_and_Operators__button"><A href="?showone=Default_Iterators_and_Operators#Default_Iterators_and_Operators">
link
</A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Default_Iterators_and_Operators__body','Default_Iterators_and_Operators__button')" name="Default_Iterators_and_Operators__button" id="Default_Iterators_and_Operators__button">▶</SPAN>
<DIV style="display:inline;" class="">
Use default iterators and operators for types that support them,
like lists, dictionaries, and files.
</DIV>
<DIV class=""><DIV class="stylepoint_body" name="Default_Iterators_and_Operators__body" id="Default_Iterators_and_Operators__body" style="display: none">
<P class="">
<SPAN class="stylepoint_section">Definition: </SPAN>
Container types, like dictionaries and lists, define default
iterators and membership test operators ("in" and "not in").
</P>
<P class="">
<SPAN class="stylepoint_section">Pros: </SPAN>
The default iterators and operators are simple and efficient.
They express the operation directly, without extra method calls.
A function that uses default operators is generic. It can be
used with any type that supports the operation.
</P>
<P class="">
<SPAN class="stylepoint_section">Cons: </SPAN>
You can't tell the type of objects by reading the method names
(e.g. has_key() means a dictionary). This is also an advantage.
</P>
<P class="">
<SPAN class="stylepoint_section">Decision: </SPAN> Use default iterators and operators for types
that support them, like lists, dictionaries, and files. The
built-in types define iterator methods, too. Prefer these
methods to methods that return lists, except that you should not
mutate a container while iterating over it.
<DIV class=""><PRE>Yes: <span class="external"></span>for key in adict: ...
<span class="external"></span>if key not in adict: ...
<span class="external"></span>if obj in alist: ...
<span class="external"></span>for line in afile: ...
<span class="external"></span>for k, v in dict.iteritems(): ...</PRE></DIV>
<DIV class=""><PRE class="badcode">No: <span class="external"></span>for key in adict.keys(): ...
<span class="external"></span>if not adict.has_key(key): ...
<span class="external"></span>for line in afile.readlines(): ...</PRE></DIV>
</P>
</DIV></DIV>
</DIV>
<DIV class="">
<H3><A name="Generators" id="Generators">Generators</A></H3>
<SPAN class="link_button" id="link-Generators__button" name="link-Generators__button"><A href="?showone=Generators#Generators">
link
</A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Generators__body','Generators__button')" name="Generators__button" id="Generators__button">▶</SPAN>
<DIV style="display:inline;" class="">
Use generators as needed.
</DIV>
<DIV class=""><DIV class="stylepoint_body" name="Generators__body" id="Generators__body" style="display: none">
<P class="">
<SPAN class="stylepoint_section">Definition: </SPAN>
A generator function returns an iterator that yields a value each
time it executes a yield statement. After it yields a value, the
runtime state of the generator function is suspended until the
next value is needed.
</P>
<P class="">
<SPAN class="stylepoint_section">Pros: </SPAN>
Simpler code, because the state of local variables and control flow
are preserved for each call. A generator uses less memory than a
function that creates an entire list of values at once.
</P>
<P class="">
<SPAN class="stylepoint_section">Cons: </SPAN>
None.
</P>
<P class="">
<SPAN class="stylepoint_section">Decision: </SPAN>
Fine. Use "Yields:" rather than "Returns:" in the
doc string for generator functions.
</P>
</DIV></DIV>
</DIV>
<DIV class="">
<H3><A name="Lambda_Functions" id="Lambda_Functions">Lambda Functions</A></H3>
<SPAN class="link_button" id="link-Lambda_Functions__button" name="link-Lambda_Functions__button"><A href="?showone=Lambda_Functions#Lambda_Functions">
link
</A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Lambda_Functions__body','Lambda_Functions__button')" name="Lambda_Functions__button" id="Lambda_Functions__button">▶</SPAN>
<DIV style="display:inline;" class="">
Okay for one-liners.
</DIV>
<DIV class=""><DIV class="stylepoint_body" name="Lambda_Functions__body" id="Lambda_Functions__body" style="display: none">
<P class="">
<SPAN class="stylepoint_section">Definition: </SPAN>
Lambdas define anonymous functions in an expression, as
opposed to a statement. They are often used to define callbacks or
operators for higher-order functions like <code>map()</code> and
<code>filter()</code>.
</P>
<P class="">
<SPAN class="stylepoint_section">Pros: </SPAN>
Convenient.
</P>
<P class="">
<SPAN class="stylepoint_section">Cons: </SPAN> Harder to read and debug than local functions. The
lack of names means stack traces are more difficult to
understand. Expressiveness is limited because the function may
only contain an expression.
</P>
<P class="">
<SPAN class="stylepoint_section">Decision: </SPAN>
Okay to use them for one-liners. If the code inside the lambda
function is any longer than 60–80 chars, it's probably better to
define it as a regular (nested) function.
<p>
For common operations like multiplication, use the functions from the
<code>operator</code> module instead of lambda functions. For
example, prefer <code>operator.mul</code> to <code>lambda
x, y: x * y</code>.
</p>
</P>
</DIV></DIV>
</DIV>
<DIV class="">
<H3><A name="Default_Argument_Values" id="Default_Argument_Values">Default Argument Values</A></H3>
<SPAN class="link_button" id="link-Default_Argument_Values__button" name="link-Default_Argument_Values__button"><A href="?showone=Default_Argument_Values#Default_Argument_Values">
link
</A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Default_Argument_Values__body','Default_Argument_Values__button')" name="Default_Argument_Values__button" id="Default_Argument_Values__button">▶</SPAN>
<DIV style="display:inline;" class="">
Okay in most cases.
</DIV>
<DIV class=""><DIV class="stylepoint_body" name="Default_Argument_Values__body" id="Default_Argument_Values__body" style="display: none">
<P class="">
<SPAN class="stylepoint_section">Definition: </SPAN>
You can specify values for variables at the end of a function's
parameter list, e.g., <code>def foo(a, b=0):</code>. If
<code>foo</code> is called with only one argument,
<code>b</code> is set to 0. If it is called with two arguments,
<code>b</code> has the value of the second argument.
</P>
<P class="">
<SPAN class="stylepoint_section">Pros: </SPAN>
Often you have a function that uses lots of default values,
but—rarely—you want to override the
defaults. Default argument values provide an easy way to do this,
without having to define lots of functions for the rare
exceptions. Also, Python does not support overloaded
methods/functions and default arguments are an easy way of
"faking" the overloading behavior.
</P>
<P class="">
<SPAN class="stylepoint_section">Cons: </SPAN>
Default arguments are evaluated once at module load
time. This may cause problems if the argument is a mutable
object such as a list or a dictionary. If the function modifies
the object (e.g., by appending an item to a list), the default
value is modified.
</P>
<P class="">
<SPAN class="stylepoint_section">Decision: </SPAN>
Okay to use with the following caveats:
<p>
Do not use mutable objects as default values in the function or method
definition.
</p>
<DIV class=""><PRE>Yes: <span class="external"></span>def foo(a, b=None):
<span class="external"> </span>if b is None:
<span class="external"> </span>b = []</PRE></DIV>
<DIV class=""><PRE class="badcode">No: <span class="external"></span>def foo(a, b=[]):
<span class="external"> </span>...</PRE></DIV>
<p>
Calling code must use named values for arguments with a default value.
This helps document the code somewhat and helps prevent and detect
interface breakage when more arguments are added.
</p>
<DIV class=""><PRE>
<span class="external"></span>def foo(a, b=1):
<span class="external"> </span>...</PRE></DIV>
<DIV class=""><PRE>Yes: <span class="external"></span>foo(1)
<span class="external"></span>foo(1, b=2)</PRE></DIV>
<DIV class=""><PRE class="badcode">No: <span class="external"></span>foo(1, 2)</PRE></DIV>
</P>
</DIV></DIV>
</DIV>
<DIV class="">
<H3><A name="Properties" id="Properties">Properties</A></H3>
<SPAN class="link_button" id="link-Properties__button" name="link-Properties__button"><A href="?showone=Properties#Properties">
link
</A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Properties__body','Properties__button')" name="Properties__button" id="Properties__button">▶</SPAN>
<DIV style="display:inline;" class="">
Use properties for accessing or setting data where you would
normally have used simple, lightweight accessor or setter methods.
</DIV>
<DIV class=""><DIV class="stylepoint_body" name="Properties__body" id="Properties__body" style="display: none">
<P class="">
<SPAN class="stylepoint_section">Definition: </SPAN> A way to wrap method calls for getting and
setting an attribute as a standard attribute access when the
computation is lightweight.
</P>
<P class="">
<SPAN class="stylepoint_section">Pros: </SPAN> Readability is increased by eliminating explicit
get and set method calls for simple attribute access. Allows
calculations to be lazy. Considered the Pythonic way to
maintain the interface of a class. In terms of performance,
allowing properties bypasses needing trivial accessor methods
when a direct variable access is reasonable. This also allows
accessor methods to be added in the future without breaking the
interface.
</P>
<P class="">
<SPAN class="stylepoint_section">Cons: </SPAN> Properties are specified after the getter and
setter methods are declared, requiring one to notice they are
used for properties farther down in the code (except for readonly
properties created with the <code>@property</code> decorator - see
below). Must inherit from
<code>object</code>. Can hide side-effects much like operator
overloading. Can be confusing for subclasses.
</P>
<P class="">
<SPAN class="stylepoint_section">Decision: </SPAN> Use properties in new code to access or
set data where you would normally have used simple, lightweight
accessor or setter methods. Read-only properties should be created
with the <code>@property</code>
<a HREF="#Function_and_Method_Decorators">decorator</a>.
<p><a id="properties-template-dp">
Inheritance with properties can be non-obvious if the property itself is
not overridden. Thus one must make sure that accessor methods are
called indirectly to ensure methods overridden in subclasses are called
by the property (using the Template Method DP).
</a></p>
<DIV class=""><PRE>Yes: <span class="external"></span>import math
<span class="external"></span>class Square(object):
<span class="external"> </span>"""A square with two properties: a writable area and a read-only perimeter.
<span class="external"> </span>To use:
<span class="external"> </span>>>> sq = Square(3)
<span class="external"> </span>>>> sq.area
<span class="external"> </span>9
<span class="external"> </span>>>> sq.perimeter
<span class="external"> </span>12
<span class="external"> </span>>>> sq.area = 16
<span class="external"> </span>>>> sq.side
<span class="external"> </span>4
<span class="external"> </span>>>> sq.perimeter
<span class="external"> </span>16
<span class="external"> </span>"""
<span class="external"> </span>def __init__(self, side):
<span class="external"> </span>self.side = side
<span class="external"> </span>def __get_area(self):
<span class="external"> </span>"""Calculates the 'area' property."""
<span class="external"> </span>return self.side ** 2
<span class="external"> </span>def ___get_area(self):
<span class="external"> </span>"""Indirect accessor for 'area' property."""
<span class="external"> </span>return self.__get_area()
<span class="external"> </span>def __set_area(self, area):
<span class="external"> </span>"""Sets the 'area' property."""
<span class="external"> </span>self.side = math.sqrt(area)
<span class="external"> </span>def ___set_area(self, area):
<span class="external"> </span>"""Indirect setter for 'area' property."""
<span class="external"> </span>self._SetArea(area)
<span class="external"> </span>area = property(___get_area, ___set_area,
<span class="external"> </span> doc="""Gets or sets the area of the square.""")
<span class="external"> </span>@property
<span class="external"> </span>def perimeter(self):
<span class="external"> </span>return self.side * 4
<span class="external"></span>
</PRE></DIV>
</P>
</DIV></DIV>
</DIV>
<DIV class="">
<H3><A name="True/False_evaluations" id="True/False_evaluations">True/False evaluations</A></H3>
<SPAN class="link_button" id="link-True/False_evaluations__button" name="link-True/False_evaluations__button"><A href="?showone=True/False_evaluations#True/False_evaluations">
link
</A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('True/False_evaluations__body','True/False_evaluations__button')" name="True/False_evaluations__button" id="True/False_evaluations__button">▶</SPAN>
<DIV style="display:inline;" class="">
Use the "implicit" false if at all possible.
</DIV>
<DIV class=""><DIV class="stylepoint_body" name="True/False_evaluations__body" id="True/False_evaluations__body" style="display: none">
<P class="">
<SPAN class="stylepoint_section">Definition: </SPAN> Python evaluates certain values as <code>false</code>
when in a boolean context. A quick "rule of thumb" is that all
"empty" values are considered <code>false</code> so <code>0, None, [], {},
""</code> all evaluate as <code>false</code> in a boolean context.
</P>
<P class="">
<SPAN class="stylepoint_section">Pros: </SPAN> Conditions using Python booleans are easier to read
and less error-prone. In most cases, they're also faster.
</P>
<P class="">
<SPAN class="stylepoint_section">Cons: </SPAN>
May look strange to C/C++ developers.
</P>
<P class="">
<SPAN class="stylepoint_section">Decision: </SPAN>
Use the "implicit" false if at all possible, e.g., <code>if
foo:</code> rather than <code>if foo != []:</code>. There are a
few caveats that you should keep in mind though:
<ul>
<li>
Never use <code>==</code> or <code>!=</code> to compare
singletons like <code>None</code>. Use <code>is</code>
or <code>is not</code>.</li>
<li>Beware of writing <code>if x:</code> when you really mean
<code>if x is not None:</code>—e.g., when testing whether
a variable or argument that defaults to <code>None</code> was
set to some other value. The other value might be a value
that's false in a boolean context!</li>
<li>
Never compare a boolean variable to <code>False</code> using
<code>==</code>. Use <code>if not x:</code> instead. If
you need to distinguish <code>False</code> from
<code>None</code> then chain the expressions,
such as <code>if not x and x is not None:</code>.
</li>
<li>
For sequences (strings, lists, tuples), use the fact that
empty sequences are false, so <code>if not seq:</code> or
<code>if seq:</code> is preferable to <code>if
len(seq):</code> or <code>if not
len(seq):</code>.</li>
<li>
When handling integers, implicit false may involve more risk than
benefit (i.e., accidentally handling <code>None</code> as 0). You may
compare a value which is known to be an integer (and is not the
result of <code>len()</code>) against the integer 0.
<DIV class=""><PRE>Yes: <span class="external"></span>if not users:
<span class="external"> </span>print 'no users'
<span class="external"></span>if foo == 0:
<span class="external"> </span>self.handle_zero()
<span class="external"></span>if i % 10 == 0:
<span class="external"> </span>self.handle_multiple_of_ten()</PRE></DIV>
<DIV class=""><PRE class="badcode">No: <span class="external"></span>if len(users) == 0:
<span class="external"> </span>print 'no users'
<span class="external"></span>if foo is not None and not foo:
<span class="external"> </span>self.handle_zero()
<span class="external"></span>if not i % 10:
<span class="external"> </span>self.handle_multiple_of_ten()</PRE></DIV>
</li>
<li>
Note that <code>'0'</code> (i.e., <code>0</code> as string)
evaluates to true.</li>
</ul>
</P>
</DIV></DIV>
</DIV>
<DIV class="">
<H3><A name="Deprecated_Language_Features" id="Deprecated_Language_Features">Deprecated Language Features</A></H3>
<SPAN class="link_button" id="link-Deprecated_Language_Features__button" name="link-Deprecated_Language_Features__button"><A href="?showone=Deprecated_Language_Features#Deprecated_Language_Features">
link
</A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Deprecated_Language_Features__body','Deprecated_Language_Features__button')" name="Deprecated_Language_Features__button" id="Deprecated_Language_Features__button">▶</SPAN>
<DIV style="display:inline;" class="">
Use string methods instead of the <code>string</code> module
where possible. Use function call syntax instead
of <code>apply</code>. Use list comprehensions
and <code>for</code> loops instead of <code>filter</code>,
<code>map</code>, and <code>reduce</code>.
</DIV>
<DIV class=""><DIV class="stylepoint_body" name="Deprecated_Language_Features__body" id="Deprecated_Language_Features__body" style="display: none">
<P class="">
<SPAN class="stylepoint_section">Definition: </SPAN>
Current versions of Python provide alternative constructs
that people find generally preferable.
</P>
<P class="">
<SPAN class="stylepoint_section">Decision: </SPAN>
We do not use any Python version which does not support
these features, so there is no reason not to use the new
styles.
<DIV class=""><PRE class="badcode">No: <span class="external"></span>words = string.split(foo, ':')
<span class="external"></span>map(lambda x: x[1], filter(lambda x: x[2] == 5, my_list))
<span class="external"></span>apply(fn, args, kwargs)</PRE></DIV>
<DIV class=""><PRE>Yes: <span class="external"></span>words = foo.split(':')
<span class="external"></span>[x[1] for x in my_list if x[2] == 5]
<span class="external"></span>fn(*args, **kwargs)</PRE></DIV>
</P>
</DIV></DIV>
</DIV>
<DIV class="">
<H3><A name="Lexical_Scoping" id="Lexical_Scoping">Lexical Scoping</A></H3>
<SPAN class="link_button" id="link-Lexical_Scoping__button" name="link-Lexical_Scoping__button"><A href="?showone=Lexical_Scoping#Lexical_Scoping">
link
</A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Lexical_Scoping__body','Lexical_Scoping__button')" name="Lexical_Scoping__button" id="Lexical_Scoping__button">▶</SPAN>
<DIV style="display:inline;" class="">
Okay to use.
</DIV>
<DIV class=""><DIV class="stylepoint_body" name="Lexical_Scoping__body" id="Lexical_Scoping__body" style="display: none">
<P class="">
<SPAN class="stylepoint_section">Definition: </SPAN>
A nested Python function can refer to variables defined in
enclosing functions, but can not assign to them. Variable
bindings are resolved using lexical scoping, that is, based on
the static program text. Any assignment to a name in a block
will cause Python to treat all references to that name as a
local variable, even if the use precedes the assignment. If a
global declaration occurs, the name is treated as a global
variable.
<p>
An example of the use of this feature is:
</p>
<DIV class=""><PRE>
<span class="external"></span>def get_adder(summand1):
<span class="external"> </span>"""Returns a function that adds numbers to a given number."""
<span class="external"> </span>def adder(summand2):
<span class="external"> </span>return summand1 + summand2
<span class="external"> </span>return adder
<span class="external"></span>
</PRE></DIV>
</P>
<P class="">
<SPAN class="stylepoint_section">Pros: </SPAN>
Often results in clearer, more elegant code. Especially comforting
to experienced Lisp and Scheme (and Haskell and ML and …)
programmers.
</P>
<P class="">
<SPAN class="stylepoint_section">Cons: </SPAN>
Can lead to confusing bugs. Such as this example based on
<a HREF="http://www.python.org/dev/peps/pep-0227/">PEP-0227</a>:
<DIV class=""><PRE class="badcode">
<span class="external"></span>i = 4
<span class="external"></span>def foo(x):
<span class="external"> </span>def bar():
<span class="external"> </span>print i,
<span class="external"> </span># ...
<span class="external"> </span># A bunch of code here
<span class="external"> </span># ...
<span class="external"> </span>for i in x: # Ah, i *is* local to Foo, so this is what Bar sees
<span class="external"> </span>print i,
<span class="external"> </span>bar()</PRE></DIV>
<p>
So <code>foo([1, 2, 3])</code> will print <code>1 2 3 3</code>, not
<code>1 2 3 4</code>.
</p>
</P>
<P class="">
<SPAN class="stylepoint_section">Decision: </SPAN>