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2311 lines
108 KiB
HTML
2311 lines
108 KiB
HTML
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<H1>Google Python Style Guide</H1>
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<p align="right">
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Revision 2.59
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||
</p>
|
||
|
||
<address>
|
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Amit Patel<br>
|
||
Antoine Picard<br>
|
||
Eugene Jhong<br>
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Jeremy Hylton<br>
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Matt Smart<br>
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Mike Shields<br>
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</address>
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<SPAN class="showhide_button" style="margin-left: 0; float: none">▶</SPAN>.
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Toggle all summaries
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</DIV>
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</DIV>
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<DIV class="toc">
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<DIV class="toc_title">Table of Contents</DIV>
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<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">
|
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<SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Lint">Lint</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="#Conditional_Expressions">Conditional Expressions</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="#Shebang_Line">Shebang Line</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="#Files_and_Sockets">Files and Sockets</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">
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link
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</A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Displaying_Hidden_Details_in_this_Guide')" name="Displaying_Hidden_Details_in_this_Guide__button" id="Displaying_Hidden_Details_in_this_Guide__button">▶</SPAN>
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||
<DIV style="display:inline;" class="">
|
||
This style guide contains many details that are initially
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||
hidden from view. They are marked by the triangle icon, which you
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||
see here on your left. Click it now.
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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>
|
||
|
||
<p>
|
||
To help you format code correctly, we've created a <a href="google_python_style.vim">settings
|
||
file for Vim</a>. For Emacs, the default settings should be fine.
|
||
</p>
|
||
|
||
|
||
</DIV>
|
||
|
||
<DIV class="">
|
||
<H2 name="Python_Language_Rules" id="Python_Language_Rules">Python Language Rules</H2>
|
||
<DIV class="">
|
||
<H3><A name="Lint" id="Lint">Lint</A></H3>
|
||
<SPAN class="link_button" id="link-Lint__button" name="link-Lint__button"><A href="?showone=Lint#Lint">
|
||
link
|
||
</A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Lint')" name="Lint__button" id="Lint__button">▶</SPAN>
|
||
<DIV style="display:inline;" class="">
|
||
Run <code>pylint</code> over your code.
|
||
</DIV>
|
||
<DIV class=""><DIV class="stylepoint_body" name="Lint__body" id="Lint__body" style="display: none">
|
||
<P class="">
|
||
<SPAN class="stylepoint_section">Definition: </SPAN>
|
||
pylint
|
||
is a tool for finding bugs and style problems in Python source
|
||
code. It finds
|
||
problems that are typically caught by a compiler for less dynamic
|
||
languages like C and C++.
|
||
|
||
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, using-vars-before-assignment, etc.
|
||
</P>
|
||
<P class="">
|
||
<SPAN class="stylepoint_section">Cons: </SPAN>
|
||
<code>pylint</code>
|
||
isn't perfect. To take advantage of it, we'll need to sometimes:
|
||
a) Write around it b) Suppress its warnings or c) Improve it.
|
||
</P>
|
||
<P class="">
|
||
<SPAN class="stylepoint_section">Decision: </SPAN>
|
||
Make sure you run <code>pylint</code> on your code.
|
||
Suppress warnings if they are inappropriate so that other issues are
|
||
not hidden.
|
||
</P>
|
||
|
||
<p>
|
||
To suppress warnings, you can set a line-level comment:
|
||
</p>
|
||
|
||
<DIV class=""><PRE>
|
||
<span class="external"></span>dict = 'something awful' # Bad Idea... pylint: disable=redefined-builtin</PRE></DIV>
|
||
<p>
|
||
pylint
|
||
warnings are each identified by a alphanumeric code
|
||
(<code>C0112</code>) and a symbolic name
|
||
(<code>empty-docstring</code>). Prefer the symbolic
|
||
names in new code or when updating existing code.
|
||
|
||
</p>
|
||
<p>
|
||
If the reason for the suppression is not clear from the symbolic name,
|
||
add an explanation.
|
||
</p>
|
||
<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
|
||
pylint
|
||
warnings by doing
|
||
<code>pylint --list-msgs</code>.
|
||
To get more information on a particular message, use
|
||
<code>pylint --help-msg=C6409</code>.
|
||
</p>
|
||
<p>
|
||
Prefer <code>pylint: disable</code> to the deprecated older form
|
||
<code>pylint: disable-msg</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
|
||
<span class="external"> </span>return a
|
||
<span class="external"></span>
|
||
</PRE></DIV>
|
||
</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')" 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>
|
||
The namespace management convention is simple. The source of each
|
||
identifier is indicated in a consistent way; <code>x.Obj</code> says
|
||
that object <code>Obj</code> is defined in module <code>x</code>.
|
||
</P>
|
||
<P class="">
|
||
<SPAN class="stylepoint_section">Cons: </SPAN> Module names can still collide. Some module names are
|
||
inconveniently long.
|
||
</P>
|
||
<P class="">
|
||
<SPAN class="stylepoint_section">Decision: </SPAN>
|
||
Use <code>import x</code> for importing packages and modules.
|
||
<br>
|
||
Use <code>from x import y</code> where <code>x</code> is
|
||
the package prefix and <code>y</code> is the module name with no
|
||
prefix.
|
||
<br>
|
||
Use <code>from x import y as z</code> if two modules named
|
||
<code>y</code> are to be imported or if <code>y</code> is an
|
||
inconveniently long name.
|
||
</P>
|
||
For example the module
|
||
<code>sound.effects.echo</code> may be imported as follows:
|
||
<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>
|
||
Do not use relative names in imports. Even if the module is in the
|
||
same package, use the full package name. This helps prevent
|
||
unintentionally importing a package twice.
|
||
</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')" name="Packages__button" id="Packages__button">▶</SPAN>
|
||
<DIV style="display:inline;" class="">
|
||
Import each module using the full pathname location of the 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 import each module by its full 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 (preferred).
|
||
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')" 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 misspelled names, sys.exit()
|
||
calls, Ctrl+C interrupts, unittest failures and all kinds of
|
||
other exceptions that you simply don't want to catch.</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>
|
||
<li>When capturing an exception, use <code>as</code> rather than
|
||
a comma. For example:
|
||
<DIV class=""><PRE>
|
||
<span class="external"></span>try:
|
||
<span class="external"> </span>raise Error
|
||
<span class="external"></span>except Error as error:
|
||
<span class="external"> </span>pass</PRE></DIV>
|
||
</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')" 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')" 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="https://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')" 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>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 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></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')" 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')" 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')" 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="Conditional_Expressions" id="Conditional_Expressions">Conditional Expressions</A></H3>
|
||
<SPAN class="link_button" id="link-Conditional_Expressions__button" name="link-Conditional_Expressions__button"><A href="?showone=Conditional_Expressions#Conditional_Expressions">
|
||
link
|
||
</A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Conditional_Expressions')" name="Conditional_Expressions__button" id="Conditional_Expressions__button">▶</SPAN>
|
||
<DIV style="display:inline;" class="">
|
||
Okay for one-liners.
|
||
</DIV>
|
||
<DIV class=""><DIV class="stylepoint_body" name="Conditional_Expressions__body" id="Conditional_Expressions__body" style="display: none">
|
||
<P class="">
|
||
<SPAN class="stylepoint_section">Definition: </SPAN>
|
||
Conditional expressions are mechanisms that provide a shorter syntax
|
||
for if statements. For example:
|
||
<code>x = 1 if cond else 2</code>.
|
||
</P>
|
||
<P class="">
|
||
<SPAN class="stylepoint_section">Pros: </SPAN>
|
||
Shorter and more convenient than an if statement.
|
||
</P>
|
||
<P class="">
|
||
<SPAN class="stylepoint_section">Cons: </SPAN>
|
||
May be harder to read than an if statement. The condition may be difficult
|
||
to locate if the expression is long.
|
||
</P>
|
||
<P class="">
|
||
<SPAN class="stylepoint_section">Decision: </SPAN>
|
||
Okay to use for one-liners. In other cases prefer to use a complete if
|
||
statement.
|
||
</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')" 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 caveat:
|
||
<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>...
|
||
No: <span class="external"></span>def foo(a, b=time.time()): # The time the module was loaded???
|
||
<span class="external"> </span>...
|
||
No: <span class="external"></span>def foo(a, b=FLAGS.my_thing): # sys.argv has not yet been parsed...
|
||
<span class="external"> </span>...</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')" 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.__set_area(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')" 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')" 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> and
|
||
<code>map</code> when the function argument would have been an
|
||
inlined lambda anyway. Use <code>for</code> loops instead of
|
||
<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>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>map(math.sqrt, data) # Ok. No inlined lambda expression.
|
||
|
||
<span class="external"></span>fn(*args, **kwargs)</PRE></DIV>
|
||
<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>
|
||
</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')" 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="https://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>
|
||
Okay to use.
|
||
</P>
|
||
</DIV></DIV>
|
||
</DIV>
|
||
<DIV class="">
|
||
<H3><A name="Function_and_Method_Decorators" id="Function_and_Method_Decorators">Function and Method Decorators</A></H3>
|
||
<SPAN class="link_button" id="link-Function_and_Method_Decorators__button" name="link-Function_and_Method_Decorators__button"><A href="?showone=Function_and_Method_Decorators#Function_and_Method_Decorators">
|
||
link
|
||
</A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Function_and_Method_Decorators')" name="Function_and_Method_Decorators__button" id="Function_and_Method_Decorators__button">▶</SPAN>
|
||
<DIV style="display:inline;" class="">
|
||
Use decorators judiciously when there is a clear advantage.
|
||
</DIV>
|
||
<DIV class=""><DIV class="stylepoint_body" name="Function_and_Method_Decorators__body" id="Function_and_Method_Decorators__body" style="display: none">
|
||
<P class="">
|
||
<SPAN class="stylepoint_section">Definition: </SPAN>
|
||
|
||
<a HREF="https://www.python.org/dev/peps/pep-0318">Decorators
|
||
for Functions and Methods</a>
|
||
(a.k.a "the <code>@</code> notation").
|
||
The most common decorators are <code>@classmethod</code> and
|
||
<code>@staticmethod</code>, for converting ordinary methods to class or
|
||
static methods. However, the decorator syntax allows for
|
||
user-defined decorators as well. Specifically, for some function
|
||
<code>my_decorator</code>, this:
|
||
<DIV class=""><PRE>
|
||
<span class="external"></span>class C(object):
|
||
<span class="external"> </span>@my_decorator
|
||
<span class="external"> </span>def method(self):
|
||
<span class="external"> </span># method body ...
|
||
<span class="external"></span>
|
||
</PRE></DIV>
|
||
|
||
is equivalent to:
|
||
<DIV class=""><PRE>
|
||
<span class="external"></span>class C(object):
|
||
<span class="external"> </span>def method(self):
|
||
<span class="external"> </span># method body ...
|
||
<span class="external"> </span>method = my_decorator(method)
|
||
<span class="external"></span>
|
||
</PRE></DIV>
|
||
</P>
|
||
<P class="">
|
||
<SPAN class="stylepoint_section">Pros: </SPAN> Elegantly specifies some transformation on a method; the
|
||
transformation might eliminate some repetitive code, enforce invariants,
|
||
etc.
|
||
</P>
|
||
<P class="">
|
||
<SPAN class="stylepoint_section">Cons: </SPAN> Decorators can perform arbitrary operations on a
|
||
function's arguments or return values, resulting in surprising
|
||
implicit behavior.
|
||
Additionally, decorators execute at import time. Failures in decorator
|
||
code are pretty much impossible to recover from.
|
||
</P>
|
||
<P class="">
|
||
<SPAN class="stylepoint_section">Decision: </SPAN> Use decorators judiciously when there is a clear
|
||
advantage. Decorators should follow the same import and naming
|
||
guidelines as functions. Decorator pydoc should clearly state that the
|
||
function is a decorator. Write unit tests for decorators.
|
||
|
||
<p>
|
||
Avoid external dependencies in the decorator itself (e.g. don't rely on
|
||
files, sockets, database connections, etc.), since they might not be
|
||
available when the decorator runs (at import time, perhaps from
|
||
<code>pydoc</code> or other tools). A decorator that is
|
||
called with valid parameters should (as much as possible) be guaranteed
|
||
to succeed in all cases.
|
||
</p>
|
||
<p>
|
||
Decorators are a special case of "top level code" - see
|
||
<a HREF="#Main">main</a> for more discussion.
|
||
</p>
|
||
</P>
|
||
</DIV></DIV>
|
||
</DIV>
|
||
<DIV class="">
|
||
<H3><A name="Threading" id="Threading">Threading</A></H3>
|
||
<SPAN class="link_button" id="link-Threading__button" name="link-Threading__button"><A href="?showone=Threading#Threading">
|
||
link
|
||
</A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Threading')" name="Threading__button" id="Threading__button">▶</SPAN>
|
||
<DIV style="display:inline;" class="">
|
||
Do not rely on the atomicity of built-in types.
|
||
</DIV>
|
||
<DIV class=""><DIV class="stylepoint_body" name="Threading__body" id="Threading__body" style="display: none">
|
||
<p>
|
||
While Python's built-in data types such as dictionaries appear
|
||
to have atomic operations, there are corner cases where they
|
||
aren't atomic (e.g. if <code>__hash__</code> or
|
||
<code>__eq__</code> are implemented as Python methods) and their
|
||
atomicity should not be relied upon. Neither should you rely on
|
||
atomic variable assignment (since this in turn depends on
|
||
dictionaries).
|
||
</p>
|
||
|
||
<p>
|
||
Use the Queue module's <code>Queue</code> data type as the preferred
|
||
way to
|
||
communicate data between threads. Otherwise, use the threading
|
||
module and its locking primitives. Learn about the proper use
|
||
of condition variables so you can use
|
||
<code>threading.Condition</code> instead of using lower-level
|
||
locks.
|
||
</p>
|
||
</DIV></DIV>
|
||
</DIV>
|
||
<DIV class="">
|
||
<H3><A name="Power_Features" id="Power_Features">Power Features</A></H3>
|
||
<SPAN class="link_button" id="link-Power_Features__button" name="link-Power_Features__button"><A href="?showone=Power_Features#Power_Features">
|
||
link
|
||
</A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Power_Features')" name="Power_Features__button" id="Power_Features__button">▶</SPAN>
|
||
<DIV style="display:inline;" class="">
|
||
Avoid these features.
|
||
</DIV>
|
||
<DIV class=""><DIV class="stylepoint_body" name="Power_Features__body" id="Power_Features__body" style="display: none">
|
||
<P class="">
|
||
<SPAN class="stylepoint_section">Definition: </SPAN> Python is an extremely flexible language and
|
||
gives you many fancy features such as metaclasses, access to bytecode,
|
||
on-the-fly compilation, dynamic inheritance, object reparenting,
|
||
import hacks, reflection, modification of system internals,
|
||
etc.
|
||
</P>
|
||
<P class="">
|
||
<SPAN class="stylepoint_section">Pros: </SPAN> These are powerful language features. They can
|
||
make your code more compact.
|
||
</P>
|
||
<P class="">
|
||
<SPAN class="stylepoint_section">Cons: </SPAN> It's very tempting to use these "cool" features
|
||
when they're not absolutely necessary. It's harder to read,
|
||
understand, and debug code that's using unusual features
|
||
underneath. It doesn't seem that way at first (to the original
|
||
author), but when revisiting the code, it tends to be more
|
||
difficult than code that is longer but is straightforward.
|
||
</P>
|
||
<P class="">
|
||
<SPAN class="stylepoint_section">Decision: </SPAN>
|
||
Avoid these features in
|
||
your code.
|
||
</P>
|
||
</DIV></DIV>
|
||
</DIV>
|
||
</DIV>
|
||
<DIV class="">
|
||
<H2 name="Python_Style_Rules" id="Python_Style_Rules">Python Style Rules</H2>
|
||
<DIV class="">
|
||
<H3><A name="Semicolons" id="Semicolons">Semicolons</A></H3>
|
||
<SPAN class="link_button" id="link-Semicolons__button" name="link-Semicolons__button"><A href="?showone=Semicolons#Semicolons">
|
||
link
|
||
</A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Semicolons')" name="Semicolons__button" id="Semicolons__button">▶</SPAN>
|
||
<DIV style="display:inline;" class="">
|
||
Do not terminate your lines with semi-colons and do not use
|
||
semi-colons to put two commands on the same line.
|
||
</DIV>
|
||
<DIV class=""><DIV class="stylepoint_body" name="Semicolons__body" id="Semicolons__body" style="display: none">
|
||
</DIV></DIV>
|
||
</DIV>
|
||
<DIV class="">
|
||
<H3><A name="Line_length" id="Line_length">Line length</A></H3>
|
||
<SPAN class="link_button" id="link-Line_length__button" name="link-Line_length__button"><A href="?showone=Line_length#Line_length">
|
||
link
|
||
</A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Line_length')" name="Line_length__button" id="Line_length__button">▶</SPAN>
|
||
<DIV style="display:inline;" class="">
|
||
Maximum line length is <em>80 characters</em>.
|
||
</DIV>
|
||
<DIV class=""><DIV class="stylepoint_body" name="Line_length__body" id="Line_length__body" style="display: none">
|
||
<p>
|
||
Exceptions:
|
||
<ul>
|
||
<li>Long import statements.</li>
|
||
<li>URLs in comments.</li>
|
||
|
||
</ul>
|
||
</p>
|
||
|
||
<p>
|
||
Do not use backslash line continuation.
|
||
</p>
|
||
|
||
<p>
|
||
Make use of Python's
|
||
|
||
<a HREF="https://docs.python.org/reference/lexical_analysis.html#implicit-line-joining">implicit
|
||
line joining inside parentheses, brackets and braces</a>.
|
||
If necessary, you can add an extra pair of parentheses around an
|
||
expression.
|
||
</p>
|
||
|
||
|
||
<DIV class=""><PRE>Yes: foo_bar(self, width, height, color='black', design=None, x='foo',
|
||
emphasis=None, highlight=0)
|
||
|
||
if (width == 0 and height == 0 and
|
||
color == 'red' and emphasis == 'strong'):</PRE></DIV>
|
||
|
||
|
||
<p>
|
||
When a literal string won't fit on a single line, use parentheses for
|
||
implicit line joining.
|
||
</p>
|
||
|
||
<DIV class=""><PRE>
|
||
<span class="external"></span>x = ('This will build a very long long '
|
||
<span class="external"></span> 'long long long long long long string')</PRE></DIV>
|
||
|
||
<p>
|
||
Within comments, put long URLs on their own line if necessary.
|
||
</p>
|
||
|
||
<DIV class=""><PRE>Yes: <span class="external"></span># See details at
|
||
<span class="external"></span># https://www.example.com/us/developer/documentation/api/content/v2.0/csv_file_name_extension_full_specification.html</PRE></DIV>
|
||
|
||
<DIV class=""><PRE class="badcode">No: <span class="external"></span># See details at
|
||
<span class="external"></span># https://www.example.com/us/developer/documentation/api/content/\
|
||
<span class="external"></span># v2.0/csv_file_name_extension_full_specification.html</PRE></DIV>
|
||
|
||
<p>
|
||
Make note of the indentation of the elements in the line
|
||
continuation examples above; see the
|
||
<a HREF="#Indentation">indentation</a>
|
||
section for explanation.
|
||
</p>
|
||
</DIV></DIV>
|
||
</DIV>
|
||
<DIV class="">
|
||
<H3><A name="Parentheses" id="Parentheses">Parentheses</A></H3>
|
||
<SPAN class="link_button" id="link-Parentheses__button" name="link-Parentheses__button"><A href="?showone=Parentheses#Parentheses">
|
||
link
|
||
</A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Parentheses')" name="Parentheses__button" id="Parentheses__button">▶</SPAN>
|
||
<DIV style="display:inline;" class="">
|
||
Use parentheses sparingly.
|
||
</DIV>
|
||
<DIV class=""><DIV class="stylepoint_body" name="Parentheses__body" id="Parentheses__body" style="display: none">
|
||
<p>
|
||
Do not use them in return statements or conditional statements unless
|
||
using parentheses for implied line continuation. (See above.)
|
||
It is however fine to use parentheses around tuples.
|
||
</p>
|
||
|
||
<DIV class=""><PRE>Yes: <span class="external"></span>if foo:
|
||
<span class="external"> </span>bar()
|
||
<span class="external"></span>while x:
|
||
<span class="external"> </span>x = bar()
|
||
<span class="external"></span>if x and y:
|
||
<span class="external"> </span>bar()
|
||
<span class="external"></span>if not x:
|
||
<span class="external"> </span>bar()
|
||
<span class="external"></span>return foo
|
||
<span class="external"></span>for (x, y) in dict.items(): ...</PRE></DIV>
|
||
<DIV class=""><PRE class="badcode">No: <span class="external"></span>if (x):
|
||
<span class="external"> </span>bar()
|
||
<span class="external"></span>if not(x):
|
||
<span class="external"> </span>bar()
|
||
<span class="external"></span>return (foo)</PRE></DIV>
|
||
</DIV></DIV>
|
||
</DIV>
|
||
<DIV class="">
|
||
<H3><A name="Indentation" id="Indentation">Indentation</A></H3>
|
||
<SPAN class="link_button" id="link-Indentation__button" name="link-Indentation__button"><A href="?showone=Indentation#Indentation">
|
||
link
|
||
</A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Indentation')" name="Indentation__button" id="Indentation__button">▶</SPAN>
|
||
<DIV style="display:inline;" class="">
|
||
Indent your code blocks with <em>4 spaces</em>.
|
||
</DIV>
|
||
<DIV class=""><DIV class="stylepoint_body" name="Indentation__body" id="Indentation__body" style="display: none">
|
||
<p>
|
||
Never use tabs or mix tabs and spaces.
|
||
In cases of implied line continuation, you should align wrapped elements
|
||
either vertically, as per the examples in the
|
||
<a HREF="#Line_length">line length</a> section; or using a hanging
|
||
indent of 4 spaces, in which case there should be no argument on
|
||
the first line.
|
||
</p>
|
||
|
||
|
||
<DIV class=""><PRE>Yes: # Aligned with opening delimiter
|
||
foo = long_function_name(var_one, var_two,
|
||
var_three, var_four)
|
||
|
||
# Aligned with opening delimiter in a dictionary
|
||
foo = {
|
||
long_dictionary_key: value1 +
|
||
value2,
|
||
...
|
||
}
|
||
|
||
# 4-space hanging indent; nothing on first line
|
||
foo = long_function_name(
|
||
var_one, var_two, var_three,
|
||
var_four)
|
||
|
||
# 4-space hanging indent in a dictionary
|
||
foo = {
|
||
long_dictionary_key:
|
||
long_dictionary_value,
|
||
...
|
||
}</PRE></DIV>
|
||
<DIV class=""><PRE class="badcode">No: <span class="external"></span># Stuff on first line forbidden
|
||
<span class="external"></span>foo = long_function_name(var_one, var_two,
|
||
<span class="external"></span> var_three, var_four)
|
||
|
||
<span class="external"></span># 2-space hanging indent forbidden
|
||
<span class="external"></span>foo = long_function_name(
|
||
<span class="external"></span> var_one, var_two, var_three,
|
||
<span class="external"></span> var_four)
|
||
|
||
<span class="external"></span># No hanging indent in a dictionary
|
||
<span class="external"></span>foo = {
|
||
<span class="external"></span> long_dictionary_key:
|
||
<span class="external"> </span>long_dictionary_value,
|
||
<span class="external"> </span>...
|
||
<span class="external"></span>}</PRE></DIV>
|
||
</DIV></DIV>
|
||
</DIV>
|
||
<DIV class="">
|
||
<H3><A name="Blank_Lines" id="Blank_Lines">Blank Lines</A></H3>
|
||
<SPAN class="link_button" id="link-Blank_Lines__button" name="link-Blank_Lines__button"><A href="?showone=Blank_Lines#Blank_Lines">
|
||
link
|
||
</A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Blank_Lines')" name="Blank_Lines__button" id="Blank_Lines__button">▶</SPAN>
|
||
<DIV style="display:inline;" class="">
|
||
Two blank lines between top-level definitions, one blank line
|
||
between method definitions.
|
||
</DIV>
|
||
<DIV class=""><DIV class="stylepoint_body" name="Blank_Lines__body" id="Blank_Lines__body" style="display: none">
|
||
<p>
|
||
Two blank lines between top-level definitions, be they function
|
||
or class definitions. One blank line between method definitions
|
||
and between the <code>class</code> line and the first method.
|
||
Use single blank lines as you judge appropriate within functions or
|
||
methods.
|
||
</p>
|
||
</DIV></DIV>
|
||
</DIV>
|
||
<DIV class="">
|
||
<H3><A name="Whitespace" id="Whitespace">Whitespace</A></H3>
|
||
<SPAN class="link_button" id="link-Whitespace__button" name="link-Whitespace__button"><A href="?showone=Whitespace#Whitespace">
|
||
link
|
||
</A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Whitespace')" name="Whitespace__button" id="Whitespace__button">▶</SPAN>
|
||
<DIV style="display:inline;" class="">
|
||
Follow standard typographic rules for the use of spaces around
|
||
punctuation.
|
||
</DIV>
|
||
<DIV class=""><DIV class="stylepoint_body" name="Whitespace__body" id="Whitespace__body" style="display: none">
|
||
<p>
|
||
No whitespace inside parentheses, brackets or braces.
|
||
</p>
|
||
<DIV class=""><PRE>Yes: <span class="external"></span>spam(ham[1], {eggs: 2}, [])</PRE></DIV>
|
||
<DIV class=""><PRE class="badcode">No: <span class="external"></span>spam( ham[ 1 ], { eggs: 2 }, [ ] )</PRE></DIV>
|
||
<p>
|
||
No whitespace before a comma, semicolon, or colon. Do use
|
||
whitespace after a comma, semicolon, or colon except at the end
|
||
of the line.
|
||
</p>
|
||
<DIV class=""><PRE>Yes: <span class="external"></span>if x == 4:
|
||
<span class="external"> </span>print x, y
|
||
<span class="external"></span>x, y = y, x</PRE></DIV>
|
||
<DIV class=""><PRE class="badcode">No: <span class="external"></span>if x == 4 :
|
||
<span class="external"> </span>print x , y
|
||
<span class="external"></span>x , y = y , x</PRE></DIV>
|
||
<p>
|
||
No whitespace before the open paren/bracket that starts an argument list,
|
||
indexing or slicing.
|
||
</p>
|
||
<DIV class=""><PRE>Yes: <span class="external"></span>spam(1)</PRE></DIV>
|
||
<DIV class=""><PRE class="badcode">No: <span class="external"></span>spam (1)</PRE></DIV>
|
||
<DIV class=""><PRE>Yes: <span class="external"></span>dict['key'] = list[index]</PRE></DIV>
|
||
<DIV class=""><PRE class="badcode">No: <span class="external"></span>dict ['key'] = list [index]</PRE></DIV>
|
||
|
||
<p>
|
||
Surround binary operators with a single space on either side for
|
||
assignment (<code>=</code>), comparisons (<code>==, <, >, !=,
|
||
<>, <=, >=, in, not in, is, is not</code>), and Booleans
|
||
(<code>and, or, not</code>). Use your better judgment for the
|
||
insertion of spaces around arithmetic operators but always be
|
||
consistent about whitespace on either side of a binary operator.
|
||
</p>
|
||
<DIV class=""><PRE>Yes: <span class="external"></span>x == 1</PRE></DIV>
|
||
<DIV class=""><PRE class="badcode">No: <span class="external"></span>x<1</PRE></DIV>
|
||
<p>
|
||
Don't use spaces around the '=' sign when used to indicate a
|
||
keyword argument or a default parameter value.
|
||
</p>
|
||
<DIV class=""><PRE>Yes: <span class="external"></span>def complex(real, imag=0.0): return magic(r=real, i=imag)</PRE></DIV>
|
||
<DIV class=""><PRE class="badcode">No: <span class="external"></span>def complex(real, imag = 0.0): return magic(r = real, i = imag)</PRE></DIV>
|
||
|
||
<p>
|
||
Don't use spaces to vertically align tokens on consecutive lines, since it
|
||
becomes a maintenance burden (applies to <code>:</code>, <code>#</code>,
|
||
<code>=</code>, etc.):
|
||
</p>
|
||
<DIV class=""><PRE>Yes:
|
||
foo = 1000 # comment
|
||
long_name = 2 # comment that should not be aligned
|
||
|
||
dictionary = {
|
||
'foo': 1,
|
||
'long_name': 2,
|
||
}</PRE></DIV>
|
||
<DIV class=""><PRE class="badcode">No:
|
||
foo = 1000 # comment
|
||
long_name = 2 # comment that should not be aligned
|
||
|
||
dictionary = {
|
||
'foo' : 1,
|
||
'long_name': 2,
|
||
}</PRE></DIV>
|
||
|
||
|
||
</DIV></DIV>
|
||
</DIV>
|
||
|
||
<a name="Python_Interpreter"></a>
|
||
<DIV class="">
|
||
<H3><A name="Shebang_Line" id="Shebang_Line">Shebang Line</A></H3>
|
||
<SPAN class="link_button" id="link-Shebang_Line__button" name="link-Shebang_Line__button"><A href="?showone=Shebang_Line#Shebang_Line">
|
||
link
|
||
</A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Shebang_Line')" name="Shebang_Line__button" id="Shebang_Line__button">▶</SPAN>
|
||
<DIV style="display:inline;" class="">
|
||
Most <code>.py</code> files do not need to start with a
|
||
<code>#!</code> line. Start the main file of a
|
||
program with
|
||
<code>#!/usr/bin/env python</code> with an optional single digit
|
||
<code>2</code> or <code>3</code> suffix.
|
||
</DIV>
|
||
<DIV class=""><DIV class="stylepoint_body" name="Shebang_Line__body" id="Shebang_Line__body" style="display: none">
|
||
|
||
<p>
|
||
This line is used by the kernel to find the Python interpreter, but is
|
||
ignored by Python when importing modules. It is only necessary on a
|
||
file that will be executed directly.
|
||
</p>
|
||
</DIV></DIV>
|
||
</DIV>
|
||
|
||
<DIV class="">
|
||
<H3><A name="Comments" id="Comments">Comments</A></H3>
|
||
<SPAN class="link_button" id="link-Comments__button" name="link-Comments__button"><A href="?showone=Comments#Comments">
|
||
link
|
||
</A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Comments')" name="Comments__button" id="Comments__button">▶</SPAN>
|
||
<DIV style="display:inline;" class="">
|
||
Be sure to use the right style for module, function, method and in-line
|
||
comments.
|
||
</DIV>
|
||
<DIV class=""><DIV class="stylepoint_body" name="Comments__body" id="Comments__body" style="display: none">
|
||
|
||
<P class="">
|
||
<SPAN class="stylepoint_subsection">Doc Strings</SPAN>
|
||
|
||
<p>
|
||
Python has a unique commenting style using doc strings. A doc
|
||
string is a string that is the first statement in a package,
|
||
module, class or function. These strings can be extracted
|
||
automatically through the <code>__doc__</code> member of the
|
||
object and are used by <code>pydoc</code>. (Try running
|
||
<code>pydoc</code> on your module to see how it looks.) We
|
||
always use the three double-quote <code>"""</code> format for doc strings
|
||
(per <a href="https://www.python.org/dev/peps/pep-0257/">PEP 257</a>).
|
||
A doc string should be organized as a
|
||
summary line (one physical line) terminated by a period,
|
||
question mark, or exclamation point, followed by a blank line,
|
||
followed by the rest of the doc string starting at the same
|
||
cursor position as the first quote of the first line. There are
|
||
more formatting guidelines for doc strings below.
|
||
</p>
|
||
|
||
</P>
|
||
<P class="">
|
||
<SPAN class="stylepoint_subsection">Modules</SPAN>
|
||
|
||
|
||
|
||
<p>
|
||
Every file should contain license boilerplate.
|
||
Choose the appropriate boilerplate for the license used by the project
|
||
(for example, Apache 2.0, BSD, LGPL, GPL)
|
||
</p>
|
||
</P>
|
||
<P class="">
|
||
<SPAN class="stylepoint_subsection">Functions and Methods</SPAN>
|
||
|
||
<p>
|
||
As used in this section "function" applies to methods, function, and
|
||
generators.
|
||
</p>
|
||
|
||
<p>
|
||
A function must have a docstring, unless it meets all of the following
|
||
criteria:
|
||
<ul>
|
||
<li>not externally visible</li>
|
||
<li>very short</li>
|
||
<li>obvious</li>
|
||
</ul>
|
||
</p>
|
||
|
||
<p>
|
||
A docstring should give enough information to write a call to the function
|
||
without reading the function's code. A docstring should describe the
|
||
function's calling syntax and its semantics, not its implementation. For
|
||
tricky code, comments alongside the code are more appropriate than using
|
||
docstrings.
|
||
</p>
|
||
|
||
<p>
|
||
Certain aspects of a function should be documented in special sections,
|
||
listed below. Each section begins with a heading line, which ends with a
|
||
colon. Sections should be indented two spaces, except for the heading.
|
||
</p>
|
||
|
||
<dl>
|
||
<dt>Args:</dt>
|
||
<dd>
|
||
List each parameter by name. A description should follow the name, and
|
||
be separated by a colon and a space. If the description is too long to
|
||
fit on a single 80-character line, use a hanging indent of 2 or 4 spaces
|
||
(be consistent with the rest of the file).
|
||
|
||
<p>
|
||
The description should mention required type(s) and the meaning of
|
||
the argument.
|
||
</p>
|
||
|
||
<p>
|
||
If a function accepts *foo (variable length argument lists) and/or
|
||
**bar (arbitrary keyword arguments), they should be listed as *foo and
|
||
**bar.
|
||
</p>
|
||
</dd>
|
||
|
||
<dt>Returns: (or Yields: for generators)</dt>
|
||
<dd>
|
||
Describe the type and semantics of the return value. If the function
|
||
only returns None, this section is not required.
|
||
</dd>
|
||
|
||
<dt>Raises:</dt>
|
||
<dd>
|
||
List all exceptions that are relevant to the interface.
|
||
</dd>
|
||
</dl>
|
||
|
||
<DIV class=""><PRE>
|
||
<span class="external"></span>def fetch_bigtable_rows(big_table, keys, other_silly_variable=None):
|
||
<span class="external"> </span>"""Fetches rows from a Bigtable.
|
||
|
||
<span class="external"> </span>Retrieves rows pertaining to the given keys from the Table instance
|
||
<span class="external"> </span>represented by big_table. Silly things may happen if
|
||
<span class="external"> </span>other_silly_variable is not None.
|
||
|
||
<span class="external"> </span>Args:
|
||
<span class="external"> </span>big_table: An open Bigtable Table instance.
|
||
<span class="external"> </span>keys: A sequence of strings representing the key of each table row
|
||
<span class="external"> </span> to fetch.
|
||
<span class="external"> </span>other_silly_variable: Another optional variable, that has a much
|
||
<span class="external"> </span> longer name than the other args, and which does nothing.
|
||
|
||
<span class="external"> </span>Returns:
|
||
<span class="external"> </span>A dict mapping keys to the corresponding table row data
|
||
<span class="external"> </span>fetched. Each row is represented as a tuple of strings. For
|
||
<span class="external"> </span>example:
|
||
|
||
<span class="external"> </span>{'Serak': ('Rigel VII', 'Preparer'),
|
||
<span class="external"> </span> 'Zim': ('Irk', 'Invader'),
|
||
<span class="external"> </span> 'Lrrr': ('Omicron Persei 8', 'Emperor')}
|
||
|
||
<span class="external"> </span>If a key from the keys argument is missing from the dictionary,
|
||
<span class="external"> </span>then that row was not found in the table.
|
||
|
||
<span class="external"> </span>Raises:
|
||
<span class="external"> </span>IOError: An error occurred accessing the bigtable.Table object.
|
||
<span class="external"> </span>"""
|
||
<span class="external"> </span>pass
|
||
<span class="external"></span>
|
||
</PRE></DIV>
|
||
</P>
|
||
<P class="">
|
||
<SPAN class="stylepoint_subsection">Classes</SPAN>
|
||
|
||
<p>
|
||
Classes should have a doc string below the class definition describing
|
||
the class. If your class has public attributes, they should be documented
|
||
here in an Attributes section and follow the same formatting as a
|
||
function's Args section.
|
||
</p>
|
||
|
||
<DIV class=""><PRE>
|
||
<span class="external"></span>class SampleClass(object):
|
||
<span class="external"> </span>"""Summary of class here.
|
||
|
||
<span class="external"> </span>Longer class information....
|
||
<span class="external"> </span>Longer class information....
|
||
|
||
<span class="external"> </span>Attributes:
|
||
<span class="external"> </span>likes_spam: A boolean indicating if we like SPAM or not.
|
||
<span class="external"> </span>eggs: An integer count of the eggs we have laid.
|
||
<span class="external"> </span>"""
|
||
|
||
<span class="external"> </span>def __init__(self, likes_spam=False):
|
||
<span class="external"> </span>"""Inits SampleClass with blah."""
|
||
<span class="external"> </span>self.likes_spam = likes_spam
|
||
<span class="external"> </span>self.eggs = 0
|
||
|
||
<span class="external"> </span>def public_method(self):
|
||
<span class="external"> </span>"""Performs operation blah."""
|
||
<span class="external"></span>
|
||
</PRE></DIV>
|
||
|
||
</P>
|
||
<P class="">
|
||
<SPAN class="stylepoint_subsection">Block and Inline Comments</SPAN>
|
||
|
||
<p>
|
||
The final place to have comments is in tricky parts of the
|
||
code. If you're going to have to explain it at the next
|
||
<a HREF="https://en.wikipedia.org/wiki/Code_review">code review</a>,
|
||
you should comment it now. Complicated operations get a few lines of
|
||
comments before the operations
|
||
commence. Non-obvious ones get comments at the end of the line.
|
||
</p>
|
||
|
||
<DIV class=""><PRE>
|
||
<span class="external"></span># We use a weighted dictionary search to find out where i is in
|
||
<span class="external"></span># the array. We extrapolate position based on the largest num
|
||
<span class="external"></span># in the array and the array size and then do binary search to
|
||
<span class="external"></span># get the exact number.
|
||
|
||
<span class="external"></span>if i & (i-1) == 0: # true iff i is a power of 2
|
||
<span class="external"></span>
|
||
</PRE></DIV>
|
||
|
||
<p>
|
||
To improve legibility, these comments should be at least 2 spaces away
|
||
from the code.
|
||
</p>
|
||
|
||
<p>
|
||
On the other hand, never describe the code. Assume the person
|
||
reading the code knows Python (though not what you're trying to
|
||
do) better than you do.
|
||
</p>
|
||
|
||
<DIV class=""><PRE class="badcode">
|
||
<span class="external"></span># BAD COMMENT: Now go through the b array and make sure whenever i occurs
|
||
<span class="external"></span># the next element is i+1
|
||
<span class="external"></span>
|
||
</PRE></DIV>
|
||
|
||
</P>
|
||
</DIV></DIV>
|
||
</DIV>
|
||
<DIV class="">
|
||
<H3><A name="Classes" id="Classes">Classes</A></H3>
|
||
<SPAN class="link_button" id="link-Classes__button" name="link-Classes__button"><A href="?showone=Classes#Classes">
|
||
link
|
||
</A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Classes')" name="Classes__button" id="Classes__button">▶</SPAN>
|
||
<DIV style="display:inline;" class="">
|
||
If a class inherits from no other base classes, explicitly inherit
|
||
from <code>object</code>. This also applies to nested classes.
|
||
</DIV>
|
||
<DIV class=""><DIV class="stylepoint_body" name="Classes__body" id="Classes__body" style="display: none">
|
||
<DIV class=""><PRE>Yes: <span class="external"></span>class SampleClass(object):
|
||
<span class="external"> </span>pass
|
||
|
||
|
||
<span class="external"></span>class OuterClass(object):
|
||
|
||
<span class="external"> </span>class InnerClass(object):
|
||
<span class="external"> </span>pass
|
||
|
||
|
||
<span class="external"></span>class ChildClass(ParentClass):
|
||
<span class="external"> </span>"""Explicitly inherits from another class already."""
|
||
<span class="external"></span>
|
||
</PRE></DIV>
|
||
<DIV class=""><PRE class="badcode">No: <span class="external"></span>class SampleClass:
|
||
<span class="external"> </span>pass
|
||
|
||
|
||
<span class="external"></span>class OuterClass:
|
||
|
||
<span class="external"> </span>class InnerClass:
|
||
<span class="external"> </span>pass
|
||
<span class="external"></span>
|
||
</PRE></DIV>
|
||
<p>Inheriting from <code>object</code> is needed to make properties work
|
||
properly, and it will protect your code from one particular potential
|
||
incompatibility with Python 3000. It also defines
|
||
special methods that implement the default semantics of objects including
|
||
<code>__new__</code>, <code>__init__</code>, <code>__delattr__</code>,
|
||
<code>__getattribute__</code>, <code>__setattr__</code>,
|
||
<code>__hash__</code>, <code>__repr__</code>, and <code>__str__</code>.
|
||
</p>
|
||
</DIV></DIV>
|
||
</DIV>
|
||
<DIV class="">
|
||
<H3><A name="Strings" id="Strings">Strings</A></H3>
|
||
<SPAN class="link_button" id="link-Strings__button" name="link-Strings__button"><A href="?showone=Strings#Strings">
|
||
link
|
||
</A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Strings')" name="Strings__button" id="Strings__button">▶</SPAN>
|
||
<DIV style="display:inline;" class="">
|
||
Use the <code>format</code> method or the <code>%</code> operator for
|
||
formatting strings, even when the parameters are all strings. Use your
|
||
best judgement to decide between <code>+</code> and <code>%</code>
|
||
(or <code>format</code>) though.
|
||
</DIV>
|
||
<DIV class=""><DIV class="stylepoint_body" name="Strings__body" id="Strings__body" style="display: none">
|
||
|
||
<DIV class=""><PRE>Yes: <span class="external"></span>x = a + b
|
||
<span class="external"></span>x = '%s, %s!' % (imperative, expletive)
|
||
<span class="external"></span>x = '{}, {}!'.format(imperative, expletive)
|
||
<span class="external"></span>x = 'name: %s; score: %d' % (name, n)
|
||
<span class="external"></span>x = 'name: {}; score: {}'.format(name, n)</PRE></DIV>
|
||
<DIV class=""><PRE class="badcode">No: <span class="external"></span>x = '%s%s' % (a, b) # use + in this case
|
||
<span class="external"></span>x = '{}{}'.format(a, b) # use + in this case
|
||
<span class="external"></span>x = imperative + ', ' + expletive + '!'
|
||
<span class="external"></span>x = 'name: ' + name + '; score: ' + str(n)</PRE></DIV>
|
||
|
||
<p>
|
||
Avoid using the <code>+</code> and <code>+=</code> operators to
|
||
accumulate a string within a loop. Since strings are immutable, this
|
||
creates unnecessary temporary objects and results in quadratic rather
|
||
than linear running time. Instead, add each substring to a list
|
||
and <code>''.join</code> the list after the loop terminates (or, write
|
||
each substring to a <code>io.BytesIO</code> buffer).
|
||
</p>
|
||
|
||
<DIV class=""><PRE>Yes: <span class="external"></span>items = ['<table>']
|
||
<span class="external"></span>for last_name, first_name in employee_list:
|
||
<span class="external"> </span>items.append('<tr><td>%s, %s</td></tr>' % (last_name, first_name))
|
||
<span class="external"></span>items.append('</table>')
|
||
<span class="external"></span>employee_table = ''.join(items)</PRE></DIV>
|
||
<DIV class=""><PRE class="badcode">No: <span class="external"></span>employee_table = '<table>'
|
||
<span class="external"></span>for last_name, first_name in employee_list:
|
||
<span class="external"> </span>employee_table += '<tr><td>%s, %s</td></tr>' % (last_name, first_name)
|
||
<span class="external"></span>employee_table += '</table>'</PRE></DIV>
|
||
|
||
<p>
|
||
Be consistent with your choice of string quote character within a file.
|
||
Pick <code>'</code> or <code>"</code> and stick with it.
|
||
It is okay to use the other quote character on a string to avoid the
|
||
need to <code>\</code> escape within the string.
|
||
GPyLint enforces this.
|
||
</p>
|
||
|
||
<DIV class=""><PRE>Ye<span class="external"></span>s:
|
||
<span class="external"></span>Python('Why are you hiding your eyes?')
|
||
<span class="external"></span>Gollum("I'm scared of lint errors.")
|
||
<span class="external"></span>Narrator('"Good!" thought a happy Python reviewer.')</PRE></DIV>
|
||
<DIV class=""><PRE class="badcode">No<span class="external"></span>:
|
||
<span class="external"></span>Python("Why are you hiding your eyes?")
|
||
<span class="external"></span>Gollum('The lint. It burns. It burns us.')
|
||
<span class="external"></span>Gollum("Always the great lint. Watching. Watching.")</PRE></DIV>
|
||
|
||
<p>
|
||
Prefer <code>"""</code> for multi-line strings rather than
|
||
<code>'''</code>. Projects may choose to use <code>'''</code> for
|
||
all non-docstring multi-line strings if and only if they also use
|
||
<code>'</code> for regular strings.
|
||
Doc strings must use <code>"""</code> regardless.
|
||
Note that it is often cleaner to
|
||
use implicit line joining since multi-line strings do
|
||
not flow with the indentation of the rest of the program:
|
||
</p>
|
||
|
||
<DIV class=""><PRE>Ye<span class="external"></span>s:
|
||
<span class="external"></span>print ("This is much nicer.\n"
|
||
<span class="external"></span> "Do it this way.\n")</PRE></DIV>
|
||
<DIV class=""><PRE class="badcode"> No<span class="external"></span>:
|
||
<span class="external"></span>print """This is pretty ugly.
|
||
Don'<span class="external"></span>t do this.
|
||
"""<span class="external"></span>
|
||
</PRE></DIV>
|
||
|
||
</DIV></DIV>
|
||
</DIV>
|
||
<DIV class="">
|
||
<H3><A name="Files_and_Sockets" id="Files_and_Sockets">Files and Sockets</A></H3>
|
||
<SPAN class="link_button" id="link-Files_and_Sockets__button" name="link-Files_and_Sockets__button"><A href="?showone=Files_and_Sockets#Files_and_Sockets">
|
||
link
|
||
</A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Files_and_Sockets')" name="Files_and_Sockets__button" id="Files_and_Sockets__button">▶</SPAN>
|
||
<DIV style="display:inline;" class="">
|
||
Explicitly close files and sockets when done with them.
|
||
</DIV>
|
||
<DIV class=""><DIV class="stylepoint_body" name="Files_and_Sockets__body" id="Files_and_Sockets__body" style="display: none">
|
||
<p>
|
||
Leaving files, sockets or other file-like objects open unnecessarily
|
||
has many downsides, including:
|
||
|
||
<ul>
|
||
<li>They may consume limited system resources, such as file
|
||
descriptors. Code that deals with many such objects may exhaust
|
||
those resources unnecessarily if they're not returned to the
|
||
system promptly after use.</li>
|
||
<li>Holding files open may prevent other actions being performed on
|
||
them, such as moves or deletion.</li>
|
||
<li>Files and sockets that are shared throughout a program may
|
||
inadvertantly be read from or written to after logically being
|
||
closed. If they are actually closed, attempts to read or write
|
||
from them will throw exceptions, making the problem known
|
||
sooner.</li>
|
||
</ul>
|
||
</p>
|
||
|
||
<p>
|
||
Furthermore, while files and sockets are automatically closed when the
|
||
file object is destructed, tying the life-time of the file object to
|
||
the state of the file is poor practice, for several reasons:
|
||
|
||
<ul>
|
||
<li>There are no guarantees as to when the runtime will actually run
|
||
the file's destructor. Different Python implementations use
|
||
different memory management techniques, such as delayed Garbage
|
||
Collection, which may increase the object's lifetime arbitrarily
|
||
and indefinitely.</li>
|
||
<li>Unexpected references to the file may keep it around longer than
|
||
intended (e.g. in tracebacks of exceptions, inside globals,
|
||
etc).</li>
|
||
</ul>
|
||
</p>
|
||
|
||
<p>
|
||
The preferred way to manage files is using the <a HREF="https://docs.python.org/reference/compound_stmts.html#the-with-statement">
|
||
"with" statement</a>:
|
||
</p>
|
||
|
||
<DIV class=""><PRE>
|
||
<span class="external"></span>with open("hello.txt") as hello_file:
|
||
<span class="external"> </span>for line in hello_file:
|
||
<span class="external"> </span>print line</PRE></DIV>
|
||
|
||
<p>
|
||
For file-like objects that do not support the "with" statement, use
|
||
contextlib.closing():
|
||
</p>
|
||
|
||
<DIV class=""><PRE>
|
||
<span class="external"></span>import contextlib
|
||
|
||
<span class="external"></span>with contextlib.closing(urllib.urlopen("https://www.python.org/")) as front_page:
|
||
<span class="external"> </span>for line in front_page:
|
||
<span class="external"> </span>print line</PRE></DIV>
|
||
|
||
<p>
|
||
Legacy AppEngine code using Python 2.5 may enable the "with" statement
|
||
using "from __future__ import with_statement".
|
||
</p>
|
||
</DIV></DIV>
|
||
</DIV>
|
||
<DIV class="">
|
||
<H3><A name="TODO_Comments" id="TODO_Comments">TODO Comments</A></H3>
|
||
<SPAN class="link_button" id="link-TODO_Comments__button" name="link-TODO_Comments__button"><A href="?showone=TODO_Comments#TODO_Comments">
|
||
link
|
||
</A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('TODO_Comments')" name="TODO_Comments__button" id="TODO_Comments__button">▶</SPAN>
|
||
<DIV style="display:inline;" class="">
|
||
Use <code>TODO</code> comments for code that is temporary, a
|
||
short-term solution, or good-enough but not perfect.
|
||
</DIV>
|
||
<DIV class=""><DIV class="stylepoint_body" name="TODO_Comments__body" id="TODO_Comments__body" style="display: none">
|
||
<p>
|
||
<code>TODO</code>s should include the string <code>TODO</code> in
|
||
all caps, followed by the
|
||
|
||
name, e-mail address, or other
|
||
identifier
|
||
of the person who can best provide context about the problem
|
||
referenced by the <code>TODO</code>,
|
||
in parentheses. A colon is optional. A comment explaining what there
|
||
is to do is required. The main purpose is to have
|
||
a consistent <code>TODO</code> format that can be searched to find the
|
||
person who can provide more details upon request. A
|
||
<code>TODO</code> is not a commitment that the person referenced
|
||
will fix the problem. Thus when you create a <code>TODO</code>, it is
|
||
almost always your
|
||
|
||
name
|
||
that is given.
|
||
</p>
|
||
|
||
<DIV class=""><PRE># TODO(kl@gmail.com): Use a "*" here for string repetition.
|
||
# TODO(Zeke) Change this to use relations.</PRE></DIV>
|
||
<p>
|
||
If your <code>TODO</code> is of the form "At a future date do
|
||
something" make sure that you either include a very specific
|
||
date ("Fix by November 2009") or a very specific event
|
||
("Remove this code when all clients can handle XML responses.").
|
||
</p>
|
||
</DIV></DIV>
|
||
</DIV>
|
||
<DIV class="">
|
||
<H3><A name="Imports_formatting" id="Imports_formatting">Imports formatting</A></H3>
|
||
<SPAN class="link_button" id="link-Imports_formatting__button" name="link-Imports_formatting__button"><A href="?showone=Imports_formatting#Imports_formatting">
|
||
link
|
||
</A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Imports_formatting')" name="Imports_formatting__button" id="Imports_formatting__button">▶</SPAN>
|
||
<DIV style="display:inline;" class="">
|
||
Imports should be on separate lines.
|
||
</DIV>
|
||
<DIV class=""><DIV class="stylepoint_body" name="Imports_formatting__body" id="Imports_formatting__body" style="display: none">
|
||
<p>
|
||
E.g.:
|
||
</p>
|
||
|
||
<DIV class=""><PRE>Yes: <span class="external"></span>import os
|
||
<span class="external"></span>import sys</PRE></DIV>
|
||
<DIV class=""><PRE class="badcode">No: <span class="external"></span>import os, sys</PRE></DIV>
|
||
<p>
|
||
Imports are always put at the top of the file, just after any
|
||
module comments and doc strings and before module globals and
|
||
constants. Imports should be grouped with the order being most generic
|
||
to least generic:
|
||
</p>
|
||
<ul>
|
||
<li>standard library imports</li>
|
||
<li>third-party imports</li>
|
||
|
||
<li>application-specific imports</li>
|
||
</ul>
|
||
<p>
|
||
Within each grouping, imports should be sorted lexicographically,
|
||
ignoring case, according to each module's full package path.
|
||
</p>
|
||
<DIV class=""><PRE>
|
||
<span class="external"></span>import foo
|
||
<span class="external"></span>from foo import bar
|
||
<span class="external"></span>from foo.bar import baz
|
||
<span class="external"></span>from foo.bar import Quux
|
||
<span class="external"></span>from Foob import ar</PRE></DIV>
|
||
|
||
|
||
|
||
</DIV></DIV>
|
||
</DIV>
|
||
<DIV class="">
|
||
<H3><A name="Statements" id="Statements">Statements</A></H3>
|
||
<SPAN class="link_button" id="link-Statements__button" name="link-Statements__button"><A href="?showone=Statements#Statements">
|
||
link
|
||
</A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Statements')" name="Statements__button" id="Statements__button">▶</SPAN>
|
||
<DIV style="display:inline;" class="">
|
||
Generally only one statement per line.
|
||
</DIV>
|
||
<DIV class=""><DIV class="stylepoint_body" name="Statements__body" id="Statements__body" style="display: none">
|
||
<p>
|
||
However, you may put the
|
||
result of a test on the same line as the test only if the entire
|
||
statement fits on one line. In particular, you can never do so
|
||
with <code>try</code>/<code>except</code> since the
|
||
<code>try</code> and <code>except</code> can't both fit on the
|
||
same line, and you can only do so with an <code>if</code> if
|
||
there is no <code>else</code>.
|
||
</p>
|
||
|
||
<DIV class=""><PRE>Ye<span class="external"></span>s:
|
||
|
||
<span class="external"></span>if foo: bar(foo)</PRE></DIV>
|
||
<DIV class=""><PRE class="badcode">No<span class="external"></span>:
|
||
|
||
<span class="external"></span>if foo: bar(foo)
|
||
<span class="external"></span>else: baz(foo)
|
||
|
||
<span class="external"></span>try: bar(foo)
|
||
<span class="external"></span>except ValueError: baz(foo)
|
||
|
||
<span class="external"></span>try:
|
||
<span class="external"> </span>bar(foo)
|
||
<span class="external"></span>except ValueError: baz(foo)
|
||
<span class="external"></span>
|
||
</PRE></DIV>
|
||
</DIV></DIV>
|
||
</DIV>
|
||
<DIV class="">
|
||
<H3><A name="Access_Control" id="Access_Control">Access Control</A></H3>
|
||
<SPAN class="link_button" id="link-Access_Control__button" name="link-Access_Control__button"><A href="?showone=Access_Control#Access_Control">
|
||
link
|
||
</A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Access_Control')" name="Access_Control__button" id="Access_Control__button">▶</SPAN>
|
||
<DIV style="display:inline;" class="">
|
||
If an accessor function would be trivial you should use public variables
|
||
instead of accessor functions to avoid the extra cost of function
|
||
calls in Python. When more functionality is added you can use
|
||
<code>property</code> to keep the syntax consistent.
|
||
</DIV>
|
||
<DIV class=""><DIV class="stylepoint_body" name="Access_Control__body" id="Access_Control__body" style="display: none">
|
||
<p>
|
||
On the other hand, if access is more complex, or the cost of accessing
|
||
the variable is significant, you should use function calls (following the
|
||
<a HREF="#naming">Naming</a> guidelines) such as <code>get_foo()</code>
|
||
and <code>set_foo()</code>. If the past behavior allowed access through a
|
||
property, do not bind the new accessor functions to the property. Any
|
||
code still attempting to access the variable by the old method should
|
||
break visibly so they are made aware of the change in complexity.
|
||
</p>
|
||
</DIV></DIV>
|
||
</DIV>
|
||
<DIV class="">
|
||
<H3><A name="Naming" id="Naming">Naming</A></H3>
|
||
<SPAN class="link_button" id="link-Naming__button" name="link-Naming__button"><A href="?showone=Naming#Naming">
|
||
link
|
||
</A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Naming')" name="Naming__button" id="Naming__button">▶</SPAN>
|
||
<DIV style="display:inline;" class="">
|
||
<code>module_name, package_name, ClassName,
|
||
method_name, ExceptionName,
|
||
function_name, GLOBAL_CONSTANT_NAME,
|
||
global_var_name, instance_var_name, function_parameter_name,
|
||
local_var_name.</code>
|
||
</DIV>
|
||
<DIV class=""><DIV class="stylepoint_body" name="Naming__body" id="Naming__body" style="display: none">
|
||
<P class="">
|
||
<SPAN class="stylepoint_subsection">Names to Avoid</SPAN>
|
||
|
||
<ul>
|
||
<li>single character names except for counters or iterators</li>
|
||
<li>dashes (<code>-</code>) in any package/module name</li>
|
||
<li>
|
||
<code>__double_leading_and_trailing_underscore__</code> names
|
||
(reserved by Python)</li>
|
||
</ul>
|
||
|
||
</P>
|
||
<P class="">
|
||
<SPAN class="stylepoint_subsection">Naming Convention</SPAN>
|
||
|
||
<ul>
|
||
<li>
|
||
"Internal" means internal to a module or protected
|
||
or private within a class.</li>
|
||
<li>
|
||
Prepending a single underscore (<code>_</code>) has some
|
||
support for protecting module variables and functions (not included
|
||
with <code>import * from</code>). Prepending a double underscore
|
||
(<code>__</code>) to an instance variable or method
|
||
effectively serves to make the variable or method private to its class
|
||
(using name mangling).</li>
|
||
<li>
|
||
Place related classes and top-level functions together in a
|
||
module. Unlike Java,
|
||
there is no need to limit yourself to one class per module.</li>
|
||
<li>
|
||
Use CapWords for class names, but lower_with_under.py for module names.
|
||
Although there are many existing modules named CapWords.py, this is now
|
||
discouraged because it's confusing when the module happens to be
|
||
named after a class. ("wait -- did I write
|
||
<code>import StringIO</code> or <code>from StringIO import
|
||
StringIO</code>?")</li>
|
||
</ul>
|
||
|
||
</P>
|
||
<P class="">
|
||
<SPAN class="stylepoint_subsection">Guidelines derived from Guido's Recommendations</SPAN>
|
||
|
||
<table rules="all" border="1" cellspacing="2" cellpadding="2">
|
||
|
||
<tr>
|
||
<th>Type</th>
|
||
<th>Public</th>
|
||
<th>Internal</th>
|
||
</tr>
|
||
|
||
|
||
|
||
<tr>
|
||
<td>Packages</td>
|
||
<td><code>lower_with_under</code></td>
|
||
<td></td>
|
||
</tr>
|
||
|
||
<tr>
|
||
<td>Modules</td>
|
||
<td><code>lower_with_under</code></td>
|
||
<td><code>_lower_with_under</code></td>
|
||
</tr>
|
||
|
||
<tr>
|
||
<td>Classes</td>
|
||
<td><code>CapWords</code></td>
|
||
<td><code>_CapWords</code></td>
|
||
</tr>
|
||
|
||
<tr>
|
||
<td>Exceptions</td>
|
||
<td><code>CapWords</code></td>
|
||
<td></td>
|
||
</tr>
|
||
|
||
|
||
|
||
<tr>
|
||
<td>Functions</td>
|
||
<td><code>lower_with_under()</code></td>
|
||
<td><code>_lower_with_under()</code></td>
|
||
</tr>
|
||
|
||
<tr>
|
||
<td>Global/Class Constants</td>
|
||
<td><code>CAPS_WITH_UNDER</code></td>
|
||
<td><code>_CAPS_WITH_UNDER</code></td>
|
||
</tr>
|
||
|
||
<tr>
|
||
<td>Global/Class Variables</td>
|
||
<td><code>lower_with_under</code></td>
|
||
<td><code>_lower_with_under</code></td>
|
||
</tr>
|
||
|
||
<tr>
|
||
<td>Instance Variables</td>
|
||
<td><code>lower_with_under</code></td>
|
||
<td><code>_lower_with_under (protected) or __lower_with_under (private)</code></td>
|
||
</tr>
|
||
|
||
|
||
|
||
<tr>
|
||
<td>Method Names</td>
|
||
<td><code>lower_with_under()</code></td>
|
||
<td><code>_lower_with_under() (protected) or __lower_with_under() (private)</code></td>
|
||
</tr>
|
||
|
||
<tr>
|
||
<td>Function/Method Parameters</td>
|
||
<td><code>lower_with_under</code></td>
|
||
<td></td>
|
||
</tr>
|
||
|
||
<tr>
|
||
<td>Local Variables</td>
|
||
<td><code>lower_with_under</code></td>
|
||
<td></td>
|
||
</tr>
|
||
|
||
|
||
</table>
|
||
|
||
|
||
</P>
|
||
</DIV></DIV>
|
||
</DIV>
|
||
<DIV class="">
|
||
<H3><A name="Main" id="Main">Main</A></H3>
|
||
<SPAN class="link_button" id="link-Main__button" name="link-Main__button"><A href="?showone=Main#Main">
|
||
link
|
||
</A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Main')" name="Main__button" id="Main__button">▶</SPAN>
|
||
<DIV style="display:inline;" class="">
|
||
Even a file meant to be used as a script should be importable and a
|
||
mere import should not have the side effect of executing the script's
|
||
main functionality. The main functionality should be in a main()
|
||
function.
|
||
</DIV>
|
||
<DIV class=""><DIV class="stylepoint_body" name="Main__body" id="Main__body" style="display: none">
|
||
<p>
|
||
In Python,
|
||
<code>pydoc</code> as well as unit tests
|
||
require modules to be importable. Your code should always check
|
||
<code>if __name__ == '__main__'</code> before executing your
|
||
main program so that the main program is not executed when the
|
||
module is imported.
|
||
|
||
</p>
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
<DIV class=""><PRE>
|
||
<span class="external"></span>def main():
|
||
<span class="external"> </span>...
|
||
|
||
<span class="external"></span>if __name__ == '__main__':
|
||
<span class="external"> </span>main()
|
||
<span class="external"></span>
|
||
</PRE></DIV>
|
||
|
||
<p>
|
||
All code at the top level will be executed when the module is
|
||
imported. Be careful not to call functions, create objects, or
|
||
perform other operations that should not be executed when the
|
||
file is being <code>pydoc</code>ed.
|
||
</p>
|
||
</DIV></DIV>
|
||
</DIV>
|
||
</DIV>
|
||
|
||
<H2>Parting Words</H2>
|
||
<p>
|
||
<em>BE CONSISTENT</em>.
|
||
</p>
|
||
|
||
<p>
|
||
If you're editing code, take a few minutes to look at the code
|
||
around you and determine its style. If they use spaces around
|
||
all their arithmetic operators, you should too. If their
|
||
comments have little boxes of hash marks around them, make your
|
||
comments have little boxes of hash marks around them too.
|
||
</p>
|
||
|
||
<p>
|
||
The point of having style guidelines is to have a common vocabulary
|
||
of coding so people can concentrate on what you're saying rather
|
||
than on how you're saying it. We present global style rules here so
|
||
people know the vocabulary, but local style is also important. If
|
||
code you add to a file looks drastically different from the existing
|
||
code around it, it throws readers out of their rhythm when they go to
|
||
read it. Avoid this.
|
||
</p>
|
||
|
||
|
||
|
||
<p align="right">
|
||
Revision 2.59
|
||
</p>
|
||
|
||
|
||
<address>
|
||
Amit Patel<br>
|
||
Antoine Picard<br>
|
||
Eugene Jhong<br>
|
||
Gregory P. Smith<br>
|
||
Jeremy Hylton<br>
|
||
Matt Smart<br>
|
||
Mike Shields<br>
|
||
Shane Liebling<br>
|
||
</address>
|
||
</BODY>
|
||
</HTML>
|