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Updating python style guide
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pyguide.html
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@ -136,7 +136,7 @@
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<H1>Google Python Style Guide</H1>
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<p align="right">
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Revision 2.29
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Revision 2.39
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</p>
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<address>
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|
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@ -165,12 +165,12 @@
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<TR valign="top" class="">
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<TD><DIV class="toc_category"><A href="#Python_Language_Rules">Python Language Rules</A></DIV></TD>
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<TD><DIV class="toc_stylepoint">
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<SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#pychecker">pychecker</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Imports">Imports</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Packages">Packages</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Exceptions">Exceptions</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Global_variables">Global variables</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Nested/Local/Inner_Classes_and_Functions">Nested/Local/Inner Classes and Functions</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#List_Comprehensions">List Comprehensions</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Default_Iterators_and_Operators">Default Iterators and Operators</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Generators">Generators</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Lambda_Functions">Lambda Functions</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Default_Argument_Values">Default Argument Values</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Properties">Properties</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#True/False_evaluations">True/False evaluations</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Deprecated_Language_Features">Deprecated Language Features</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Lexical_Scoping">Lexical Scoping</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Function_and_Method_Decorators">Function and Method Decorators</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Threading">Threading</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Power_Features">Power Features</A></SPAN> </DIV></TD>
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<SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#pychecker">pychecker</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Imports">Imports</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Packages">Packages</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Exceptions">Exceptions</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Global_variables">Global variables</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Nested/Local/Inner_Classes_and_Functions">Nested/Local/Inner Classes and Functions</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#List_Comprehensions">List Comprehensions</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Default_Iterators_and_Operators">Default Iterators and Operators</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Generators">Generators</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Lambda_Functions">Lambda Functions</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#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>
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</TR>
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<TR valign="top" class="">
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<TD><DIV class="toc_category"><A href="#Python_Style_Rules">Python Style Rules</A></DIV></TD>
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<TD><DIV class="toc_stylepoint">
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<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="#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>
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<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>
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</TR>
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</TABLE>
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</DIV>
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@ -276,7 +276,7 @@
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</p>
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<DIV class=""><PRE>
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<span class="external"></span>def foo(a, unused_b, unused_c, d=None, e=None):
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<span class="external"> </span>(d, e) = (d, e) # Silence pychecker
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<span class="external"> </span>_ = d, e
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<span class="external"> </span>return a
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<span class="external"></span>
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</PRE></DIV>
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@ -406,11 +406,11 @@ from sound.effects import echo
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Exceptions must follow certain conditions:
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<ul>
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<li>Raise exceptions like this: <code>raise MyException("Error
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message")</code> or <code>raise MyException</code>. Do not
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use the two-argument form (<code>raise MyException, "Error
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message"</code>) or deprecated string-based exceptions
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(<code>raise "Error message"</code>).</li>
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<li>Raise exceptions like this: <code>raise MyException('Error
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message')</code> or <code>raise MyException</code>. Do not
|
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use the two-argument form (<code>raise MyException, 'Error
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message'</code>) or deprecated string-based exceptions
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(<code>raise 'Error message'</code>).</li>
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<li>Modules or packages should define their own domain-specific
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base exception class, which should inherit from the built-in
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Exception class. The base exception for a module should be called
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@ -436,6 +436,14 @@ from sound.effects import echo
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<li>Use the <code>finally</code> clause to execute code whether
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or not an exception is raised in the <code>try</code> block.
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This is often useful for cleanup, i.e., closing a file.</li>
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<li>When capturing an exception, use <code>as</code> rather than
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a comma. For example:
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<DIV class=""><PRE>
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<span class="external"></span>try:
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<span class="external"> </span>raise Error
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<span class="external"></span>except Error as error:
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<span class="external"> </span>pass</PRE></DIV>
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</li>
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</ul>
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</P>
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</DIV></DIV>
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@ -547,15 +555,6 @@ from sound.effects import echo
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permitted. Use loops instead when things get more complicated.
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</P>
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<DIV class=""><PRE class="badcode">No<span class="external"></span>:
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<span class="external"></span>result = [(x, y) for x in range(10) for y in range(5) if x * y > 10]
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<span class="external"></span>return ((x, y, z)
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<span class="external"></span> for x in xrange(5)
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<span class="external"></span> for y in xrange(5)
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<span class="external"></span> if x != y
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<span class="external"></span> for z in xrange(5)
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<span class="external"></span> if y != z)</PRE></DIV>
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<DIV class=""><PRE>Ye<span class="external"></span>s:
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<span class="external"></span>result = []
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<span class="external"></span>for x in range(10):
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@ -578,6 +577,15 @@ from sound.effects import echo
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<span class="external"></span>eat(jelly_bean for jelly_bean in jelly_beans
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<span class="external"></span> if jelly_bean.color == 'black')</PRE></DIV>
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<DIV class=""><PRE class="badcode">No<span class="external"></span>:
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<span class="external"></span>result = [(x, y) for x in range(10) for y in range(5) if x * y > 10]
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<span class="external"></span>return ((x, y, z)
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<span class="external"></span> for x in xrange(5)
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<span class="external"></span> for y in xrange(5)
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<span class="external"></span> if x != y
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<span class="external"></span> for z in xrange(5)
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<span class="external"></span> if y != z)</PRE></DIV>
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</DIV></DIV>
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</DIV>
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<DIV class="">
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|
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@ -699,6 +707,37 @@ from sound.effects import echo
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</DIV></DIV>
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</DIV>
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<DIV class="">
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<H3><A name="Conditional_Expressions" id="Conditional_Expressions">Conditional Expressions</A></H3>
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<SPAN class="link_button" id="link-Conditional_Expressions__button" name="link-Conditional_Expressions__button"><A href="?showone=Conditional_Expressions#Conditional_Expressions">
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link
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</A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Conditional_Expressions')" name="Conditional_Expressions__button" id="Conditional_Expressions__button">▶</SPAN>
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<DIV style="display:inline;" class="">
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Okay for one-liners.
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</DIV>
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<DIV class=""><DIV class="stylepoint_body" name="Conditional_Expressions__body" id="Conditional_Expressions__body" style="display: none">
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<P class="">
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<SPAN class="stylepoint_section">Definition: </SPAN>
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Conditional expressions are mechanisms that provide a shorter syntax
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for if statements. For example:
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<code>x = 1 if cond else 2</code>.
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</P>
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<P class="">
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<SPAN class="stylepoint_section">Pros: </SPAN>
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Shorter and more convenient than an if statement.
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</P>
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<P class="">
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<SPAN class="stylepoint_section">Cons: </SPAN>
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May be harder to read than an if statement. The condition may be difficult
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to locate if the expression is long.
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</P>
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<P class="">
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<SPAN class="stylepoint_section">Decision: </SPAN>
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Okay to use for one-liners. In other cases prefer to use a complete if
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statement.
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</P>
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</DIV></DIV>
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</DIV>
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<DIV class="">
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<H3><A name="Default_Argument_Values" id="Default_Argument_Values">Default Argument Values</A></H3>
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||||
<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
|
||||
|
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@ -868,7 +907,7 @@ from sound.effects import echo
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|||
<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.
|
||||
''</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
|
||||
|
|
@ -963,16 +1002,16 @@ from sound.effects import echo
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|||
We do not use any Python version which does not support
|
||||
these features, so there is no reason not to use the new
|
||||
styles.
|
||||
<DIV class=""><PRE class="badcode">No: <span class="external"></span>words = string.split(foo, ':')
|
||||
|
||||
<span class="external"></span>map(lambda x: x[1], filter(lambda x: x[2] == 5, my_list))
|
||||
|
||||
<span class="external"></span>apply(fn, args, kwargs)</PRE></DIV>
|
||||
<DIV class=""><PRE>Yes: <span class="external"></span>words = foo.split(':')
|
||||
|
||||
<span class="external"></span>[x[1] for x in my_list if x[2] == 5]
|
||||
|
||||
<span class="external"></span>fn(*args, **kwargs)</PRE></DIV>
|
||||
<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>
|
||||
|
|
@ -1395,16 +1434,16 @@ from sound.effects import echo
|
|||
long_name = 2 # comment that should not be aligned
|
||||
|
||||
dictionary = {
|
||||
"foo": 1,
|
||||
"long_name": 2,
|
||||
'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,
|
||||
'foo' : 1,
|
||||
'long_name': 2,
|
||||
}</PRE></DIV>
|
||||
|
||||
|
||||
|
|
@ -1469,15 +1508,9 @@ from sound.effects import echo
|
|||
|
||||
|
||||
<p>
|
||||
Every file should contain the following items, in order:
|
||||
<ul>
|
||||
<li>a copyright statement (for example,
|
||||
<code>Copyright 2008 Google Inc.</code>)</li>
|
||||
<li>a license boilerplate. Choose the appropriate boilerplate
|
||||
for the license used by the project (for example, Apache 2.0, BSD,
|
||||
LGPL, GPL)</li>
|
||||
<li>an author line to identify the original author of the file</li>
|
||||
</ul>
|
||||
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="">
|
||||
|
|
@ -1663,17 +1696,6 @@ from sound.effects import echo
|
|||
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 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>
|
||||
|
||||
<DIV class=""><PRE>Yes: <span class="external"></span>class SampleClass(object):
|
||||
<span class="external"> </span>pass
|
||||
|
||||
|
|
@ -1688,7 +1710,16 @@ from sound.effects import echo
|
|||
<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
|
||||
|
|
@ -1710,12 +1741,13 @@ from sound.effects import echo
|
|||
to decide between <code>+</code> and <code>%</code> though.
|
||||
</DIV>
|
||||
<DIV class=""><DIV class="stylepoint_body" name="Strings__body" id="Strings__body" style="display: none">
|
||||
<DIV class=""><PRE class="badcode">No: <span class="external"></span>x = '%s%s' % (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>
|
||||
|
||||
<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 = 'name: %s; score: %d' % (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 = 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
|
||||
|
|
@ -1726,15 +1758,15 @@ from sound.effects import echo
|
|||
substring to a <code>cStringIO.StringIO</code> buffer).
|
||||
</p>
|
||||
|
||||
<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>
|
||||
<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>
|
||||
Use <code>"""</code> for multi-line strings rather than
|
||||
|
|
@ -1743,14 +1775,88 @@ from sound.effects import echo
|
|||
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 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></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="http://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("http://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="">
|
||||
|
|
@ -2112,7 +2218,7 @@ Don'<span class="external"></span>t do this.
|
|||
|
||||
|
||||
<p align="right">
|
||||
Revision 2.29
|
||||
Revision 2.39
|
||||
</p>
|
||||
|
||||
|
||||
|
|
|
|||
Loading…
Reference in New Issue
Block a user