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apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]3<TITLE>Google Python Style Guide</TITLE>
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99<BODY>
100<H1>Google Python Style Guide</H1>
101 <p align="right">
102
mshields@google.com222e6da2010-11-29 20:32:06 +0000[diff] [blame^]103 Revision 2.19
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]104 </p>
105
106 <address>
107 Amit Patel<br>
108 Antoine Picard<br>
109 Eugene Jhong<br>
110 Jeremy Hylton<br>
111 Matt Smart<br>
112 Mike Shields<br>
113 </address>
114 <DIV style="margin-left: 50%; font-size: 75%;">
115<P>
116 Each style point has a summary for which additional information is available
117 by toggling the accompanying arrow button that looks this way:
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]118 <SPAN class="showhide_button" style="margin-left: 0; float: none">▶</SPAN>.
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]119 You may toggle all summaries with the big arrow button:
120 </P>
121<DIV style=" font-size: larger; margin-left: +2em;">
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]122<SPAN class="showhide_button" style="font-size: 180%; float: none" onclick="javascript:ShowHideAll()" name="show_hide_all_button" id="show_hide_all_button">▶</SPAN>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]123 Toggle all summaries
124 </DIV>
125</DIV>
126<DIV class="toc">
127<DIV class="toc_title">Table of Contents</DIV>
128<TABLE>
129<TR valign="top" class="">
130<TD><DIV class="toc_category"><A href="#Python_Language_Rules">Python Language Rules</A></DIV></TD>
131<TD><DIV class="toc_stylepoint">
mmentovaif7facf92009-10-23 21:01:49 +0000[diff] [blame]132<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>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]133</TR>
134<TR valign="top" class="">
135<TD><DIV class="toc_category"><A href="#Python_Style_Rules">Python Style Rules</A></DIV></TD>
136<TD><DIV class="toc_stylepoint">
137<SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Semicolons">Semicolons</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Line_length">Line length</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Parentheses">Parentheses</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Indentation">Indentation</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Blank_Lines">Blank Lines</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Whitespace">Whitespace</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Python_Interpreter">Python Interpreter</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Comments">Comments</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Classes">Classes</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Strings">Strings</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#TODO_Comments">TODO Comments</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Imports_formatting">Imports formatting</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Statements">Statements</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Access_Control">Access Control</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Naming">Naming</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Main">Main</A></SPAN> </DIV></TD>
138</TR>
139</TABLE>
140</DIV>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]141 <DIV class="">
142<H2 name="Important_Note" id="Important_Note">Important Note</H2>
143 <DIV class="">
144<H3><A name="Displaying_Hidden_Details_in_this_Guide" id="Displaying_Hidden_Details_in_this_Guide">Displaying Hidden Details in this Guide</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]145<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">
146 link
147 </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Displaying_Hidden_Details_in_this_Guide__body','Displaying_Hidden_Details_in_this_Guide__button')" name="Displaying_Hidden_Details_in_this_Guide__button" id="Displaying_Hidden_Details_in_this_Guide__button">▶</SPAN>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]148 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]149 This style guide contains many details that are initially
150 hidden from view. They are marked by the triangle icon, which you
151 see here on your left. Click it now.
152 You should see "Hooray" appear below.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]153 </DIV>
154 <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">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]155 <p>
156 Hooray! Now you know you can expand points to get more
157 details. Alternatively, there's a "toggle all" at the
158 top of this document.
159 </p>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]160 </DIV></DIV>
161 </DIV>
162 </DIV>
163 <DIV class="">
164<H2 name="Background" id="Background">Background</H2>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]165 <p>
166 Python is the main scripting language used at Google. This
167 style guide is a list of <em>do</em>s and <em>don't</em>s for Python
168 programs.
169 </p>
170
mshields@google.com222e6da2010-11-29 20:32:06 +0000[diff] [blame^]171 <p>
172 To help you format code correctly, we've created a <a href="google_python_style.vim">settings
173 file for Vim</a>. For Emacs, the default settings should be fine.
174 </p>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]175
176
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]177 </DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]178
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]179 <DIV class="">
180<H2 name="Python_Language_Rules" id="Python_Language_Rules">Python Language Rules</H2>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]181
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]182 <DIV class="">
183<H3><A name="pychecker" id="pychecker">pychecker</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]184<SPAN class="link_button" id="link-pychecker__button" name="link-pychecker__button"><A href="?showone=pychecker#pychecker">
185 link
186 </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('pychecker__body','pychecker__button')" name="pychecker__button" id="pychecker__button">▶</SPAN>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]187 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]188 Run <code>pychecker</code> over your code.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]189 </DIV>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]190 <DIV class=""><DIV class="stylepoint_body" name="pychecker__body" id="pychecker__body" style="display: none">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]191 <P class="">
192<SPAN class="stylepoint_section">Definition: </SPAN>
193 PyChecker is a tool for finding bugs in Python source code. It finds
194 problems that are typically caught by a compiler for less dynamic
195 languages like C and C++. It is similar to lint. Because of the
196 dynamic nature of Python, some warnings may be incorrect; however,
197 spurious warnings should be fairly infrequent.
198 </P>
199 <P class="">
200<SPAN class="stylepoint_section">Pros: </SPAN>
201 Catches easy-to-miss errors like typos, use-vars-before-assignment, etc.
202 </P>
203 <P class="">
204<SPAN class="stylepoint_section">Cons: </SPAN>
205 <code>pychecker</code> isn't perfect. To take
206 advantage of it, we'll need to sometimes: a) Write around it b)
207 Suppress its warnings c) Improve it or d) Ignore it.
208 </P>
209 <P class="">
210<SPAN class="stylepoint_section">Decision: </SPAN>
211 Make sure you run <code>pychecker</code> on your code.
212 </P>
213
214 <p>
215 For information on how to run <code>pychecker</code>, see the
216 <a HREF="http://pychecker.sourceforge.net">pychecker
217 homepage</a>
218 </p>
219 <p>
220 To suppress warnings, you can set a module-level variable named
221 <code>__pychecker__</code> to suppress appropriate warnings.
222 For example:
223 </p>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]224 <DIV class=""><PRE>
225<span class="external"></span>__pychecker__ = 'no-callinit no-classattr'</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]226 <p>
227 Suppressing in this way has the advantage that we can easily search
228 for suppressions and revisit them.
229 </p>
230 <p>
231 You can get a list of pychecker warnings by doing
232 <code>pychecker --help</code>.
233 </p>
234 <p>
235 Unused argument warnings can be suppressed by using `_' as the
236 identifier for the unused argument or prefixing the argument name with
237 `unused_'. In situations where changing the argument names is
238 infeasible, you can mention them at the beginning of the function.
239 For example:
240 </p>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]241 <DIV class=""><PRE>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]242<span class="external"></span>def foo(a, unused_b, unused_c, d=None, e=None):
243 <span class="external"> </span>(d, e) = (d, e) # Silence pychecker
244 <span class="external"> </span>return a
245<span class="external"></span>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]246</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]247 <p>
248 Ideally, pychecker would be extended to ensure that such `unused
249 declarations' were true.
250 </p>
251
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]252 </DIV></DIV>
253 </DIV>
254 <DIV class="">
255<H3><A name="Imports" id="Imports">Imports</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]256<SPAN class="link_button" id="link-Imports__button" name="link-Imports__button"><A href="?showone=Imports#Imports">
257 link
258 </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Imports__body','Imports__button')" name="Imports__button" id="Imports__button">▶</SPAN>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]259 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]260 Use <code>import</code>s for packages and modules only.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]261 </DIV>
262 <DIV class=""><DIV class="stylepoint_body" name="Imports__body" id="Imports__body" style="display: none">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]263 <P class="">
264<SPAN class="stylepoint_section">Definition: </SPAN>
265 Reusability mechanism for sharing code from one module to another.
266 </P>
267 <P class="">
268<SPAN class="stylepoint_section">Pros: </SPAN>
mmentovaidb989ec2010-11-23 18:02:36 +0000[diff] [blame]269 The namespace management convention is simple. The source of each
270 identifier is indicated in a consistent way; <code>x.Obj</code> says
271 that object <code>Obj</code> is defined in module <code>x</code>.
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]272 </P>
273 <P class="">
mmentovaidb989ec2010-11-23 18:02:36 +0000[diff] [blame]274<SPAN class="stylepoint_section">Cons: </SPAN> Module names can still collide. Some module names are
275 inconveniently long.
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]276 </P>
277 <P class="">
278<SPAN class="stylepoint_section">Decision: </SPAN>
mmentovaidb989ec2010-11-23 18:02:36 +0000[diff] [blame]279 Use <code>import x</code> for importing packages and modules.
280 <br>
281 Use <code>from x import y</code> where <code>x</code> is
282 the package prefix and <code>y</code> is the module name with no
283 prefix.
284 <br>
285 Use <code>from x import y as z</code> if two modules named
286 <code>z</code> are to be imported or if <code>y</code> is an
287 inconveniently long name.
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]288 </P>
mmentovaidb989ec2010-11-23 18:02:36 +0000[diff] [blame]289 For example the module
290 <code>sound.effects.echo</code> may be imported as follows:
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]291 <DIV class=""><PRE>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]292<span class="external"></span>from sound.effects import echo
293<span class="external"></span>...
mmentovaidb989ec2010-11-23 18:02:36 +0000[diff] [blame]294<span class="external"></span>echo.EchoFilter(input, output, delay=0.7, atten=4)
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]295<span class="external"></span>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]296</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]297 <p>
mmentovaidb989ec2010-11-23 18:02:36 +0000[diff] [blame]298 Do not use relative names in imports. Even if the module is in the
299 same package, use the full package name. This helps prevent
300 unintentionally importing a package twice.
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]301 </p>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]302 </DIV></DIV>
303 </DIV>
304 <DIV class="">
305<H3><A name="Packages" id="Packages">Packages</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]306<SPAN class="link_button" id="link-Packages__button" name="link-Packages__button"><A href="?showone=Packages#Packages">
307 link
308 </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Packages__body','Packages__button')" name="Packages__button" id="Packages__button">▶</SPAN>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]309 <DIV style="display:inline;" class="">
mmentovaidb989ec2010-11-23 18:02:36 +0000[diff] [blame]310 Import each module using the full pathname location of the module.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]311 </DIV>
312 <DIV class=""><DIV class="stylepoint_body" name="Packages__body" id="Packages__body" style="display: none">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]313 <P class="">
314<SPAN class="stylepoint_section">Pros: </SPAN>
315 Avoids conflicts in module names. Makes it easier to find modules.
316 </P>
317 <P class="">
318<SPAN class="stylepoint_section">Cons: </SPAN>
319 Makes it harder to deploy code because you have to replicate the
320 package hierarchy.
321 </P>
322 <P class="">
323<SPAN class="stylepoint_section">Decision: </SPAN>
mmentovaidb989ec2010-11-23 18:02:36 +0000[diff] [blame]324 All new code should import each module by its full package name.
325
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]326 </P>
327 <p>
328 Imports should be as follows:
329 </p>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]330
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]331 <DIV class=""><PRE># Reference in code with complete name.
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]332import sound.effects.echo
333
mmentovaidb989ec2010-11-23 18:02:36 +0000[diff] [blame]334# Reference in code with just module name (preferred).
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]335from sound.effects import echo
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]336</PRE></DIV>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]337 </DIV></DIV>
338 </DIV>
339 <DIV class="">
340<H3><A name="Exceptions" id="Exceptions">Exceptions</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]341<SPAN class="link_button" id="link-Exceptions__button" name="link-Exceptions__button"><A href="?showone=Exceptions#Exceptions">
342 link
343 </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Exceptions__body','Exceptions__button')" name="Exceptions__button" id="Exceptions__button">▶</SPAN>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]344 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]345 Exceptions are allowed but must be used carefully.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]346 </DIV>
347 <DIV class=""><DIV class="stylepoint_body" name="Exceptions__body" id="Exceptions__body" style="display: none">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]348 <P class="">
349<SPAN class="stylepoint_section">Definition: </SPAN>
350 Exceptions are a means of breaking out of the normal flow of control
351 of a code block to handle errors or other exceptional conditions.
352 </P>
353 <P class="">
354<SPAN class="stylepoint_section">Pros: </SPAN>
355 The control flow of normal operation code is not cluttered by
356 error-handling code. It also allows the control flow to skip multiple
357 frames when a certain condition occurs, e.g., returning from N
358 nested functions in one step instead of having to carry-through
359 error codes.
360 </P>
361 <P class="">
362<SPAN class="stylepoint_section">Cons: </SPAN>
363 May cause the control flow to be confusing. Easy to miss error
364 cases when making library calls.
365 </P>
366 <P class="">
367<SPAN class="stylepoint_section">Decision: </SPAN>
368
369
370 Exceptions must follow certain conditions:
371
372 <ul>
373 <li>Raise exceptions like this: <code>raise MyException("Error
374 message")</code> or <code>raise MyException</code>. Do not
375 use the two-argument form (<code>raise MyException, "Error
376 message"</code>) or deprecated string-based exceptions
377 (<code>raise "Error message"</code>).</li>
378 <li>Modules or packages should define their own domain-specific
379 base exception class, which should inherit from the built-in
380 Exception class. The base exception for a module should be called
381 <code>Error</code>.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]382 <DIV class=""><PRE>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]383<span class="external"></span>class Error(Exception):
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]384 <span class="external"> </span>pass</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]385</li>
386 <li>Never use catch-all <code>except:</code> statements, or
387 catch <code>Exception</code> or <code>StandardError</code>,
388 unless you are re-raising the exception or in the outermost
389 block in your thread (and printing an error message). Python
390 is very tolerant in this regard and <code>except:</code> will
391 really catch everything including Python syntax errors. It is
392 easy to hide real bugs using <code>except:</code>.</li>
393 <li>Minimize the amount of code in a
394 <code>try</code>/<code>except</code> block. The larger the
395 body of the <code>try</code>, the more likely that an
396 exception will be raised by a line of code that you didn't
397 expect to raise an exception. In those cases,
398 the <code>try</code>/<code>except</code> block hides a real
399 error.</li>
400 <li>Use the <code>finally</code> clause to execute code whether
401 or not an exception is raised in the <code>try</code> block.
402 This is often useful for cleanup, i.e., closing a file.</li>
403 </ul>
404 </P>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]405 </DIV></DIV>
406 </DIV>
407 <DIV class="">
408<H3><A name="Global_variables" id="Global_variables">Global variables</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]409<SPAN class="link_button" id="link-Global_variables__button" name="link-Global_variables__button"><A href="?showone=Global_variables#Global_variables">
410 link
411 </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Global_variables__body','Global_variables__button')" name="Global_variables__button" id="Global_variables__button">▶</SPAN>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]412 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]413 Avoid global variables.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]414 </DIV>
415 <DIV class=""><DIV class="stylepoint_body" name="Global_variables__body" id="Global_variables__body" style="display: none">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]416 <P class="">
417<SPAN class="stylepoint_section">Definition: </SPAN>
418 Variables that are declared at the module level.
419 </P>
420 <P class="">
421<SPAN class="stylepoint_section">Pros: </SPAN>
422 Occasionally useful.
423 </P>
424 <P class="">
425<SPAN class="stylepoint_section">Cons: </SPAN>
426 Has the potential to change module behavior during the import,
427 because assignments to module-level variables are done when the
428 module is imported.
429 </P>
430 <P class="">
431<SPAN class="stylepoint_section">Decision: </SPAN>
432 Avoid global variables in favor of class variables. Some
433 exceptions are:
434 <ul>
435 <li>Default options for scripts.</li>
436 <li>Module-level constants. For example: <code>PI = 3.14159</code>.
437 Constants should be named using all caps with underscores;
438 see <a HREF="#Naming">Naming</a> below.</li>
439 <li>It is sometimes useful for globals to cache values needed
440 or returned by functions.</li>
441 <li>If needed, globals should be made internal to the module
442 and accessed through public module level functions;
443 see <a HREF="#Naming">Naming</a> below.</li>
444 </ul>
445 </P>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]446 </DIV></DIV>
447 </DIV>
448 <DIV class="">
449<H3><A name="Nested/Local/Inner_Classes_and_Functions" id="Nested/Local/Inner_Classes_and_Functions">Nested/Local/Inner Classes and Functions</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]450<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">
451 link
452 </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Nested/Local/Inner_Classes_and_Functions__body','Nested/Local/Inner_Classes_and_Functions__button')" name="Nested/Local/Inner_Classes_and_Functions__button" id="Nested/Local/Inner_Classes_and_Functions__button">▶</SPAN>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]453 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]454 Nested/local/inner classes and functions are fine.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]455 </DIV>
456 <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">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]457 <P class="">
458<SPAN class="stylepoint_section">Definition: </SPAN>
mmentovaif7facf92009-10-23 21:01:49 +0000[diff] [blame]459 A class can be defined inside of a method, function, or class. A
460 function can be defined inside a method or function. Nested functions
461 have read-only access to variables defined in enclosing scopes.
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]462 </P>
463 <P class="">
464<SPAN class="stylepoint_section">Pros: </SPAN>
465 Allows definition of utility classes and functions that are only
466 used inside of a very limited scope. Very <a HREF="http://en.wikipedia.org/wiki/Abstract_data_type">ADT</a>-y.
467 </P>
468 <P class="">
469<SPAN class="stylepoint_section">Cons: </SPAN>
470 Instances of nested or local classes cannot be pickled.
471 </P>
472 <P class="">
473<SPAN class="stylepoint_section">Decision: </SPAN>
474 They are fine.
475 </P>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]476 </DIV></DIV>
477 </DIV>
478 <DIV class="">
479<H3><A name="List_Comprehensions" id="List_Comprehensions">List Comprehensions</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]480<SPAN class="link_button" id="link-List_Comprehensions__button" name="link-List_Comprehensions__button"><A href="?showone=List_Comprehensions#List_Comprehensions">
481 link
482 </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('List_Comprehensions__body','List_Comprehensions__button')" name="List_Comprehensions__button" id="List_Comprehensions__button">▶</SPAN>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]483 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]484 Okay to use for simple cases.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]485 </DIV>
486 <DIV class=""><DIV class="stylepoint_body" name="List_Comprehensions__body" id="List_Comprehensions__body" style="display: none">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]487 <P class="">
488<SPAN class="stylepoint_section">Definition: </SPAN>
489 List comprehensions and generator expressions provide a concise
490 and efficient way to create lists and iterators without
491 resorting to the use of <code>map()</code>,
492 <code>filter()</code>, or <code>lambda</code>.
493 </P>
494 <P class="">
495<SPAN class="stylepoint_section">Pros: </SPAN>
496 Simple list comprehensions can be clearer and simpler than
497 other list creation techniques. Generator expressions can be
498 very efficient, since they avoid the creation of a list
499 entirely.
500 </P>
501 <P class="">
502<SPAN class="stylepoint_section">Cons: </SPAN>
503 Complicated list comprehensions or generator expressions can be
504 hard to read.
505 </P>
506 <P class="">
507<SPAN class="stylepoint_section">Decision: </SPAN>
508 Okay to use for simple cases. Each portion must fit on one line:
509 mapping expression, <code>for</code> clause, filter expression.
510 Multiple <code>for</code> clauses or filter expressions are not
511 permitted. Use loops instead when things get more complicated.
512 </P>
513
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]514<DIV class=""><PRE class="badcode">No<span class="external"></span>:
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]515 <span class="external"></span>result = [(x, y) for x in range(10) for y in range(5) if x * y &gt; 10]
516
517 <span class="external"></span>return ((x, y, z)
518 <span class="external"></span> for x in xrange(5)
519 <span class="external"></span> for y in xrange(5)
520 <span class="external"></span> if x != y
521 <span class="external"></span> for z in xrange(5)
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]522 <span class="external"></span> if y != z)</PRE></DIV>
523<DIV class=""><PRE>Ye<span class="external"></span>s:
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]524 <span class="external"></span>result = []
525 <span class="external"></span>for x in range(10):
526 <span class="external"> </span>for y in range(5):
527 <span class="external"> </span>if x * y &gt; 10:
528 <span class="external"> </span>result.append((x, y))
529
530 <span class="external"></span>for x in xrange(5):
531 <span class="external"> </span>for y in xrange(5):
532 <span class="external"> </span>if x != y:
533 <span class="external"> </span>for z in xrange(5):
534 <span class="external"> </span>if y != z:
535 <span class="external"> </span>yield (x, y, z)
536
537 <span class="external"></span>return ((x, complicated_transform(x))
538 <span class="external"></span> for x in long_generator_function(parameter)
539 <span class="external"></span> if x is not None)
540
541 <span class="external"></span>squares = [x * x for x in range(10)]
542
543 <span class="external"></span>eat(jelly_bean for jelly_bean in jelly_beans
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]544 <span class="external"></span> if jelly_bean.color == 'black')</PRE></DIV>
545 </DIV></DIV>
546 </DIV>
547 <DIV class="">
548<H3><A name="Default_Iterators_and_Operators" id="Default_Iterators_and_Operators">Default Iterators and Operators</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]549<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">
550 link
551 </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Default_Iterators_and_Operators__body','Default_Iterators_and_Operators__button')" name="Default_Iterators_and_Operators__button" id="Default_Iterators_and_Operators__button">▶</SPAN>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]552 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]553 Use default iterators and operators for types that support them,
554 like lists, dictionaries, and files.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]555 </DIV>
556 <DIV class=""><DIV class="stylepoint_body" name="Default_Iterators_and_Operators__body" id="Default_Iterators_and_Operators__body" style="display: none">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]557 <P class="">
558<SPAN class="stylepoint_section">Definition: </SPAN>
559 Container types, like dictionaries and lists, define default
560 iterators and membership test operators ("in" and "not in").
561 </P>
562 <P class="">
563<SPAN class="stylepoint_section">Pros: </SPAN>
564 The default iterators and operators are simple and efficient.
565 They express the operation directly, without extra method calls.
566 A function that uses default operators is generic. It can be
567 used with any type that supports the operation.
568 </P>
569 <P class="">
570<SPAN class="stylepoint_section">Cons: </SPAN>
571 You can't tell the type of objects by reading the method names
572 (e.g. has_key() means a dictionary). This is also an advantage.
573 </P>
574 <P class="">
575<SPAN class="stylepoint_section">Decision: </SPAN> Use default iterators and operators for types
576 that support them, like lists, dictionaries, and files. The
577 built-in types define iterator methods, too. Prefer these
578 methods to methods that return lists, except that you should not
579 mutate a container while iterating over it.
580
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]581<DIV class=""><PRE>Yes: <span class="external"></span>for key in adict: ...
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]582 <span class="external"></span>if key not in adict: ...
583 <span class="external"></span>if obj in alist: ...
584 <span class="external"></span>for line in afile: ...
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]585 <span class="external"></span>for k, v in dict.iteritems(): ...</PRE></DIV>
586<DIV class=""><PRE class="badcode">No: <span class="external"></span>for key in adict.keys(): ...
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]587 <span class="external"></span>if not adict.has_key(key): ...
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]588 <span class="external"></span>for line in afile.readlines(): ...</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]589 </P>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]590 </DIV></DIV>
591 </DIV>
592 <DIV class="">
593<H3><A name="Generators" id="Generators">Generators</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]594<SPAN class="link_button" id="link-Generators__button" name="link-Generators__button"><A href="?showone=Generators#Generators">
595 link
596 </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Generators__body','Generators__button')" name="Generators__button" id="Generators__button">▶</SPAN>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]597 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]598 Use generators as needed.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]599 </DIV>
600 <DIV class=""><DIV class="stylepoint_body" name="Generators__body" id="Generators__body" style="display: none">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]601 <P class="">
602<SPAN class="stylepoint_section">Definition: </SPAN>
603 A generator function returns an iterator that yields a value each
604 time it executes a yield statement. After it yields a value, the
605 runtime state of the generator function is suspended until the
606 next value is needed.
607 </P>
608 <P class="">
609<SPAN class="stylepoint_section">Pros: </SPAN>
610 Simpler code, because the state of local variables and control flow
611 are preserved for each call. A generator uses less memory than a
612 function that creates an entire list of values at once.
613 </P>
614 <P class="">
615<SPAN class="stylepoint_section">Cons: </SPAN>
616 None.
617 </P>
618 <P class="">
619<SPAN class="stylepoint_section">Decision: </SPAN>
620 Fine. Use "Yields:" rather than "Returns:" in the
621 doc string for generator functions.
622 </P>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]623 </DIV></DIV>
624 </DIV>
625 <DIV class="">
626<H3><A name="Lambda_Functions" id="Lambda_Functions">Lambda Functions</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]627<SPAN class="link_button" id="link-Lambda_Functions__button" name="link-Lambda_Functions__button"><A href="?showone=Lambda_Functions#Lambda_Functions">
628 link
629 </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Lambda_Functions__body','Lambda_Functions__button')" name="Lambda_Functions__button" id="Lambda_Functions__button">▶</SPAN>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]630 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]631 Okay for one-liners.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]632 </DIV>
633 <DIV class=""><DIV class="stylepoint_body" name="Lambda_Functions__body" id="Lambda_Functions__body" style="display: none">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]634 <P class="">
635<SPAN class="stylepoint_section">Definition: </SPAN>
636 Lambdas define anonymous functions in an expression, as
637 opposed to a statement. They are often used to define callbacks or
638 operators for higher-order functions like <code>map()</code> and
639 <code>filter()</code>.
640 </P>
641 <P class="">
642<SPAN class="stylepoint_section">Pros: </SPAN>
643 Convenient.
644 </P>
645 <P class="">
646<SPAN class="stylepoint_section">Cons: </SPAN> Harder to read and debug than local functions. The
647 lack of names means stack traces are more difficult to
648 understand. Expressiveness is limited because the function may
649 only contain an expression.
650 </P>
651 <P class="">
652<SPAN class="stylepoint_section">Decision: </SPAN>
653 Okay to use them for one-liners. If the code inside the lambda
654 function is any longer than 60–80 chars, it's probably better to
655 define it as a regular (nested) function.
656 <p>
657 For common operations like multiplication, use the functions from the
658 <code>operator</code> module instead of lambda functions. For
659 example, prefer <code>operator.mul</code> to <code>lambda
660 x, y: x * y</code>.
661 </p>
662 </P>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]663 </DIV></DIV>
664 </DIV>
665 <DIV class="">
666<H3><A name="Default_Argument_Values" id="Default_Argument_Values">Default Argument Values</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]667<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">
668 link
669 </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Default_Argument_Values__body','Default_Argument_Values__button')" name="Default_Argument_Values__button" id="Default_Argument_Values__button">▶</SPAN>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]670 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]671 Okay in most cases.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]672 </DIV>
673 <DIV class=""><DIV class="stylepoint_body" name="Default_Argument_Values__body" id="Default_Argument_Values__body" style="display: none">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]674 <P class="">
675<SPAN class="stylepoint_section">Definition: </SPAN>
676 You can specify values for variables at the end of a function's
677 parameter list, e.g., <code>def foo(a, b=0):</code>. If
678 <code>foo</code> is called with only one argument,
679 <code>b</code> is set to 0. If it is called with two arguments,
680 <code>b</code> has the value of the second argument.
681 </P>
682 <P class="">
683<SPAN class="stylepoint_section">Pros: </SPAN>
684 Often you have a function that uses lots of default values,
685 but—rarely—you want to override the
686 defaults. Default argument values provide an easy way to do this,
687 without having to define lots of functions for the rare
688 exceptions. Also, Python does not support overloaded
689 methods/functions and default arguments are an easy way of
690 "faking" the overloading behavior.
691 </P>
692 <P class="">
693<SPAN class="stylepoint_section">Cons: </SPAN>
694 Default arguments are evaluated once at module load
695 time. This may cause problems if the argument is a mutable
696 object such as a list or a dictionary. If the function modifies
697 the object (e.g., by appending an item to a list), the default
698 value is modified.
699 </P>
700 <P class="">
701<SPAN class="stylepoint_section">Decision: </SPAN>
702 Okay to use with the following caveats:
703 <p>
704 Do not use mutable objects as default values in the function or method
705 definition.
706 </p>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]707<DIV class=""><PRE>Yes: <span class="external"></span>def foo(a, b=None):
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]708 <span class="external"> </span>if b is None:
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]709 <span class="external"> </span>b = []</PRE></DIV>
710<DIV class=""><PRE class="badcode">No: <span class="external"></span>def foo(a, b=[]):
711 <span class="external"> </span>...</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]712 <p>
mmentovaif7facf92009-10-23 21:01:49 +0000[diff] [blame]713 Calling code must use named values for arguments with a default value.
714 This helps document the code somewhat and helps prevent and detect
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]715 interface breakage when more arguments are added.
716 </p>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]717<DIV class=""><PRE>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]718<span class="external"></span>def foo(a, b=1):
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]719 <span class="external"> </span>...</PRE></DIV>
720<DIV class=""><PRE>Yes: <span class="external"></span>foo(1)
721 <span class="external"></span>foo(1, b=2)</PRE></DIV>
722<DIV class=""><PRE class="badcode">No: <span class="external"></span>foo(1, 2)</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]723 </P>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]724 </DIV></DIV>
725 </DIV>
726 <DIV class="">
727<H3><A name="Properties" id="Properties">Properties</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]728<SPAN class="link_button" id="link-Properties__button" name="link-Properties__button"><A href="?showone=Properties#Properties">
729 link
730 </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Properties__body','Properties__button')" name="Properties__button" id="Properties__button">▶</SPAN>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]731 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]732 Use properties for accessing or setting data where you would
733 normally have used simple, lightweight accessor or setter methods.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]734 </DIV>
735 <DIV class=""><DIV class="stylepoint_body" name="Properties__body" id="Properties__body" style="display: none">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]736 <P class="">
737<SPAN class="stylepoint_section">Definition: </SPAN> A way to wrap method calls for getting and
738 setting an attribute as a standard attribute access when the
739 computation is lightweight.
740 </P>
741 <P class="">
742<SPAN class="stylepoint_section">Pros: </SPAN> Readability is increased by eliminating explicit
743 get and set method calls for simple attribute access. Allows
744 calculations to be lazy. Considered the Pythonic way to
745 maintain the interface of a class. In terms of performance,
746 allowing properties bypasses needing trivial accessor methods
747 when a direct variable access is reasonable. This also allows
748 accessor methods to be added in the future without breaking the
749 interface.
750 </P>
751 <P class="">
752<SPAN class="stylepoint_section">Cons: </SPAN> Properties are specified after the getter and
753 setter methods are declared, requiring one to notice they are
754 used for properties farther down in the code (except for readonly
755 properties created with the <code>@property</code> decorator - see
756 below). Must inherit from
757 <code>object</code>. Can hide side-effects much like operator
758 overloading. Can be confusing for subclasses.
759 </P>
760 <P class="">
761<SPAN class="stylepoint_section">Decision: </SPAN> Use properties in new code to access or
762 set data where you would normally have used simple, lightweight
763 accessor or setter methods. Read-only properties should be created
764 with the <code>@property</code>
765 <a HREF="#Function_and_Method_Decorators">decorator</a>.
766
767 <p><a id="properties-template-dp">
768 Inheritance with properties can be non-obvious if the property itself is
769 not overridden. Thus one must make sure that accessor methods are
770 called indirectly to ensure methods overridden in subclasses are called
771 by the property (using the Template Method DP).
772 </a></p>
773
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]774 <DIV class=""><PRE>Yes: <span class="external"></span>import math
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]775
776 <span class="external"></span>class Square(object):
777 <span class="external"> </span>"""A square with two properties: a writable area and a read-only perimeter.
778
779 <span class="external"> </span>To use:
780 <span class="external"> </span>&gt;&gt;&gt; sq = Square(3)
781 <span class="external"> </span>&gt;&gt;&gt; sq.area
782 <span class="external"> </span>9
783 <span class="external"> </span>&gt;&gt;&gt; sq.perimeter
784 <span class="external"> </span>12
785 <span class="external"> </span>&gt;&gt;&gt; sq.area = 16
786 <span class="external"> </span>&gt;&gt;&gt; sq.side
787 <span class="external"> </span>4
788 <span class="external"> </span>&gt;&gt;&gt; sq.perimeter
789 <span class="external"> </span>16
790 <span class="external"> </span>"""
791
792 <span class="external"> </span>def __init__(self, side):
793 <span class="external"> </span>self.side = side
794
795 <span class="external"> </span>def __get_area(self):
796 <span class="external"> </span>"""Calculates the 'area' property."""
797 <span class="external"> </span>return self.side ** 2
798
799 <span class="external"> </span>def ___get_area(self):
800 <span class="external"> </span>"""Indirect accessor for 'area' property."""
801 <span class="external"> </span>return self.__get_area()
802
803 <span class="external"> </span>def __set_area(self, area):
804 <span class="external"> </span>"""Sets the 'area' property."""
805 <span class="external"> </span>self.side = math.sqrt(area)
806
807 <span class="external"> </span>def ___set_area(self, area):
808 <span class="external"> </span>"""Indirect setter for 'area' property."""
809 <span class="external"> </span>self._SetArea(area)
810
811 <span class="external"> </span>area = property(___get_area, ___set_area,
812 <span class="external"> </span> doc="""Gets or sets the area of the square.""")
813
814 <span class="external"> </span>@property
815 <span class="external"> </span>def perimeter(self):
816 <span class="external"> </span>return self.side * 4
817<span class="external"></span>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]818</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]819 </P>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]820 </DIV></DIV>
821 </DIV>
822 <DIV class="">
823<H3><A name="True/False_evaluations" id="True/False_evaluations">True/False evaluations</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]824<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">
825 link
826 </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('True/False_evaluations__body','True/False_evaluations__button')" name="True/False_evaluations__button" id="True/False_evaluations__button">▶</SPAN>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]827 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]828 Use the "implicit" false if at all possible.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]829 </DIV>
830 <DIV class=""><DIV class="stylepoint_body" name="True/False_evaluations__body" id="True/False_evaluations__body" style="display: none">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]831 <P class="">
832<SPAN class="stylepoint_section">Definition: </SPAN> Python evaluates certain values as <code>false</code>
833 when in a boolean context. A quick "rule of thumb" is that all
834 "empty" values are considered <code>false</code> so <code>0, None, [], {},
835 ""</code> all evaluate as <code>false</code> in a boolean context.
836 </P>
837 <P class="">
838<SPAN class="stylepoint_section">Pros: </SPAN> Conditions using Python booleans are easier to read
839 and less error-prone. In most cases, they're also faster.
840 </P>
841 <P class="">
842<SPAN class="stylepoint_section">Cons: </SPAN>
843 May look strange to C/C++ developers.
844 </P>
845 <P class="">
846<SPAN class="stylepoint_section">Decision: </SPAN>
847 Use the "implicit" false if at all possible, e.g., <code>if
848 foo:</code> rather than <code>if foo != []:</code>. There are a
849 few caveats that you should keep in mind though:
850 <ul>
851 <li>
852 Never use <code>==</code> or <code>!=</code> to compare
853 singletons like <code>None</code>. Use <code>is</code>
854 or <code>is not</code>.</li>
855
856 <li>Beware of writing <code>if x:</code> when you really mean
857 <code>if x is not None:</code>—e.g., when testing whether
858 a variable or argument that defaults to <code>None</code> was
859 set to some other value. The other value might be a value
860 that's false in a boolean context!</li>
861
862 <li>
863 Never compare a boolean variable to <code>False</code> using
864 <code>==</code>. Use <code>if not x:</code> instead. If
865 you need to distinguish <code>False</code> from
866 <code>None</code> then chain the expressions,
867 such as <code>if not x and x is not None:</code>.
868 </li>
869
870 <li>
871 For sequences (strings, lists, tuples), use the fact that
872 empty sequences are false, so <code>if not seq:</code> or
873 <code>if seq:</code> is preferable to <code>if
874 len(seq):</code> or <code>if not
875 len(seq):</code>.</li>
876
877 <li>
878 When handling integers, implicit false may involve more risk than
879 benefit (i.e., accidentally handling <code>None</code> as 0). You may
880 compare a value which is known to be an integer (and is not the
881 result of <code>len()</code>) against the integer 0.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]882<DIV class=""><PRE>Yes: <span class="external"></span>if not users:
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]883 <span class="external"> </span>print 'no users'
884
885 <span class="external"></span>if foo == 0:
886 <span class="external"> </span>self.handle_zero()
887
888 <span class="external"></span>if i % 10 == 0:
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]889 <span class="external"> </span>self.handle_multiple_of_ten()</PRE></DIV>
890<DIV class=""><PRE class="badcode">No: <span class="external"></span>if len(users) == 0:
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]891 <span class="external"> </span>print 'no users'
892
893 <span class="external"></span>if foo is not None and not foo:
894 <span class="external"> </span>self.handle_zero()
895
896 <span class="external"></span>if not i % 10:
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]897 <span class="external"> </span>self.handle_multiple_of_ten()</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]898</li>
899
900 <li>
901 Note that <code>'0'</code> (i.e., <code>0</code> as string)
902 evaluates to true.</li>
903 </ul>
904 </P>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]905 </DIV></DIV>
906 </DIV>
907 <DIV class="">
908<H3><A name="Deprecated_Language_Features" id="Deprecated_Language_Features">Deprecated Language Features</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]909<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">
910 link
911 </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Deprecated_Language_Features__body','Deprecated_Language_Features__button')" name="Deprecated_Language_Features__button" id="Deprecated_Language_Features__button">▶</SPAN>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]912 <DIV style="display:inline;" class="">
mmentovaif7facf92009-10-23 21:01:49 +0000[diff] [blame]913 Use string methods instead of the <code>string</code> module
914 where possible. Use function call syntax instead
915 of <code>apply</code>. Use list comprehensions
916 and <code>for</code> loops instead of <code>filter</code>,
917 <code>map</code>, and <code>reduce</code>.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]918 </DIV>
919 <DIV class=""><DIV class="stylepoint_body" name="Deprecated_Language_Features__body" id="Deprecated_Language_Features__body" style="display: none">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]920 <P class="">
mmentovaif7facf92009-10-23 21:01:49 +0000[diff] [blame]921<SPAN class="stylepoint_section">Definition: </SPAN>
922 Current versions of Python provide alternative constructs
923 that people find generally preferable.
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]924 </P>
925 <P class="">
mmentovaif7facf92009-10-23 21:01:49 +0000[diff] [blame]926<SPAN class="stylepoint_section">Decision: </SPAN>
927 We do not use any Python version which does not support
928 these features, so there is no reason not to use the new
929 styles.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]930<DIV class=""><PRE class="badcode">No: <span class="external"></span>words = string.split(foo, ':')
mmentovaif7facf92009-10-23 21:01:49 +0000[diff] [blame]931
932 <span class="external"></span>map(lambda x: x[1], filter(lambda x: x[2] == 5, my_list))
933
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]934 <span class="external"></span>apply(fn, args, kwargs)</PRE></DIV>
935<DIV class=""><PRE>Yes: <span class="external"></span>words = foo.split(':')
mmentovaif7facf92009-10-23 21:01:49 +0000[diff] [blame]936
937 <span class="external"></span>[x[1] for x in my_list if x[2] == 5]
938
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]939 <span class="external"></span>fn(*args, **kwargs)</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]940 </P>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]941 </DIV></DIV>
942 </DIV>
943 <DIV class="">
944<H3><A name="Lexical_Scoping" id="Lexical_Scoping">Lexical Scoping</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]945<SPAN class="link_button" id="link-Lexical_Scoping__button" name="link-Lexical_Scoping__button"><A href="?showone=Lexical_Scoping#Lexical_Scoping">
946 link
947 </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Lexical_Scoping__body','Lexical_Scoping__button')" name="Lexical_Scoping__button" id="Lexical_Scoping__button">▶</SPAN>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]948 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]949 Okay to use.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]950 </DIV>
951 <DIV class=""><DIV class="stylepoint_body" name="Lexical_Scoping__body" id="Lexical_Scoping__body" style="display: none">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]952 <P class="">
953<SPAN class="stylepoint_section">Definition: </SPAN>
954 A nested Python function can refer to variables defined in
955 enclosing functions, but can not assign to them. Variable
956 bindings are resolved using lexical scoping, that is, based on
957 the static program text. Any assignment to a name in a block
958 will cause Python to treat all references to that name as a
959 local variable, even if the use precedes the assignment. If a
960 global declaration occurs, the name is treated as a global
961 variable.
962
963 <p>
964 An example of the use of this feature is:
965 </p>
966
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]967 <DIV class=""><PRE>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]968<span class="external"></span>def get_adder(summand1):
969 <span class="external"> </span>"""Returns a function that adds numbers to a given number."""
970 <span class="external"> </span>def adder(summand2):
971 <span class="external"> </span>return summand1 + summand2
972
973 <span class="external"> </span>return adder
974<span class="external"></span>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]975</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]976 </P>
977 <P class="">
978<SPAN class="stylepoint_section">Pros: </SPAN>
979 Often results in clearer, more elegant code. Especially comforting
980 to experienced Lisp and Scheme (and Haskell and ML and …)
981 programmers.
982 </P>
983 <P class="">
984<SPAN class="stylepoint_section">Cons: </SPAN>
985 Can lead to confusing bugs. Such as this example based on
986 <a HREF="http://www.python.org/dev/peps/pep-0227/">PEP-0227</a>:
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]987<DIV class=""><PRE class="badcode">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]988<span class="external"></span>i = 4
989<span class="external"></span>def foo(x):
990 <span class="external"> </span>def bar():
991 <span class="external"> </span>print i,
992 <span class="external"> </span># ...
993 <span class="external"> </span># A bunch of code here
994 <span class="external"> </span># ...
995 <span class="external"> </span>for i in x: # Ah, i *is* local to Foo, so this is what Bar sees
996 <span class="external"> </span>print i,
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]997 <span class="external"> </span>bar()</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]998 <p>
999 So <code>foo([1, 2, 3])</code> will print <code>1 2 3 3</code>, not
1000 <code>1 2 3 4</code>.
1001 </p>
1002 </P>
1003 <P class="">
1004<SPAN class="stylepoint_section">Decision: </SPAN>
1005 Okay to use.
1006 </P>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1007 </DIV></DIV>
1008 </DIV>
1009 <DIV class="">
1010<H3><A name="Function_and_Method_Decorators" id="Function_and_Method_Decorators">Function and Method Decorators</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]1011<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">
1012 link
1013 </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Function_and_Method_Decorators__body','Function_and_Method_Decorators__button')" name="Function_and_Method_Decorators__button" id="Function_and_Method_Decorators__button">▶</SPAN>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1014 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1015 Use decorators judiciously when there is a clear advantage.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1016 </DIV>
1017 <DIV class=""><DIV class="stylepoint_body" name="Function_and_Method_Decorators__body" id="Function_and_Method_Decorators__body" style="display: none">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1018 <P class="">
1019<SPAN class="stylepoint_section">Definition: </SPAN>
1020
1021 <a HREF="http://www.python.org/doc/2.4.3/whatsnew/node6.html">Decorators
1022 for Functions and Methods</a>
1023 (a.k.a "the <code>@</code> notation").
1024 The most common decorators are <code>@classmethod</code> and
1025 <code>@staticmethod</code>, for converting ordinary methods to class or
1026 static methods. However, the decorator syntax allows for
1027 user-defined decorators as well. Specifically, for some function
1028 <code>my_decorator</code>, this:
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1029 <DIV class=""><PRE>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1030<span class="external"></span>class C(object):
1031 <span class="external"> </span>@my_decorator
1032 <span class="external"> </span>def method(self):
1033 <span class="external"> </span># method body ...
1034<span class="external"></span>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1035</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1036
1037 is equivalent to:
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1038 <DIV class=""><PRE>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1039<span class="external"></span>class C(object):
1040 <span class="external"> </span>def method(self):
1041 <span class="external"> </span># method body ...
1042 <span class="external"> </span>method = my_decorator(method)
1043<span class="external"></span>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1044</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1045 </P>
1046 <P class="">
1047<SPAN class="stylepoint_section">Pros: </SPAN> Elegantly specifies some transformation on a method; the
1048 transformation might eliminate some repetitive code, enforce invariants,
1049 etc.
1050 </P>
1051 <P class="">
1052<SPAN class="stylepoint_section">Cons: </SPAN> Decorators can perform arbitrary operations on a
1053 function's arguments or return values, resulting in surprising
1054 implicit behavior.
1055 Additionally, decorators execute at import time. Failures in decorator
1056 code are pretty much impossible to recover from.
1057 </P>
1058 <P class="">
1059<SPAN class="stylepoint_section">Decision: </SPAN> Use decorators judiciously when there is a clear
1060 advantage. Decorators should follow the same import and naming
1061 guidelines as functions. Decorator pydoc should clearly state that the
1062 function is a decorator. Write unit tests for decorators.
1063
1064 <p>
1065 Avoid external dependencies in the decorator itself (e.g. don't rely on
1066 files, sockets, database connections, etc.), since they might not be
1067 available when the decorator runs (at import time, perhaps from
1068 <code>pychecker</code> or other tools). A decorator that is
1069 called with valid parameters should (as much as possible) be guaranteed
1070 to succeed in all cases.
1071 </p>
1072 <p>
1073 Decorators are a special case of "top level code" - see
1074 <a HREF="#Main">main</a> for more discussion.
1075 </p>
1076 </P>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1077 </DIV></DIV>
1078 </DIV>
1079 <DIV class="">
1080<H3><A name="Threading" id="Threading">Threading</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]1081<SPAN class="link_button" id="link-Threading__button" name="link-Threading__button"><A href="?showone=Threading#Threading">
1082 link
1083 </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Threading__body','Threading__button')" name="Threading__button" id="Threading__button">▶</SPAN>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1084 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1085 Do not rely on the atomicity of built-in types.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1086 </DIV>
1087 <DIV class=""><DIV class="stylepoint_body" name="Threading__body" id="Threading__body" style="display: none">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1088 <p>
1089 While Python's built-in data types such as dictionaries appear
1090 to have atomic operations, there are corner cases where they
1091 aren't atomic (e.g. if <code>__hash__</code> or
1092 <code>__eq__</code> are implemented as Python methods) and their
1093 atomicity should not be relied upon. Neither should you rely on
1094 atomic variable assignment (since this in turn depends on
1095 dictionaries).
1096 </p>
1097
1098 <p>
1099 Use the Queue module's <code>Queue</code> data type as the preferred
1100 way to
1101 communicate data between threads. Otherwise, use the threading
1102 module and its locking primitives. Learn about the proper use
1103 of condition variables so you can use
1104 <code>threading.Condition</code> instead of using lower-level
1105 locks.
1106 </p>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1107 </DIV></DIV>
1108 </DIV>
1109 <DIV class="">
1110<H3><A name="Power_Features" id="Power_Features">Power Features</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]1111<SPAN class="link_button" id="link-Power_Features__button" name="link-Power_Features__button"><A href="?showone=Power_Features#Power_Features">
1112 link
1113 </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Power_Features__body','Power_Features__button')" name="Power_Features__button" id="Power_Features__button">▶</SPAN>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1114 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1115 Avoid these features.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1116 </DIV>
1117 <DIV class=""><DIV class="stylepoint_body" name="Power_Features__body" id="Power_Features__body" style="display: none">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1118 <P class="">
1119<SPAN class="stylepoint_section">Definition: </SPAN> Python is an extremely flexible language and
1120 gives you many fancy features such as metaclasses, access to bytecode,
1121 on-the-fly compilation, dynamic inheritance, object reparenting,
1122 import hacks, reflection, modification of system internals,
1123 etc.
1124 </P>
1125 <P class="">
1126<SPAN class="stylepoint_section">Pros: </SPAN> These are powerful language features. They can
1127 make your code more compact.
1128 </P>
1129 <P class="">
1130<SPAN class="stylepoint_section">Cons: </SPAN> It's very tempting to use these "cool" features
1131 when they're not absolutely necessary. It's harder to read,
1132 understand, and debug code that's using unusual features
1133 underneath. It doesn't seem that way at first (to the original
1134 author), but when revisiting the code, it tends to be more
1135 difficult than code that is longer but is straightforward.
1136 </P>
1137 <P class="">
1138<SPAN class="stylepoint_section">Decision: </SPAN>
1139 Avoid these features in
1140 your code.
1141 </P>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1142 </DIV></DIV>
1143 </DIV>
1144 </DIV>
1145 <DIV class="">
1146<H2 name="Python_Style_Rules" id="Python_Style_Rules">Python Style Rules</H2>
1147 <DIV class="">
1148<H3><A name="Semicolons" id="Semicolons">Semicolons</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]1149<SPAN class="link_button" id="link-Semicolons__button" name="link-Semicolons__button"><A href="?showone=Semicolons#Semicolons">
1150 link
1151 </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Semicolons__body','Semicolons__button')" name="Semicolons__button" id="Semicolons__button">▶</SPAN>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1152 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1153 Do not terminate your lines with semi-colons and do not use
1154 semi-colons to put two commands on the same line.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1155 </DIV>
1156 <DIV class=""><DIV class="stylepoint_body" name="Semicolons__body" id="Semicolons__body" style="display: none">
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1157 </DIV></DIV>
1158 </DIV>
1159 <DIV class="">
1160<H3><A name="Line_length" id="Line_length">Line length</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]1161<SPAN class="link_button" id="link-Line_length__button" name="link-Line_length__button"><A href="?showone=Line_length#Line_length">
1162 link
1163 </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Line_length__body','Line_length__button')" name="Line_length__button" id="Line_length__button">▶</SPAN>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1164 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1165 Maximum line length is <em>80 characters</em>.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1166 </DIV>
1167 <DIV class=""><DIV class="stylepoint_body" name="Line_length__body" id="Line_length__body" style="display: none">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1168 <p>
1169 Exception: lines importing modules may end up longer than 80
1170 characters only if using Python 2.4 or
1171 earlier.
1172 </p>
1173
1174 <p>
1175 Make use of Python's
1176
1177 <a HREF="http://www.python.org/doc/ref/implicit-joining.html">implicit
1178 line joining inside parentheses, brackets and braces</a>.
1179 If necessary, you can add an extra pair of parentheses around an
1180 expression.
1181 </p>
1182
1183
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1184 <DIV class=""><PRE>Yes: foo_bar(self, width, height, color='black', design=None, x='foo',
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1185 emphasis=None, highlight=0)
1186
1187 if (width == 0 and height == 0 and
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1188 color == 'red' and emphasis == 'strong'):</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1189
1190
1191 <p>
1192 When a literal string won't fit on a single line, use parentheses for
1193 implicit line joining.
1194 </p>
1195
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1196 <DIV class=""><PRE>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1197<span class="external"></span>x = ('This will build a very long long '
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1198<span class="external"></span> 'long long long long long long string')</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1199
1200 <p>
1201 Make note of the indentation of the elements in the line
1202 continuation examples above; see the
1203 <a HREF="#indentation">indentation</a>
1204 section for explanation.
1205 </p>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1206 </DIV></DIV>
1207 </DIV>
1208 <DIV class="">
1209<H3><A name="Parentheses" id="Parentheses">Parentheses</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]1210<SPAN class="link_button" id="link-Parentheses__button" name="link-Parentheses__button"><A href="?showone=Parentheses#Parentheses">
1211 link
1212 </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Parentheses__body','Parentheses__button')" name="Parentheses__button" id="Parentheses__button">▶</SPAN>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1213 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1214 Use parentheses sparingly.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1215 </DIV>
1216 <DIV class=""><DIV class="stylepoint_body" name="Parentheses__body" id="Parentheses__body" style="display: none">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1217 <p>
1218 Do not use them in return statements or conditional statements unless
1219 using parentheses for implied line continuation. (See above.)
1220 It is however fine to use parentheses around tuples.
1221 </p>
1222
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1223<DIV class=""><PRE>Yes: <span class="external"></span>if foo:
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1224 <span class="external"> </span>bar()
1225 <span class="external"></span>while x:
1226 <span class="external"> </span>x = bar()
1227 <span class="external"></span>if x and y:
1228 <span class="external"> </span>bar()
1229 <span class="external"></span>if not x:
1230 <span class="external"> </span>bar()
1231 <span class="external"></span>return foo
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1232 <span class="external"></span>for (x, y) in dict.items(): ...</PRE></DIV>
1233<DIV class=""><PRE class="badcode">No: <span class="external"></span>if (x):
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1234 <span class="external"> </span>bar()
1235 <span class="external"></span>if not(x):
1236 <span class="external"> </span>bar()
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1237 <span class="external"></span>return (foo)</PRE></DIV>
1238 </DIV></DIV>
1239 </DIV>
1240 <DIV class="">
1241<H3><A name="Indentation" id="Indentation">Indentation</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]1242<SPAN class="link_button" id="link-Indentation__button" name="link-Indentation__button"><A href="?showone=Indentation#Indentation">
1243 link
1244 </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Indentation__body','Indentation__button')" name="Indentation__button" id="Indentation__button">▶</SPAN>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1245 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1246 Indent your code blocks with <em>4 spaces</em>.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1247 </DIV>
1248 <DIV class=""><DIV class="stylepoint_body" name="Indentation__body" id="Indentation__body" style="display: none">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1249 <p>
1250 Never use tabs or mix tabs and spaces.
1251 In cases of implied line continuation, you should align wrapped elements
1252 either vertically, as per the examples in the
1253 <a HREF="#Line_length">line length</a> section; or using a hanging
1254 indent of 4 spaces, in which case there should be no argument on
1255 the first line.
1256 </p>
1257
1258
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1259<DIV class=""><PRE>Yes: # Aligned with opening delimiter
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1260 foo = long_function_name(var_one, var_two,
1261 var_three, var_four)
1262
1263 # 4-space hanging indent; nothing on first line
1264 foo = long_function_name(
1265 var_one, var_two, var_three,
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1266 var_four)</PRE></DIV>
1267<DIV class=""><PRE class="badcode">No: <span class="external"></span># Stuff on first line forbidden
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1268 <span class="external"></span>foo = long_function_name(var_one, var_two,
1269 <span class="external"></span> var_three, var_four)
1270
1271 <span class="external"></span># 2-space hanging indent forbidden
1272 <span class="external"></span>foo = long_function_name(
1273 <span class="external"></span> var_one, var_two, var_three,
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1274 <span class="external"></span> var_four)</PRE></DIV>
1275 </DIV></DIV>
1276 </DIV>
1277 <DIV class="">
1278<H3><A name="Blank_Lines" id="Blank_Lines">Blank Lines</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]1279<SPAN class="link_button" id="link-Blank_Lines__button" name="link-Blank_Lines__button"><A href="?showone=Blank_Lines#Blank_Lines">
1280 link
1281 </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Blank_Lines__body','Blank_Lines__button')" name="Blank_Lines__button" id="Blank_Lines__button">▶</SPAN>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1282 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1283 Two blank lines between top-level definitions, one blank line
1284 between method definitions.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1285 </DIV>
1286 <DIV class=""><DIV class="stylepoint_body" name="Blank_Lines__body" id="Blank_Lines__body" style="display: none">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1287 <p>
1288 Two blank lines between top-level definitions, be they function
1289 or class definitions. One blank line between method definitions
1290 and between the <code>class</code> line and the first method.
1291 Use single blank lines as you judge appropriate within functions or
1292 methods.
1293 </p>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1294 </DIV></DIV>
1295 </DIV>
1296 <DIV class="">
1297<H3><A name="Whitespace" id="Whitespace">Whitespace</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]1298<SPAN class="link_button" id="link-Whitespace__button" name="link-Whitespace__button"><A href="?showone=Whitespace#Whitespace">
1299 link
1300 </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Whitespace__body','Whitespace__button')" name="Whitespace__button" id="Whitespace__button">▶</SPAN>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1301 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1302 Follow standard typographic rules for the use of spaces around
1303 punctuation.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1304 </DIV>
1305 <DIV class=""><DIV class="stylepoint_body" name="Whitespace__body" id="Whitespace__body" style="display: none">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1306 <p>
1307 No whitespace inside parentheses, brackets or braces.
1308 </p>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1309<DIV class=""><PRE>Yes: <span class="external"></span>spam(ham[1], {eggs: 2}, [])</PRE></DIV>
1310<DIV class=""><PRE class="badcode">No: <span class="external"></span>spam( ham[ 1 ], { eggs: 2 }, [ ] )</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1311 <p>
1312 No whitespace before a comma, semicolon, or colon. Do use
1313 whitespace after a comma, semicolon, or colon except at the end
1314 of the line.
1315 </p>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1316<DIV class=""><PRE>Yes: <span class="external"></span>if x == 4:
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1317 <span class="external"> </span>print x, y
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1318 <span class="external"></span>x, y = y, x</PRE></DIV>
1319<DIV class=""><PRE class="badcode">No: <span class="external"></span>if x == 4 :
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1320 <span class="external"> </span>print x , y
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1321 <span class="external"></span>x , y = y , x</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1322 <p>
1323 No whitespace before the open paren/bracket that starts an argument list,
1324 indexing or slicing.
1325 </p>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1326 <DIV class=""><PRE>Yes: <span class="external"></span>spam(1)</PRE></DIV>
1327<DIV class=""><PRE class="badcode">No: <span class="external"></span>spam (1)</PRE></DIV>
1328<DIV class=""><PRE>Yes: <span class="external"></span>dict['key'] = list[index]</PRE></DIV>
1329<DIV class=""><PRE class="badcode">No: <span class="external"></span>dict ['key'] = list [index]</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1330
1331 <p>
1332 Surround binary operators with a single space on either side for
1333 assignment (<code>=</code>), comparisons (<code>==, &lt;, &gt;, !=,
1334 &lt;&gt;, &lt;=, &gt;=, in, not in, is, is not</code>), and Booleans
1335 (<code>and, or, not</code>). Use your better judgment for the
1336 insertion of spaces around arithmetic operators but always be
1337 consistent about whitespace on either side of a binary operator.
1338 </p>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1339<DIV class=""><PRE>Yes: <span class="external"></span>x == 1</PRE></DIV>
1340<DIV class=""><PRE class="badcode">No: <span class="external"></span>x&lt;1</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1341 <p>
1342 Don't use spaces around the '=' sign when used to indicate a
1343 keyword argument or a default parameter value.
1344 </p>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1345<DIV class=""><PRE>Yes: <span class="external"></span>def complex(real, imag=0.0): return magic(r=real, i=imag)</PRE></DIV>
1346<DIV class=""><PRE class="badcode">No: <span class="external"></span>def complex(real, imag = 0.0): return magic(r = real, i = imag)</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1347
1348 <p>
1349 Don't use spaces to vertically align tokens on consecutive lines, since it
1350 becomes a maintenance burden (applies to <code>:</code>, <code>#</code>,
1351 <code>=</code>, etc.):
1352 </p>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1353<DIV class=""><PRE>Yes:
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1354 foo = 1000 # comment
1355 long_name = 2 # comment that should not be aligned
1356
1357 dictionary = {
1358 "foo": 1,
1359 "long_name": 2,
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1360 }</PRE></DIV>
1361<DIV class=""><PRE class="badcode">No:
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1362 foo = 1000 # comment
1363 long_name = 2 # comment that should not be aligned
1364
1365 dictionary = {
1366 "foo" : 1,
1367 "long_name": 2,
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1368 }</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1369
1370
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1371 </DIV></DIV>
1372 </DIV>
1373 <DIV class="">
1374<H3><A name="Python_Interpreter" id="Python_Interpreter">Python Interpreter</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]1375<SPAN class="link_button" id="link-Python_Interpreter__button" name="link-Python_Interpreter__button"><A href="?showone=Python_Interpreter#Python_Interpreter">
1376 link
1377 </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Python_Interpreter__body','Python_Interpreter__button')" name="Python_Interpreter__button" id="Python_Interpreter__button">▶</SPAN>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1378 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1379 Modules should begin with
1380
1381 <code>#!/usr/bin/env python&lt;version&gt;</code>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1382 </DIV>
1383 <DIV class=""><DIV class="stylepoint_body" name="Python_Interpreter__body" id="Python_Interpreter__body" style="display: none">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1384 <p>
1385 Modules should begin with a "shebang" line specifying the Python
1386 interpreter used to execute the program:
1387 </p>
1388
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1389<DIV class=""><PRE>
1390<span class="external"></span>#!/usr/bin/env python2.4</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1391
1392 <p>
1393 Always use the most specific version you can use, e.g.,
1394 <code>/usr/bin/python2.4</code>, not
1395 <code>/usr/bin/python2</code>. This makes it easier to find
mmentovaidb989ec2010-11-23 18:02:36 +0000[diff] [blame]1396 dependencies when upgrading to a different Python version
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1397 and also avoids confusion and breakage during use. E.g., Does
mmentovaidb989ec2010-11-23 18:02:36 +0000[diff] [blame]1398 <code>/usr/bin/python2</code> mean Python 2.0.1 or Python 2.3.0?
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1399 </p>
1400
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1401 </DIV></DIV>
1402 </DIV>
1403 <DIV class="">
1404<H3><A name="Comments" id="Comments">Comments</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]1405<SPAN class="link_button" id="link-Comments__button" name="link-Comments__button"><A href="?showone=Comments#Comments">
1406 link
1407 </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Comments__body','Comments__button')" name="Comments__button" id="Comments__button">▶</SPAN>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1408 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1409 Be sure to use the right style for module, function, method and in-line
1410 comments.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1411 </DIV>
1412 <DIV class=""><DIV class="stylepoint_body" name="Comments__body" id="Comments__body" style="display: none">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1413
1414 <P class="">
1415<SPAN class="stylepoint_subsection">Doc Strings</SPAN>
1416
1417 <p>
1418 Python has a unique commenting style using doc strings. A doc
1419 string is a string that is the first statement in a package,
1420 module, class or function. These strings can be extracted
1421 automatically through the <code>__doc__</code> member of the
1422 object and are used by <code>pydoc</code>. (Try running
1423 <code>pydoc</code> on your module to see how it looks.) Our
1424 convention for doc strings is to use the three double-quote
1425 format for strings. A doc string should be organized as a
1426 summary line (one physical line) terminated by a period,
1427 question mark, or exclamation point, followed by a blank line,
1428 followed by the rest of the doc string starting at the same
1429 cursor position as the first quote of the first line. There are
1430 more formatting guidelines for doc strings below.
1431 </p>
1432
1433 </P>
1434 <P class="">
1435<SPAN class="stylepoint_subsection">Modules</SPAN>
1436
1437
1438
1439 <p>
1440 Every file should contain the following items, in order:
1441 <ul>
1442 <li>a copyright statement (for example,
1443 <code>Copyright 2008 Google Inc.</code>)</li>
1444 <li>a license boilerplate. Choose the appropriate boilerplate
1445 for the license used by the project (for example, Apache 2.0, BSD,
1446 LGPL, GPL)</li>
1447 <li>an author line to identify the original author of the file</li>
1448 </ul>
1449 </p>
1450 </P>
1451 <P class="">
1452<SPAN class="stylepoint_subsection">Functions and Methods</SPAN>
1453
1454 <p>
1455 Any function or method which is not both obvious and very short
1456 needs a doc string. Additionally, any externally accessible
1457 function or method regardless of length or simplicity needs a
1458 doc string. The doc string should include what the function does
1459 and have detailed descriptions of the input and output. It
1460 should not, generally, describe how it does it unless it's some
1461 complicated algorithm. For tricky code block/inline comments
1462 within the code are more appropriate. The doc string should give
1463 enough information to write a call to the function without
1464 looking at a single line of the function's code. Args should be
1465 individually documented, an explanation following after a colon,
1466 and should use a uniform hanging indent of 2 or 4 spaces. The
1467 doc string should specify the expected types where specific types
1468 are required. A "Raises:" section should list all exceptions
1469 that can be raised by the function. The doc string for generator
1470 functions should use "Yields:" rather than "Returns:".
1471 </p>
1472
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1473 <DIV class=""><PRE>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1474<span class="external"></span>def fetch_bigtable_rows(big_table, keys, other_silly_variable=None):
1475 <span class="external"> </span>"""Fetches rows from a Bigtable.
1476
1477 <span class="external"> </span>Retrieves rows pertaining to the given keys from the Table instance
1478 <span class="external"> </span>represented by big_table. Silly things may happen if
1479 <span class="external"> </span>other_silly_variable is not None.
1480
1481 <span class="external"> </span>Args:
1482 <span class="external"> </span>big_table: An open Bigtable Table instance.
1483 <span class="external"> </span>keys: A sequence of strings representing the key of each table row
1484 <span class="external"> </span> to fetch.
1485 <span class="external"> </span>other_silly_variable: Another optional variable, that has a much
1486 <span class="external"> </span> longer name than the other args, and which does nothing.
1487
1488 <span class="external"> </span>Returns:
1489 <span class="external"> </span>A dict mapping keys to the corresponding table row data
1490 <span class="external"> </span>fetched. Each row is represented as a tuple of strings. For
1491 <span class="external"> </span>example:
1492
1493 <span class="external"> </span>{'Serak': ('Rigel VII', 'Preparer'),
1494 <span class="external"> </span> 'Zim': ('Irk', 'Invader'),
1495 <span class="external"> </span> 'Lrrr': ('Omicron Persei 8', 'Emperor')}
1496
1497 <span class="external"> </span>If a key from the keys argument is missing from the dictionary,
1498 <span class="external"> </span>then that row was not found in the table.
1499
1500 <span class="external"> </span>Raises:
1501 <span class="external"> </span>IOError: An error occurred accessing the bigtable.Table object.
1502 <span class="external"> </span>"""
1503 <span class="external"> </span>pass
1504<span class="external"></span>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1505</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1506 </P>
1507 <P class="">
1508<SPAN class="stylepoint_subsection">Classes</SPAN>
1509
1510 <p>
1511 Classes should have a doc string below the class definition describing
1512 the class. If your class has public attributes, they should be documented
1513 here in an Attributes section and follow the same formatting as a
1514 function's Args section.
1515 </p>
1516
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1517 <DIV class=""><PRE>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1518<span class="external"></span>class SampleClass(object):
1519 <span class="external"> </span>"""Summary of class here.
1520
1521 <span class="external"> </span>Longer class information....
1522 <span class="external"> </span>Longer class information....
1523
1524 <span class="external"> </span>Attributes:
1525 <span class="external"> </span>likes_spam: A boolean indicating if we like SPAM or not.
1526 <span class="external"> </span>eggs: An integer count of the eggs we have laid.
1527 <span class="external"> </span>"""
1528
1529 <span class="external"> </span>def __init__(self, likes_spam=False):
1530 <span class="external"> </span>"""Inits SampleClass with blah."""
1531 <span class="external"> </span>self.likes_spam = likes_spam
1532 <span class="external"> </span>self.eggs = 0
1533
1534 <span class="external"> </span>def public_method(self):
1535 <span class="external"> </span>"""Performs operation blah."""
1536<span class="external"></span>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1537</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1538
1539 </P>
1540 <P class="">
1541<SPAN class="stylepoint_subsection">Block and Inline Comments</SPAN>
1542
1543 <p>
1544 The final place to have comments is in tricky parts of the
1545 code. If you're going to have to explain it at the next
1546 <a HREF="http://en.wikipedia.org/wiki/Code_review">code review</a>,
1547 you should comment it now. Complicated operations get a few lines of
1548 comments before the operations
1549 commence. Non-obvious ones get comments at the end of the line.
1550 </p>
1551
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1552 <DIV class=""><PRE>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1553<span class="external"></span># We use a weighted dictionary search to find out where i is in
1554<span class="external"></span># the array. We extrapolate position based on the largest num
1555<span class="external"></span># in the array and the array size and then do binary search to
1556<span class="external"></span># get the exact number.
1557
1558<span class="external"></span>if i &amp; (i-1) == 0: # true iff i is a power of 2
1559<span class="external"></span>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1560</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1561
1562 <p>
1563 To improve legibility, these comments should be at least 2 spaces away
1564 from the code.
1565 </p>
1566
1567 <p>
1568 On the other hand, never describe the code. Assume the person
1569 reading the code knows Python (though not what you're trying to
1570 do) better than you do.
1571 </p>
1572
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1573 <DIV class=""><PRE class="badcode">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1574<span class="external"></span># BAD COMMENT: Now go through the b array and make sure whenever i occurs
1575<span class="external"></span># the next element is i+1
1576<span class="external"></span>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1577</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1578
1579 </P>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1580 </DIV></DIV>
1581 </DIV>
1582 <DIV class="">
1583<H3><A name="Classes" id="Classes">Classes</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]1584<SPAN class="link_button" id="link-Classes__button" name="link-Classes__button"><A href="?showone=Classes#Classes">
1585 link
1586 </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Classes__body','Classes__button')" name="Classes__button" id="Classes__button">▶</SPAN>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1587 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1588 If a class inherits from no other base classes, explicitly inherit
1589 from <code>object</code>. This also applies to nested classes.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1590 </DIV>
1591 <DIV class=""><DIV class="stylepoint_body" name="Classes__body" id="Classes__body" style="display: none">
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1592 <DIV class=""><PRE class="badcode">No: <span class="external"></span>class SampleClass:
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1593 <span class="external"> </span>pass
1594
1595
1596 <span class="external"></span>class OuterClass:
1597
1598 <span class="external"> </span>class InnerClass:
1599 <span class="external"> </span>pass
1600<span class="external"></span>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1601</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1602
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1603 <DIV class=""><PRE>Yes: <span class="external"></span>class SampleClass(object):
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1604 <span class="external"> </span>pass
1605
1606
1607 <span class="external"></span>class OuterClass(object):
1608
1609 <span class="external"> </span>class InnerClass(object):
1610 <span class="external"> </span>pass
1611
1612
1613 <span class="external"></span>class ChildClass(ParentClass):
1614 <span class="external"> </span>"""Explicitly inherits from another class already."""
1615<span class="external"></span>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1616</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1617
1618 <p>Inheriting from <code>object</code> is needed to make properties work
1619 properly, and it will protect your code from one particular potential
1620 incompatibility with Python 3000. It also defines
1621 special methods that implement the default semantics of objects including
1622 <code>__new__</code>, <code>__init__</code>, <code>__delattr__</code>,
1623 <code>__getattribute__</code>, <code>__setattr__</code>,
1624 <code>__hash__</code>, <code>__repr__</code>, and <code>__str__</code>.
1625 </p>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1626 </DIV></DIV>
1627 </DIV>
1628 <DIV class="">
1629<H3><A name="Strings" id="Strings">Strings</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]1630<SPAN class="link_button" id="link-Strings__button" name="link-Strings__button"><A href="?showone=Strings#Strings">
1631 link
1632 </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Strings__body','Strings__button')" name="Strings__button" id="Strings__button">▶</SPAN>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1633 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1634 Use the <code>%</code> operator for formatting strings,
1635 even when the parameters are all strings. Use your best judgement
1636 to decide between <code>+</code> and <code>%</code> though.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1637 </DIV>
1638 <DIV class=""><DIV class="stylepoint_body" name="Strings__body" id="Strings__body" style="display: none">
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1639<DIV class=""><PRE class="badcode">No: <span class="external"></span>x = '%s%s' % (a, b) # use + in this case
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1640 <span class="external"></span>x = imperative + ', ' + expletive + '!'
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1641 <span class="external"></span>x = 'name: ' + name + '; score: ' + str(n)</PRE></DIV>
1642<DIV class=""><PRE>Yes: <span class="external"></span>x = a + b
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1643 <span class="external"></span>x = '%s, %s!' % (imperative, expletive)
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1644 <span class="external"></span>x = 'name: %s; score: %d' % (name, n)</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1645
1646 <p>
1647 Avoid using the <code>+</code> and <code>+=</code> operators to
1648 accumulate a string within a loop. Since strings are immutable, this
1649 creates unnecessary temporary objects and results in quadratic rather
1650 than linear running time. Instead, add each substring to a list and
1651 <code>''.join</code> the list after the loop terminates (or, write each
1652 substring to a <code>cStringIO.StringIO</code> buffer).
1653 </p>
1654
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1655<DIV class=""><PRE class="badcode">No: <span class="external"></span>employee_table = '&lt;table&gt;'
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1656 <span class="external"></span>for last_name, first_name in employee_list:
1657 <span class="external"> </span>employee_table += '&lt;tr&gt;&lt;td&gt;%s, %s&lt;/td&gt;&lt;/tr&gt;' % (last_name, first_name)
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1658 <span class="external"></span>employee_table += '&lt;/table&gt;'</PRE></DIV>
1659<DIV class=""><PRE>Yes: <span class="external"></span>items = ['&lt;table&gt;']
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1660 <span class="external"></span>for last_name, first_name in employee_list:
1661 <span class="external"> </span>items.append('&lt;tr&gt;&lt;td&gt;%s, %s&lt;/td&gt;&lt;/tr&gt;' % (last_name, first_name))
1662 <span class="external"></span>items.append('&lt;/table&gt;')
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1663 <span class="external"></span>employee_table = ''.join(items)</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1664
1665 <p>
1666 Use <code>"""</code> for multi-line strings rather than
1667 <code>'''</code>. Note, however, that it is often cleaner to
1668 use implicit line joining since multi-line strings do
1669 not flow with the indentation of the rest of the program:
1670 </p>
1671
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1672 <DIV class=""><PRE class="badcode"> No<span class="external"></span>:
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1673 <span class="external"></span>print """This is pretty ugly.
1674Don'<span class="external"></span>t do this.
1675"""<span class="external"></span>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1676</PRE></DIV>
1677<DIV class=""><PRE>Ye<span class="external"></span>s:
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1678 <span class="external"></span>print ("This is much nicer.\n"
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1679 <span class="external"></span> "Do it this way.\n")</PRE></DIV>
1680 </DIV></DIV>
1681 </DIV>
1682 <DIV class="">
1683<H3><A name="TODO_Comments" id="TODO_Comments">TODO Comments</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]1684<SPAN class="link_button" id="link-TODO_Comments__button" name="link-TODO_Comments__button"><A href="?showone=TODO_Comments#TODO_Comments">
1685 link
1686 </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('TODO_Comments__body','TODO_Comments__button')" name="TODO_Comments__button" id="TODO_Comments__button">▶</SPAN>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1687 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1688 Use <code>TODO</code> comments for code that is temporary, a
1689 short-term solution, or good-enough but not perfect.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1690 </DIV>
1691 <DIV class=""><DIV class="stylepoint_body" name="TODO_Comments__body" id="TODO_Comments__body" style="display: none">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1692 <p>
1693 <code>TODO</code>s should include the string <code>TODO</code> in
1694 all caps, followed by your
1695
1696 name, e-mail address, or other
1697 identifier
1698 in parentheses. A colon is optional. A comment explaining what there
1699 is to do is required. The main purpose is to have
1700 a consistent <code>TODO</code> format searchable by the person
1701 adding the comment (who can provide more details upon request). A
1702 <code>TODO</code> is not a commitment to provide the fix yourself.
1703 </p>
1704
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1705 <DIV class=""><PRE># TODO(kl@gmail.com): Drop the use of "has_key".
1706# TODO(Zeke) change this to use relations.</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1707 <p>
1708 If your <code>TODO</code> is of the form "At a future date do
1709 something" make sure that you either include a very specific
1710 date ("Fix by November 2009") or a very specific event
1711 ("Remove this code when all clients can handle XML responses.").
1712 </p>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1713 </DIV></DIV>
1714 </DIV>
1715 <DIV class="">
1716<H3><A name="Imports_formatting" id="Imports_formatting">Imports formatting</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]1717<SPAN class="link_button" id="link-Imports_formatting__button" name="link-Imports_formatting__button"><A href="?showone=Imports_formatting#Imports_formatting">
1718 link
1719 </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Imports_formatting__body','Imports_formatting__button')" name="Imports_formatting__button" id="Imports_formatting__button">▶</SPAN>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1720 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1721 Imports should be on separate lines.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1722 </DIV>
1723 <DIV class=""><DIV class="stylepoint_body" name="Imports_formatting__body" id="Imports_formatting__body" style="display: none">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1724 <p>
1725 E.g.:
1726 </p>
1727
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1728<DIV class=""><PRE>Yes: <span class="external"></span>import os
1729 <span class="external"></span>import sys</PRE></DIV>
1730<DIV class=""><PRE class="badcode">No: <span class="external"></span>import os, sys</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1731 <p>
1732 Imports are always put at the top of the file, just after any
1733 module comments and doc strings and before module globals and
1734 constants. Imports should be grouped with the order being most generic
1735 to least generic:
1736 </p>
1737 <ul>
1738 <li>standard library imports</li>
1739 <li>third-party imports</li>
1740
1741 <li>application-specific imports</li>
1742 </ul>
1743 <p>
1744 Within each grouping, imports should be sorted lexicographically,
1745 ignoring case, according to each module's full package path.
1746 </p>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1747 <DIV class=""><PRE>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1748<span class="external"></span>import foo
1749<span class="external"></span>from foo import bar
1750<span class="external"></span>from foo.bar import baz
1751<span class="external"></span>from foo.bar import Quux
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1752<span class="external"></span>from Foob import ar</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1753
1754
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1755
1756 </DIV></DIV>
1757 </DIV>
1758 <DIV class="">
1759<H3><A name="Statements" id="Statements">Statements</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]1760<SPAN class="link_button" id="link-Statements__button" name="link-Statements__button"><A href="?showone=Statements#Statements">
1761 link
1762 </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Statements__body','Statements__button')" name="Statements__button" id="Statements__button">▶</SPAN>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1763 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1764 Generally only one statement per line.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1765 </DIV>
1766 <DIV class=""><DIV class="stylepoint_body" name="Statements__body" id="Statements__body" style="display: none">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1767 <p>
1768 However, you may put the
1769 result of a test on the same line as the test only if the entire
1770 statement fits on one line. In particular, you can never do so
1771 with <code>try</code>/<code>except</code> since the
1772 <code>try</code> and <code>except</code> can't both fit on the
1773 same line, and you can only do so with an <code>if</code> if
1774 there is no <code>else</code>.
1775 </p>
1776
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1777 <DIV class=""><PRE>Ye<span class="external"></span>s:
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1778
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1779 <span class="external"></span>if foo: bar(foo)</PRE></DIV>
1780<DIV class=""><PRE class="badcode">No<span class="external"></span>:
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1781
1782 <span class="external"></span>if foo: bar(foo)
1783 <span class="external"></span>else: baz(foo)
1784
1785 <span class="external"></span>try: bar(foo)
1786 <span class="external"></span>except ValueError: baz(foo)
1787
1788 <span class="external"></span>try:
1789 <span class="external"> </span>bar(foo)
1790 <span class="external"></span>except ValueError: baz(foo)
1791<span class="external"></span>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1792</PRE></DIV>
1793 </DIV></DIV>
1794 </DIV>
1795 <DIV class="">
1796<H3><A name="Access_Control" id="Access_Control">Access Control</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]1797<SPAN class="link_button" id="link-Access_Control__button" name="link-Access_Control__button"><A href="?showone=Access_Control#Access_Control">
1798 link
1799 </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Access_Control__body','Access_Control__button')" name="Access_Control__button" id="Access_Control__button">▶</SPAN>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1800 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1801 If an accessor function would be trivial you should use public variables
1802 instead of accessor functions to avoid the extra cost of function
1803 calls in Python. When more functionality is added you can use
1804 <code>property</code> to keep the syntax consistent.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1805 </DIV>
1806 <DIV class=""><DIV class="stylepoint_body" name="Access_Control__body" id="Access_Control__body" style="display: none">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1807 <p>
1808 On the other hand, if access is more complex, or the cost of accessing
1809 the variable is significant, you should use function calls (following the
1810 <a HREF="#naming">Naming</a> guidelines) such as <code>get_foo()</code>
1811 and <code>set_foo()</code>. If the past behavior allowed access through a
1812 property, do not bind the new accessor functions to the property. Any
1813 code still attempting to access the variable by the old method should
1814 break visibly so they are made aware of the change in complexity.
1815 </p>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1816 </DIV></DIV>
1817 </DIV>
1818 <DIV class="">
1819<H3><A name="Naming" id="Naming">Naming</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]1820<SPAN class="link_button" id="link-Naming__button" name="link-Naming__button"><A href="?showone=Naming#Naming">
1821 link
1822 </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Naming__body','Naming__button')" name="Naming__button" id="Naming__button">▶</SPAN>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1823 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1824 <code>module_name, package_name, ClassName, method_name, ExceptionName,
1825 function_name, GLOBAL_VAR_NAME, instance_var_name,
1826 function_parameter_name, local_var_name.</code>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1827 </DIV>
1828 <DIV class=""><DIV class="stylepoint_body" name="Naming__body" id="Naming__body" style="display: none">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1829 <P class="">
1830<SPAN class="stylepoint_subsection">Names to Avoid</SPAN>
1831
1832 <ul>
1833 <li>single character names except for counters or iterators</li>
1834 <li>dashes (<code>-</code>) in any package/module name</li>
1835 <li>
1836<code>__double_leading_and_trailing_underscore__</code> names
1837 (reserved by Python)</li>
1838 </ul>
1839
1840 </P>
1841 <P class="">
1842<SPAN class="stylepoint_subsection">Naming Convention</SPAN>
1843
1844 <ul>
1845 <li>
1846 "Internal" means internal to a module or protected
1847 or private within a class.</li>
1848 <li>
1849 Prepending a single underscore (<code>_</code>) has some
1850 support for protecting module variables and functions (not included
1851 with <code>import * from</code>). Prepending a double underscore
1852 (<code>__</code>) to an instance variable or method
1853 effectively serves to make the variable or method private to its class
1854 (using name mangling).</li>
1855 <li>
1856 Place related classes and top-level functions together in a
1857 module. Unlike Java,
1858 there is no need to limit yourself to one class per module.</li>
1859 <li>
1860 Use CapWords for class names, but lower_with_under.py for module names.
1861 Although there are many existing modules named CapWords.py, this is now
1862 discouraged because it's confusing when the module happens to be
1863 named after a class. ("wait -- did I write
1864 <code>import StringIO</code> or <code>from StringIO import
1865 StringIO</code>?")</li>
1866 </ul>
1867
1868 </P>
1869 <P class="">
1870<SPAN class="stylepoint_subsection">Guidelines derived from Guido's Recommendations</SPAN>
1871
1872 <table rules="all" border="1" cellspacing="2" cellpadding="2">
1873
1874 <tr>
1875 <th>Type</th>
1876 <th>Public</th>
1877 <th>Internal</th>
1878 </tr>
1879
1880
1881
1882 <tr>
1883 <td>Packages</td>
1884 <td><code>lower_with_under</code></td>
1885 <td></td>
1886 </tr>
1887
1888 <tr>
1889 <td>Modules</td>
1890 <td><code>lower_with_under</code></td>
1891 <td><code>_lower_with_under</code></td>
1892 </tr>
1893
1894 <tr>
1895 <td>Classes</td>
1896 <td><code>CapWords</code></td>
1897 <td><code>_CapWords</code></td>
1898 </tr>
1899
1900 <tr>
1901 <td>Exceptions</td>
1902 <td><code>CapWords</code></td>
1903 <td></td>
1904 </tr>
1905
1906
1907
1908 <tr>
1909 <td>Functions</td>
1910 <td><code>lower_with_under()</code></td>
1911 <td><code>_lower_with_under()</code></td>
1912 </tr>
1913
1914 <tr>
1915 <td>Global/Class Constants</td>
1916 <td><code>CAPS_WITH_UNDER</code></td>
1917 <td><code>_CAPS_WITH_UNDER</code></td>
1918 </tr>
1919
1920 <tr>
1921 <td>Global/Class Variables</td>
1922 <td><code>lower_with_under</code></td>
1923 <td><code>_lower_with_under</code></td>
1924 </tr>
1925
1926 <tr>
1927 <td>Instance Variables</td>
1928 <td><code>lower_with_under</code></td>
1929 <td><code>_lower_with_under (protected) or __lower_with_under (private)</code></td>
1930 </tr>
1931
1932
1933
1934 <tr>
1935 <td>Method Names</td>
1936 <td><code>lower_with_under()</code></td>
1937 <td><code>_lower_with_under() (protected) or __lower_with_under() (private)</code></td>
1938 </tr>
1939
1940 <tr>
1941 <td>Function/Method Parameters</td>
1942 <td><code>lower_with_under</code></td>
1943 <td></td>
1944 </tr>
1945
1946 <tr>
1947 <td>Local Variables</td>
1948 <td><code>lower_with_under</code></td>
1949 <td></td>
1950 </tr>
1951
1952
1953 </table>
1954
1955
1956 </P>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1957 </DIV></DIV>
1958 </DIV>
1959 <DIV class="">
1960<H3><A name="Main" id="Main">Main</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]1961<SPAN class="link_button" id="link-Main__button" name="link-Main__button"><A href="?showone=Main#Main">
1962 link
1963 </A></SPAN><SPAN class="showhide_button" onclick="javascript:ShowHideByName('Main__body','Main__button')" name="Main__button" id="Main__button">▶</SPAN>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1964 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1965 Even a file meant to be used as a script should be importable and a
1966 mere import should not have the side effect of executing the script's
1967 main functionality. The main functionality should be in a main()
1968 function.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1969 </DIV>
1970 <DIV class=""><DIV class="stylepoint_body" name="Main__body" id="Main__body" style="display: none">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1971 <p>
1972 In Python,
1973 <code>pychecker</code>, <code>pydoc</code>, and unit tests
1974 require modules to be importable. Your code should always check
1975 <code>if __name__ == '__main__'</code> before executing your
1976 main program so that the main program is not executed when the
1977 module is imported.
1978
1979 </p>
1980
1981
1982
1983
1984
1985
1986
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1987 <DIV class=""><PRE>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1988<span class="external"></span>def main():
1989 <span class="external"> </span>...
1990
1991<span class="external"></span>if __name__ == '__main__':
1992 <span class="external"> </span>main()
1993<span class="external"></span>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1994</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1995
1996 <p>
1997 All code at the top level will be executed when the module is
1998 imported. Be careful not to call functions, create objects, or
1999 perform other operations that should not be executed when the
2000 file is being <code>pycheck</code>ed or <code>pydoc</code>ed.
2001 </p>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]2002 </DIV></DIV>
2003 </DIV>
2004 </DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]2005
2006<H2>Parting Words</H2>
2007 <p>
2008 <em>BE CONSISTENT</em>.
2009 </p>
2010
2011 <p>
2012 If you're editing code, take a few minutes to look at the code
2013 around you and determine its style. If they use spaces around
2014 all their arithmetic operators, you should too. If their
2015 comments have little boxes of hash marks around them, make your
2016 comments have little boxes of hash marks around them too.
2017 </p>
2018
2019 <p>
2020 The point of having style guidelines is to have a common vocabulary
2021 of coding so people can concentrate on what you're saying rather
2022 than on how you're saying it. We present global style rules here so
2023 people know the vocabulary, but local style is also important. If
2024 code you add to a file looks drastically different from the existing
2025 code around it, it throws readers out of their rhythm when they go to
2026 read it. Avoid this.
2027 </p>
2028
2029
2030
2031<p align="right">
mshields@google.com222e6da2010-11-29 20:32:06 +0000[diff] [blame^]2032Revision 2.19
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]2033</p>
2034
2035
2036<address>
2037 Amit Patel<br>
2038 Antoine Picard<br>
2039 Eugene Jhong<br>
2040 Jeremy Hylton<br>
2041 Matt Smart<br>
2042 Mike Shields<br>
2043</address>
2044</BODY>
2045</HTML>