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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
mmentovaidb989ec2010-11-23 18:02:36 +0000[diff] [blame]103 Revision 2.18
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
171
172
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]173 </DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]174
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]175 <DIV class="">
176<H2 name="Python_Language_Rules" id="Python_Language_Rules">Python Language Rules</H2>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]177
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]178 <DIV class="">
179<H3><A name="pychecker" id="pychecker">pychecker</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]180<SPAN class="link_button" id="link-pychecker__button" name="link-pychecker__button"><A href="?showone=pychecker#pychecker">
181 link
182 </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]183 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]184 Run <code>pychecker</code> over your code.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]185 </DIV>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]186 <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]187 <P class="">
188<SPAN class="stylepoint_section">Definition: </SPAN>
189 PyChecker is a tool for finding bugs in Python source code. It finds
190 problems that are typically caught by a compiler for less dynamic
191 languages like C and C++. It is similar to lint. Because of the
192 dynamic nature of Python, some warnings may be incorrect; however,
193 spurious warnings should be fairly infrequent.
194 </P>
195 <P class="">
196<SPAN class="stylepoint_section">Pros: </SPAN>
197 Catches easy-to-miss errors like typos, use-vars-before-assignment, etc.
198 </P>
199 <P class="">
200<SPAN class="stylepoint_section">Cons: </SPAN>
201 <code>pychecker</code> isn't perfect. To take
202 advantage of it, we'll need to sometimes: a) Write around it b)
203 Suppress its warnings c) Improve it or d) Ignore it.
204 </P>
205 <P class="">
206<SPAN class="stylepoint_section">Decision: </SPAN>
207 Make sure you run <code>pychecker</code> on your code.
208 </P>
209
210 <p>
211 For information on how to run <code>pychecker</code>, see the
212 <a HREF="http://pychecker.sourceforge.net">pychecker
213 homepage</a>
214 </p>
215 <p>
216 To suppress warnings, you can set a module-level variable named
217 <code>__pychecker__</code> to suppress appropriate warnings.
218 For example:
219 </p>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]220 <DIV class=""><PRE>
221<span class="external"></span>__pychecker__ = 'no-callinit no-classattr'</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]222 <p>
223 Suppressing in this way has the advantage that we can easily search
224 for suppressions and revisit them.
225 </p>
226 <p>
227 You can get a list of pychecker warnings by doing
228 <code>pychecker --help</code>.
229 </p>
230 <p>
231 Unused argument warnings can be suppressed by using `_' as the
232 identifier for the unused argument or prefixing the argument name with
233 `unused_'. In situations where changing the argument names is
234 infeasible, you can mention them at the beginning of the function.
235 For example:
236 </p>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]237 <DIV class=""><PRE>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]238<span class="external"></span>def foo(a, unused_b, unused_c, d=None, e=None):
239 <span class="external"> </span>(d, e) = (d, e) # Silence pychecker
240 <span class="external"> </span>return a
241<span class="external"></span>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]242</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]243 <p>
244 Ideally, pychecker would be extended to ensure that such `unused
245 declarations' were true.
246 </p>
247
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]248 </DIV></DIV>
249 </DIV>
250 <DIV class="">
251<H3><A name="Imports" id="Imports">Imports</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]252<SPAN class="link_button" id="link-Imports__button" name="link-Imports__button"><A href="?showone=Imports#Imports">
253 link
254 </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]255 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]256 Use <code>import</code>s for packages and modules only.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]257 </DIV>
258 <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]259 <P class="">
260<SPAN class="stylepoint_section">Definition: </SPAN>
261 Reusability mechanism for sharing code from one module to another.
262 </P>
263 <P class="">
264<SPAN class="stylepoint_section">Pros: </SPAN>
mmentovaidb989ec2010-11-23 18:02:36 +0000[diff] [blame]265 The namespace management convention is simple. The source of each
266 identifier is indicated in a consistent way; <code>x.Obj</code> says
267 that object <code>Obj</code> is defined in module <code>x</code>.
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]268 </P>
269 <P class="">
mmentovaidb989ec2010-11-23 18:02:36 +0000[diff] [blame]270<SPAN class="stylepoint_section">Cons: </SPAN> Module names can still collide. Some module names are
271 inconveniently long.
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]272 </P>
273 <P class="">
274<SPAN class="stylepoint_section">Decision: </SPAN>
mmentovaidb989ec2010-11-23 18:02:36 +0000[diff] [blame]275 Use <code>import x</code> for importing packages and modules.
276 <br>
277 Use <code>from x import y</code> where <code>x</code> is
278 the package prefix and <code>y</code> is the module name with no
279 prefix.
280 <br>
281 Use <code>from x import y as z</code> if two modules named
282 <code>z</code> are to be imported or if <code>y</code> is an
283 inconveniently long name.
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]284 </P>
mmentovaidb989ec2010-11-23 18:02:36 +0000[diff] [blame]285 For example the module
286 <code>sound.effects.echo</code> may be imported as follows:
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]287 <DIV class=""><PRE>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]288<span class="external"></span>from sound.effects import echo
289<span class="external"></span>...
mmentovaidb989ec2010-11-23 18:02:36 +0000[diff] [blame]290<span class="external"></span>echo.EchoFilter(input, output, delay=0.7, atten=4)
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]291<span class="external"></span>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]292</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]293 <p>
mmentovaidb989ec2010-11-23 18:02:36 +0000[diff] [blame]294 Do not use relative names in imports. Even if the module is in the
295 same package, use the full package name. This helps prevent
296 unintentionally importing a package twice.
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]297 </p>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]298 </DIV></DIV>
299 </DIV>
300 <DIV class="">
301<H3><A name="Packages" id="Packages">Packages</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]302<SPAN class="link_button" id="link-Packages__button" name="link-Packages__button"><A href="?showone=Packages#Packages">
303 link
304 </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]305 <DIV style="display:inline;" class="">
mmentovaidb989ec2010-11-23 18:02:36 +0000[diff] [blame]306 Import each module using the full pathname location of the module.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]307 </DIV>
308 <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]309 <P class="">
310<SPAN class="stylepoint_section">Pros: </SPAN>
311 Avoids conflicts in module names. Makes it easier to find modules.
312 </P>
313 <P class="">
314<SPAN class="stylepoint_section">Cons: </SPAN>
315 Makes it harder to deploy code because you have to replicate the
316 package hierarchy.
317 </P>
318 <P class="">
319<SPAN class="stylepoint_section">Decision: </SPAN>
mmentovaidb989ec2010-11-23 18:02:36 +0000[diff] [blame]320 All new code should import each module by its full package name.
321
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]322 </P>
323 <p>
324 Imports should be as follows:
325 </p>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]326
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]327 <DIV class=""><PRE># Reference in code with complete name.
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]328import sound.effects.echo
329
mmentovaidb989ec2010-11-23 18:02:36 +0000[diff] [blame]330# Reference in code with just module name (preferred).
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]331from sound.effects import echo
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]332</PRE></DIV>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]333 </DIV></DIV>
334 </DIV>
335 <DIV class="">
336<H3><A name="Exceptions" id="Exceptions">Exceptions</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]337<SPAN class="link_button" id="link-Exceptions__button" name="link-Exceptions__button"><A href="?showone=Exceptions#Exceptions">
338 link
339 </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]340 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]341 Exceptions are allowed but must be used carefully.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]342 </DIV>
343 <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]344 <P class="">
345<SPAN class="stylepoint_section">Definition: </SPAN>
346 Exceptions are a means of breaking out of the normal flow of control
347 of a code block to handle errors or other exceptional conditions.
348 </P>
349 <P class="">
350<SPAN class="stylepoint_section">Pros: </SPAN>
351 The control flow of normal operation code is not cluttered by
352 error-handling code. It also allows the control flow to skip multiple
353 frames when a certain condition occurs, e.g., returning from N
354 nested functions in one step instead of having to carry-through
355 error codes.
356 </P>
357 <P class="">
358<SPAN class="stylepoint_section">Cons: </SPAN>
359 May cause the control flow to be confusing. Easy to miss error
360 cases when making library calls.
361 </P>
362 <P class="">
363<SPAN class="stylepoint_section">Decision: </SPAN>
364
365
366 Exceptions must follow certain conditions:
367
368 <ul>
369 <li>Raise exceptions like this: <code>raise MyException("Error
370 message")</code> or <code>raise MyException</code>. Do not
371 use the two-argument form (<code>raise MyException, "Error
372 message"</code>) or deprecated string-based exceptions
373 (<code>raise "Error message"</code>).</li>
374 <li>Modules or packages should define their own domain-specific
375 base exception class, which should inherit from the built-in
376 Exception class. The base exception for a module should be called
377 <code>Error</code>.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]378 <DIV class=""><PRE>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]379<span class="external"></span>class Error(Exception):
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]380 <span class="external"> </span>pass</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]381</li>
382 <li>Never use catch-all <code>except:</code> statements, or
383 catch <code>Exception</code> or <code>StandardError</code>,
384 unless you are re-raising the exception or in the outermost
385 block in your thread (and printing an error message). Python
386 is very tolerant in this regard and <code>except:</code> will
387 really catch everything including Python syntax errors. It is
388 easy to hide real bugs using <code>except:</code>.</li>
389 <li>Minimize the amount of code in a
390 <code>try</code>/<code>except</code> block. The larger the
391 body of the <code>try</code>, the more likely that an
392 exception will be raised by a line of code that you didn't
393 expect to raise an exception. In those cases,
394 the <code>try</code>/<code>except</code> block hides a real
395 error.</li>
396 <li>Use the <code>finally</code> clause to execute code whether
397 or not an exception is raised in the <code>try</code> block.
398 This is often useful for cleanup, i.e., closing a file.</li>
399 </ul>
400 </P>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]401 </DIV></DIV>
402 </DIV>
403 <DIV class="">
404<H3><A name="Global_variables" id="Global_variables">Global variables</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]405<SPAN class="link_button" id="link-Global_variables__button" name="link-Global_variables__button"><A href="?showone=Global_variables#Global_variables">
406 link
407 </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]408 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]409 Avoid global variables.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]410 </DIV>
411 <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]412 <P class="">
413<SPAN class="stylepoint_section">Definition: </SPAN>
414 Variables that are declared at the module level.
415 </P>
416 <P class="">
417<SPAN class="stylepoint_section">Pros: </SPAN>
418 Occasionally useful.
419 </P>
420 <P class="">
421<SPAN class="stylepoint_section">Cons: </SPAN>
422 Has the potential to change module behavior during the import,
423 because assignments to module-level variables are done when the
424 module is imported.
425 </P>
426 <P class="">
427<SPAN class="stylepoint_section">Decision: </SPAN>
428 Avoid global variables in favor of class variables. Some
429 exceptions are:
430 <ul>
431 <li>Default options for scripts.</li>
432 <li>Module-level constants. For example: <code>PI = 3.14159</code>.
433 Constants should be named using all caps with underscores;
434 see <a HREF="#Naming">Naming</a> below.</li>
435 <li>It is sometimes useful for globals to cache values needed
436 or returned by functions.</li>
437 <li>If needed, globals should be made internal to the module
438 and accessed through public module level functions;
439 see <a HREF="#Naming">Naming</a> below.</li>
440 </ul>
441 </P>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]442 </DIV></DIV>
443 </DIV>
444 <DIV class="">
445<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]446<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">
447 link
448 </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]449 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]450 Nested/local/inner classes and functions are fine.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]451 </DIV>
452 <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]453 <P class="">
454<SPAN class="stylepoint_section">Definition: </SPAN>
mmentovaif7facf92009-10-23 21:01:49 +0000[diff] [blame]455 A class can be defined inside of a method, function, or class. A
456 function can be defined inside a method or function. Nested functions
457 have read-only access to variables defined in enclosing scopes.
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]458 </P>
459 <P class="">
460<SPAN class="stylepoint_section">Pros: </SPAN>
461 Allows definition of utility classes and functions that are only
462 used inside of a very limited scope. Very <a HREF="http://en.wikipedia.org/wiki/Abstract_data_type">ADT</a>-y.
463 </P>
464 <P class="">
465<SPAN class="stylepoint_section">Cons: </SPAN>
466 Instances of nested or local classes cannot be pickled.
467 </P>
468 <P class="">
469<SPAN class="stylepoint_section">Decision: </SPAN>
470 They are fine.
471 </P>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]472 </DIV></DIV>
473 </DIV>
474 <DIV class="">
475<H3><A name="List_Comprehensions" id="List_Comprehensions">List Comprehensions</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]476<SPAN class="link_button" id="link-List_Comprehensions__button" name="link-List_Comprehensions__button"><A href="?showone=List_Comprehensions#List_Comprehensions">
477 link
478 </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]479 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]480 Okay to use for simple cases.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]481 </DIV>
482 <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]483 <P class="">
484<SPAN class="stylepoint_section">Definition: </SPAN>
485 List comprehensions and generator expressions provide a concise
486 and efficient way to create lists and iterators without
487 resorting to the use of <code>map()</code>,
488 <code>filter()</code>, or <code>lambda</code>.
489 </P>
490 <P class="">
491<SPAN class="stylepoint_section">Pros: </SPAN>
492 Simple list comprehensions can be clearer and simpler than
493 other list creation techniques. Generator expressions can be
494 very efficient, since they avoid the creation of a list
495 entirely.
496 </P>
497 <P class="">
498<SPAN class="stylepoint_section">Cons: </SPAN>
499 Complicated list comprehensions or generator expressions can be
500 hard to read.
501 </P>
502 <P class="">
503<SPAN class="stylepoint_section">Decision: </SPAN>
504 Okay to use for simple cases. Each portion must fit on one line:
505 mapping expression, <code>for</code> clause, filter expression.
506 Multiple <code>for</code> clauses or filter expressions are not
507 permitted. Use loops instead when things get more complicated.
508 </P>
509
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]510<DIV class=""><PRE class="badcode">No<span class="external"></span>:
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]511 <span class="external"></span>result = [(x, y) for x in range(10) for y in range(5) if x * y &gt; 10]
512
513 <span class="external"></span>return ((x, y, z)
514 <span class="external"></span> for x in xrange(5)
515 <span class="external"></span> for y in xrange(5)
516 <span class="external"></span> if x != y
517 <span class="external"></span> for z in xrange(5)
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]518 <span class="external"></span> if y != z)</PRE></DIV>
519<DIV class=""><PRE>Ye<span class="external"></span>s:
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]520 <span class="external"></span>result = []
521 <span class="external"></span>for x in range(10):
522 <span class="external"> </span>for y in range(5):
523 <span class="external"> </span>if x * y &gt; 10:
524 <span class="external"> </span>result.append((x, y))
525
526 <span class="external"></span>for x in xrange(5):
527 <span class="external"> </span>for y in xrange(5):
528 <span class="external"> </span>if x != y:
529 <span class="external"> </span>for z in xrange(5):
530 <span class="external"> </span>if y != z:
531 <span class="external"> </span>yield (x, y, z)
532
533 <span class="external"></span>return ((x, complicated_transform(x))
534 <span class="external"></span> for x in long_generator_function(parameter)
535 <span class="external"></span> if x is not None)
536
537 <span class="external"></span>squares = [x * x for x in range(10)]
538
539 <span class="external"></span>eat(jelly_bean for jelly_bean in jelly_beans
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]540 <span class="external"></span> if jelly_bean.color == 'black')</PRE></DIV>
541 </DIV></DIV>
542 </DIV>
543 <DIV class="">
544<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]545<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">
546 link
547 </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]548 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]549 Use default iterators and operators for types that support them,
550 like lists, dictionaries, and files.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]551 </DIV>
552 <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]553 <P class="">
554<SPAN class="stylepoint_section">Definition: </SPAN>
555 Container types, like dictionaries and lists, define default
556 iterators and membership test operators ("in" and "not in").
557 </P>
558 <P class="">
559<SPAN class="stylepoint_section">Pros: </SPAN>
560 The default iterators and operators are simple and efficient.
561 They express the operation directly, without extra method calls.
562 A function that uses default operators is generic. It can be
563 used with any type that supports the operation.
564 </P>
565 <P class="">
566<SPAN class="stylepoint_section">Cons: </SPAN>
567 You can't tell the type of objects by reading the method names
568 (e.g. has_key() means a dictionary). This is also an advantage.
569 </P>
570 <P class="">
571<SPAN class="stylepoint_section">Decision: </SPAN> Use default iterators and operators for types
572 that support them, like lists, dictionaries, and files. The
573 built-in types define iterator methods, too. Prefer these
574 methods to methods that return lists, except that you should not
575 mutate a container while iterating over it.
576
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]577<DIV class=""><PRE>Yes: <span class="external"></span>for key in adict: ...
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]578 <span class="external"></span>if key not in adict: ...
579 <span class="external"></span>if obj in alist: ...
580 <span class="external"></span>for line in afile: ...
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]581 <span class="external"></span>for k, v in dict.iteritems(): ...</PRE></DIV>
582<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]583 <span class="external"></span>if not adict.has_key(key): ...
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]584 <span class="external"></span>for line in afile.readlines(): ...</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]585 </P>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]586 </DIV></DIV>
587 </DIV>
588 <DIV class="">
589<H3><A name="Generators" id="Generators">Generators</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]590<SPAN class="link_button" id="link-Generators__button" name="link-Generators__button"><A href="?showone=Generators#Generators">
591 link
592 </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]593 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]594 Use generators as needed.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]595 </DIV>
596 <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]597 <P class="">
598<SPAN class="stylepoint_section">Definition: </SPAN>
599 A generator function returns an iterator that yields a value each
600 time it executes a yield statement. After it yields a value, the
601 runtime state of the generator function is suspended until the
602 next value is needed.
603 </P>
604 <P class="">
605<SPAN class="stylepoint_section">Pros: </SPAN>
606 Simpler code, because the state of local variables and control flow
607 are preserved for each call. A generator uses less memory than a
608 function that creates an entire list of values at once.
609 </P>
610 <P class="">
611<SPAN class="stylepoint_section">Cons: </SPAN>
612 None.
613 </P>
614 <P class="">
615<SPAN class="stylepoint_section">Decision: </SPAN>
616 Fine. Use "Yields:" rather than "Returns:" in the
617 doc string for generator functions.
618 </P>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]619 </DIV></DIV>
620 </DIV>
621 <DIV class="">
622<H3><A name="Lambda_Functions" id="Lambda_Functions">Lambda Functions</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]623<SPAN class="link_button" id="link-Lambda_Functions__button" name="link-Lambda_Functions__button"><A href="?showone=Lambda_Functions#Lambda_Functions">
624 link
625 </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]626 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]627 Okay for one-liners.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]628 </DIV>
629 <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]630 <P class="">
631<SPAN class="stylepoint_section">Definition: </SPAN>
632 Lambdas define anonymous functions in an expression, as
633 opposed to a statement. They are often used to define callbacks or
634 operators for higher-order functions like <code>map()</code> and
635 <code>filter()</code>.
636 </P>
637 <P class="">
638<SPAN class="stylepoint_section">Pros: </SPAN>
639 Convenient.
640 </P>
641 <P class="">
642<SPAN class="stylepoint_section">Cons: </SPAN> Harder to read and debug than local functions. The
643 lack of names means stack traces are more difficult to
644 understand. Expressiveness is limited because the function may
645 only contain an expression.
646 </P>
647 <P class="">
648<SPAN class="stylepoint_section">Decision: </SPAN>
649 Okay to use them for one-liners. If the code inside the lambda
650 function is any longer than 60–80 chars, it's probably better to
651 define it as a regular (nested) function.
652 <p>
653 For common operations like multiplication, use the functions from the
654 <code>operator</code> module instead of lambda functions. For
655 example, prefer <code>operator.mul</code> to <code>lambda
656 x, y: x * y</code>.
657 </p>
658 </P>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]659 </DIV></DIV>
660 </DIV>
661 <DIV class="">
662<H3><A name="Default_Argument_Values" id="Default_Argument_Values">Default Argument Values</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]663<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">
664 link
665 </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]666 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]667 Okay in most cases.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]668 </DIV>
669 <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]670 <P class="">
671<SPAN class="stylepoint_section">Definition: </SPAN>
672 You can specify values for variables at the end of a function's
673 parameter list, e.g., <code>def foo(a, b=0):</code>. If
674 <code>foo</code> is called with only one argument,
675 <code>b</code> is set to 0. If it is called with two arguments,
676 <code>b</code> has the value of the second argument.
677 </P>
678 <P class="">
679<SPAN class="stylepoint_section">Pros: </SPAN>
680 Often you have a function that uses lots of default values,
681 but—rarely—you want to override the
682 defaults. Default argument values provide an easy way to do this,
683 without having to define lots of functions for the rare
684 exceptions. Also, Python does not support overloaded
685 methods/functions and default arguments are an easy way of
686 "faking" the overloading behavior.
687 </P>
688 <P class="">
689<SPAN class="stylepoint_section">Cons: </SPAN>
690 Default arguments are evaluated once at module load
691 time. This may cause problems if the argument is a mutable
692 object such as a list or a dictionary. If the function modifies
693 the object (e.g., by appending an item to a list), the default
694 value is modified.
695 </P>
696 <P class="">
697<SPAN class="stylepoint_section">Decision: </SPAN>
698 Okay to use with the following caveats:
699 <p>
700 Do not use mutable objects as default values in the function or method
701 definition.
702 </p>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]703<DIV class=""><PRE>Yes: <span class="external"></span>def foo(a, b=None):
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]704 <span class="external"> </span>if b is None:
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]705 <span class="external"> </span>b = []</PRE></DIV>
706<DIV class=""><PRE class="badcode">No: <span class="external"></span>def foo(a, b=[]):
707 <span class="external"> </span>...</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]708 <p>
mmentovaif7facf92009-10-23 21:01:49 +0000[diff] [blame]709 Calling code must use named values for arguments with a default value.
710 This helps document the code somewhat and helps prevent and detect
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]711 interface breakage when more arguments are added.
712 </p>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]713<DIV class=""><PRE>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]714<span class="external"></span>def foo(a, b=1):
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]715 <span class="external"> </span>...</PRE></DIV>
716<DIV class=""><PRE>Yes: <span class="external"></span>foo(1)
717 <span class="external"></span>foo(1, b=2)</PRE></DIV>
718<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]719 </P>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]720 </DIV></DIV>
721 </DIV>
722 <DIV class="">
723<H3><A name="Properties" id="Properties">Properties</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]724<SPAN class="link_button" id="link-Properties__button" name="link-Properties__button"><A href="?showone=Properties#Properties">
725 link
726 </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]727 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]728 Use properties for accessing or setting data where you would
729 normally have used simple, lightweight accessor or setter methods.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]730 </DIV>
731 <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]732 <P class="">
733<SPAN class="stylepoint_section">Definition: </SPAN> A way to wrap method calls for getting and
734 setting an attribute as a standard attribute access when the
735 computation is lightweight.
736 </P>
737 <P class="">
738<SPAN class="stylepoint_section">Pros: </SPAN> Readability is increased by eliminating explicit
739 get and set method calls for simple attribute access. Allows
740 calculations to be lazy. Considered the Pythonic way to
741 maintain the interface of a class. In terms of performance,
742 allowing properties bypasses needing trivial accessor methods
743 when a direct variable access is reasonable. This also allows
744 accessor methods to be added in the future without breaking the
745 interface.
746 </P>
747 <P class="">
748<SPAN class="stylepoint_section">Cons: </SPAN> Properties are specified after the getter and
749 setter methods are declared, requiring one to notice they are
750 used for properties farther down in the code (except for readonly
751 properties created with the <code>@property</code> decorator - see
752 below). Must inherit from
753 <code>object</code>. Can hide side-effects much like operator
754 overloading. Can be confusing for subclasses.
755 </P>
756 <P class="">
757<SPAN class="stylepoint_section">Decision: </SPAN> Use properties in new code to access or
758 set data where you would normally have used simple, lightweight
759 accessor or setter methods. Read-only properties should be created
760 with the <code>@property</code>
761 <a HREF="#Function_and_Method_Decorators">decorator</a>.
762
763 <p><a id="properties-template-dp">
764 Inheritance with properties can be non-obvious if the property itself is
765 not overridden. Thus one must make sure that accessor methods are
766 called indirectly to ensure methods overridden in subclasses are called
767 by the property (using the Template Method DP).
768 </a></p>
769
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]770 <DIV class=""><PRE>Yes: <span class="external"></span>import math
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]771
772 <span class="external"></span>class Square(object):
773 <span class="external"> </span>"""A square with two properties: a writable area and a read-only perimeter.
774
775 <span class="external"> </span>To use:
776 <span class="external"> </span>&gt;&gt;&gt; sq = Square(3)
777 <span class="external"> </span>&gt;&gt;&gt; sq.area
778 <span class="external"> </span>9
779 <span class="external"> </span>&gt;&gt;&gt; sq.perimeter
780 <span class="external"> </span>12
781 <span class="external"> </span>&gt;&gt;&gt; sq.area = 16
782 <span class="external"> </span>&gt;&gt;&gt; sq.side
783 <span class="external"> </span>4
784 <span class="external"> </span>&gt;&gt;&gt; sq.perimeter
785 <span class="external"> </span>16
786 <span class="external"> </span>"""
787
788 <span class="external"> </span>def __init__(self, side):
789 <span class="external"> </span>self.side = side
790
791 <span class="external"> </span>def __get_area(self):
792 <span class="external"> </span>"""Calculates the 'area' property."""
793 <span class="external"> </span>return self.side ** 2
794
795 <span class="external"> </span>def ___get_area(self):
796 <span class="external"> </span>"""Indirect accessor for 'area' property."""
797 <span class="external"> </span>return self.__get_area()
798
799 <span class="external"> </span>def __set_area(self, area):
800 <span class="external"> </span>"""Sets the 'area' property."""
801 <span class="external"> </span>self.side = math.sqrt(area)
802
803 <span class="external"> </span>def ___set_area(self, area):
804 <span class="external"> </span>"""Indirect setter for 'area' property."""
805 <span class="external"> </span>self._SetArea(area)
806
807 <span class="external"> </span>area = property(___get_area, ___set_area,
808 <span class="external"> </span> doc="""Gets or sets the area of the square.""")
809
810 <span class="external"> </span>@property
811 <span class="external"> </span>def perimeter(self):
812 <span class="external"> </span>return self.side * 4
813<span class="external"></span>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]814</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]815 </P>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]816 </DIV></DIV>
817 </DIV>
818 <DIV class="">
819<H3><A name="True/False_evaluations" id="True/False_evaluations">True/False evaluations</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]820<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">
821 link
822 </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]823 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]824 Use the "implicit" false if at all possible.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]825 </DIV>
826 <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]827 <P class="">
828<SPAN class="stylepoint_section">Definition: </SPAN> Python evaluates certain values as <code>false</code>
829 when in a boolean context. A quick "rule of thumb" is that all
830 "empty" values are considered <code>false</code> so <code>0, None, [], {},
831 ""</code> all evaluate as <code>false</code> in a boolean context.
832 </P>
833 <P class="">
834<SPAN class="stylepoint_section">Pros: </SPAN> Conditions using Python booleans are easier to read
835 and less error-prone. In most cases, they're also faster.
836 </P>
837 <P class="">
838<SPAN class="stylepoint_section">Cons: </SPAN>
839 May look strange to C/C++ developers.
840 </P>
841 <P class="">
842<SPAN class="stylepoint_section">Decision: </SPAN>
843 Use the "implicit" false if at all possible, e.g., <code>if
844 foo:</code> rather than <code>if foo != []:</code>. There are a
845 few caveats that you should keep in mind though:
846 <ul>
847 <li>
848 Never use <code>==</code> or <code>!=</code> to compare
849 singletons like <code>None</code>. Use <code>is</code>
850 or <code>is not</code>.</li>
851
852 <li>Beware of writing <code>if x:</code> when you really mean
853 <code>if x is not None:</code>—e.g., when testing whether
854 a variable or argument that defaults to <code>None</code> was
855 set to some other value. The other value might be a value
856 that's false in a boolean context!</li>
857
858 <li>
859 Never compare a boolean variable to <code>False</code> using
860 <code>==</code>. Use <code>if not x:</code> instead. If
861 you need to distinguish <code>False</code> from
862 <code>None</code> then chain the expressions,
863 such as <code>if not x and x is not None:</code>.
864 </li>
865
866 <li>
867 For sequences (strings, lists, tuples), use the fact that
868 empty sequences are false, so <code>if not seq:</code> or
869 <code>if seq:</code> is preferable to <code>if
870 len(seq):</code> or <code>if not
871 len(seq):</code>.</li>
872
873 <li>
874 When handling integers, implicit false may involve more risk than
875 benefit (i.e., accidentally handling <code>None</code> as 0). You may
876 compare a value which is known to be an integer (and is not the
877 result of <code>len()</code>) against the integer 0.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]878<DIV class=""><PRE>Yes: <span class="external"></span>if not users:
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]879 <span class="external"> </span>print 'no users'
880
881 <span class="external"></span>if foo == 0:
882 <span class="external"> </span>self.handle_zero()
883
884 <span class="external"></span>if i % 10 == 0:
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]885 <span class="external"> </span>self.handle_multiple_of_ten()</PRE></DIV>
886<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]887 <span class="external"> </span>print 'no users'
888
889 <span class="external"></span>if foo is not None and not foo:
890 <span class="external"> </span>self.handle_zero()
891
892 <span class="external"></span>if not i % 10:
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]893 <span class="external"> </span>self.handle_multiple_of_ten()</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]894</li>
895
896 <li>
897 Note that <code>'0'</code> (i.e., <code>0</code> as string)
898 evaluates to true.</li>
899 </ul>
900 </P>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]901 </DIV></DIV>
902 </DIV>
903 <DIV class="">
904<H3><A name="Deprecated_Language_Features" id="Deprecated_Language_Features">Deprecated Language Features</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]905<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">
906 link
907 </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]908 <DIV style="display:inline;" class="">
mmentovaif7facf92009-10-23 21:01:49 +0000[diff] [blame]909 Use string methods instead of the <code>string</code> module
910 where possible. Use function call syntax instead
911 of <code>apply</code>. Use list comprehensions
912 and <code>for</code> loops instead of <code>filter</code>,
913 <code>map</code>, and <code>reduce</code>.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]914 </DIV>
915 <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]916 <P class="">
mmentovaif7facf92009-10-23 21:01:49 +0000[diff] [blame]917<SPAN class="stylepoint_section">Definition: </SPAN>
918 Current versions of Python provide alternative constructs
919 that people find generally preferable.
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]920 </P>
921 <P class="">
mmentovaif7facf92009-10-23 21:01:49 +0000[diff] [blame]922<SPAN class="stylepoint_section">Decision: </SPAN>
923 We do not use any Python version which does not support
924 these features, so there is no reason not to use the new
925 styles.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]926<DIV class=""><PRE class="badcode">No: <span class="external"></span>words = string.split(foo, ':')
mmentovaif7facf92009-10-23 21:01:49 +0000[diff] [blame]927
928 <span class="external"></span>map(lambda x: x[1], filter(lambda x: x[2] == 5, my_list))
929
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]930 <span class="external"></span>apply(fn, args, kwargs)</PRE></DIV>
931<DIV class=""><PRE>Yes: <span class="external"></span>words = foo.split(':')
mmentovaif7facf92009-10-23 21:01:49 +0000[diff] [blame]932
933 <span class="external"></span>[x[1] for x in my_list if x[2] == 5]
934
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]935 <span class="external"></span>fn(*args, **kwargs)</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]936 </P>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]937 </DIV></DIV>
938 </DIV>
939 <DIV class="">
940<H3><A name="Lexical_Scoping" id="Lexical_Scoping">Lexical Scoping</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]941<SPAN class="link_button" id="link-Lexical_Scoping__button" name="link-Lexical_Scoping__button"><A href="?showone=Lexical_Scoping#Lexical_Scoping">
942 link
943 </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]944 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]945 Okay to use.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]946 </DIV>
947 <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]948 <P class="">
949<SPAN class="stylepoint_section">Definition: </SPAN>
950 A nested Python function can refer to variables defined in
951 enclosing functions, but can not assign to them. Variable
952 bindings are resolved using lexical scoping, that is, based on
953 the static program text. Any assignment to a name in a block
954 will cause Python to treat all references to that name as a
955 local variable, even if the use precedes the assignment. If a
956 global declaration occurs, the name is treated as a global
957 variable.
958
959 <p>
960 An example of the use of this feature is:
961 </p>
962
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]963 <DIV class=""><PRE>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]964<span class="external"></span>def get_adder(summand1):
965 <span class="external"> </span>"""Returns a function that adds numbers to a given number."""
966 <span class="external"> </span>def adder(summand2):
967 <span class="external"> </span>return summand1 + summand2
968
969 <span class="external"> </span>return adder
970<span class="external"></span>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]971</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]972 </P>
973 <P class="">
974<SPAN class="stylepoint_section">Pros: </SPAN>
975 Often results in clearer, more elegant code. Especially comforting
976 to experienced Lisp and Scheme (and Haskell and ML and …)
977 programmers.
978 </P>
979 <P class="">
980<SPAN class="stylepoint_section">Cons: </SPAN>
981 Can lead to confusing bugs. Such as this example based on
982 <a HREF="http://www.python.org/dev/peps/pep-0227/">PEP-0227</a>:
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]983<DIV class=""><PRE class="badcode">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]984<span class="external"></span>i = 4
985<span class="external"></span>def foo(x):
986 <span class="external"> </span>def bar():
987 <span class="external"> </span>print i,
988 <span class="external"> </span># ...
989 <span class="external"> </span># A bunch of code here
990 <span class="external"> </span># ...
991 <span class="external"> </span>for i in x: # Ah, i *is* local to Foo, so this is what Bar sees
992 <span class="external"> </span>print i,
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]993 <span class="external"> </span>bar()</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]994 <p>
995 So <code>foo([1, 2, 3])</code> will print <code>1 2 3 3</code>, not
996 <code>1 2 3 4</code>.
997 </p>
998 </P>
999 <P class="">
1000<SPAN class="stylepoint_section">Decision: </SPAN>
1001 Okay to use.
1002 </P>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1003 </DIV></DIV>
1004 </DIV>
1005 <DIV class="">
1006<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]1007<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">
1008 link
1009 </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]1010 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1011 Use decorators judiciously when there is a clear advantage.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1012 </DIV>
1013 <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]1014 <P class="">
1015<SPAN class="stylepoint_section">Definition: </SPAN>
1016
1017 <a HREF="http://www.python.org/doc/2.4.3/whatsnew/node6.html">Decorators
1018 for Functions and Methods</a>
1019 (a.k.a "the <code>@</code> notation").
1020 The most common decorators are <code>@classmethod</code> and
1021 <code>@staticmethod</code>, for converting ordinary methods to class or
1022 static methods. However, the decorator syntax allows for
1023 user-defined decorators as well. Specifically, for some function
1024 <code>my_decorator</code>, this:
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1025 <DIV class=""><PRE>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1026<span class="external"></span>class C(object):
1027 <span class="external"> </span>@my_decorator
1028 <span class="external"> </span>def method(self):
1029 <span class="external"> </span># method body ...
1030<span class="external"></span>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1031</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1032
1033 is equivalent to:
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1034 <DIV class=""><PRE>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1035<span class="external"></span>class C(object):
1036 <span class="external"> </span>def method(self):
1037 <span class="external"> </span># method body ...
1038 <span class="external"> </span>method = my_decorator(method)
1039<span class="external"></span>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1040</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1041 </P>
1042 <P class="">
1043<SPAN class="stylepoint_section">Pros: </SPAN> Elegantly specifies some transformation on a method; the
1044 transformation might eliminate some repetitive code, enforce invariants,
1045 etc.
1046 </P>
1047 <P class="">
1048<SPAN class="stylepoint_section">Cons: </SPAN> Decorators can perform arbitrary operations on a
1049 function's arguments or return values, resulting in surprising
1050 implicit behavior.
1051 Additionally, decorators execute at import time. Failures in decorator
1052 code are pretty much impossible to recover from.
1053 </P>
1054 <P class="">
1055<SPAN class="stylepoint_section">Decision: </SPAN> Use decorators judiciously when there is a clear
1056 advantage. Decorators should follow the same import and naming
1057 guidelines as functions. Decorator pydoc should clearly state that the
1058 function is a decorator. Write unit tests for decorators.
1059
1060 <p>
1061 Avoid external dependencies in the decorator itself (e.g. don't rely on
1062 files, sockets, database connections, etc.), since they might not be
1063 available when the decorator runs (at import time, perhaps from
1064 <code>pychecker</code> or other tools). A decorator that is
1065 called with valid parameters should (as much as possible) be guaranteed
1066 to succeed in all cases.
1067 </p>
1068 <p>
1069 Decorators are a special case of "top level code" - see
1070 <a HREF="#Main">main</a> for more discussion.
1071 </p>
1072 </P>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1073 </DIV></DIV>
1074 </DIV>
1075 <DIV class="">
1076<H3><A name="Threading" id="Threading">Threading</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]1077<SPAN class="link_button" id="link-Threading__button" name="link-Threading__button"><A href="?showone=Threading#Threading">
1078 link
1079 </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]1080 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1081 Do not rely on the atomicity of built-in types.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1082 </DIV>
1083 <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]1084 <p>
1085 While Python's built-in data types such as dictionaries appear
1086 to have atomic operations, there are corner cases where they
1087 aren't atomic (e.g. if <code>__hash__</code> or
1088 <code>__eq__</code> are implemented as Python methods) and their
1089 atomicity should not be relied upon. Neither should you rely on
1090 atomic variable assignment (since this in turn depends on
1091 dictionaries).
1092 </p>
1093
1094 <p>
1095 Use the Queue module's <code>Queue</code> data type as the preferred
1096 way to
1097 communicate data between threads. Otherwise, use the threading
1098 module and its locking primitives. Learn about the proper use
1099 of condition variables so you can use
1100 <code>threading.Condition</code> instead of using lower-level
1101 locks.
1102 </p>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1103 </DIV></DIV>
1104 </DIV>
1105 <DIV class="">
1106<H3><A name="Power_Features" id="Power_Features">Power Features</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]1107<SPAN class="link_button" id="link-Power_Features__button" name="link-Power_Features__button"><A href="?showone=Power_Features#Power_Features">
1108 link
1109 </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]1110 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1111 Avoid these features.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1112 </DIV>
1113 <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]1114 <P class="">
1115<SPAN class="stylepoint_section">Definition: </SPAN> Python is an extremely flexible language and
1116 gives you many fancy features such as metaclasses, access to bytecode,
1117 on-the-fly compilation, dynamic inheritance, object reparenting,
1118 import hacks, reflection, modification of system internals,
1119 etc.
1120 </P>
1121 <P class="">
1122<SPAN class="stylepoint_section">Pros: </SPAN> These are powerful language features. They can
1123 make your code more compact.
1124 </P>
1125 <P class="">
1126<SPAN class="stylepoint_section">Cons: </SPAN> It's very tempting to use these "cool" features
1127 when they're not absolutely necessary. It's harder to read,
1128 understand, and debug code that's using unusual features
1129 underneath. It doesn't seem that way at first (to the original
1130 author), but when revisiting the code, it tends to be more
1131 difficult than code that is longer but is straightforward.
1132 </P>
1133 <P class="">
1134<SPAN class="stylepoint_section">Decision: </SPAN>
1135 Avoid these features in
1136 your code.
1137 </P>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1138 </DIV></DIV>
1139 </DIV>
1140 </DIV>
1141 <DIV class="">
1142<H2 name="Python_Style_Rules" id="Python_Style_Rules">Python Style Rules</H2>
1143 <DIV class="">
1144<H3><A name="Semicolons" id="Semicolons">Semicolons</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]1145<SPAN class="link_button" id="link-Semicolons__button" name="link-Semicolons__button"><A href="?showone=Semicolons#Semicolons">
1146 link
1147 </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]1148 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1149 Do not terminate your lines with semi-colons and do not use
1150 semi-colons to put two commands on the same line.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1151 </DIV>
1152 <DIV class=""><DIV class="stylepoint_body" name="Semicolons__body" id="Semicolons__body" style="display: none">
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1153 </DIV></DIV>
1154 </DIV>
1155 <DIV class="">
1156<H3><A name="Line_length" id="Line_length">Line length</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]1157<SPAN class="link_button" id="link-Line_length__button" name="link-Line_length__button"><A href="?showone=Line_length#Line_length">
1158 link
1159 </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]1160 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1161 Maximum line length is <em>80 characters</em>.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1162 </DIV>
1163 <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]1164 <p>
1165 Exception: lines importing modules may end up longer than 80
1166 characters only if using Python 2.4 or
1167 earlier.
1168 </p>
1169
1170 <p>
1171 Make use of Python's
1172
1173 <a HREF="http://www.python.org/doc/ref/implicit-joining.html">implicit
1174 line joining inside parentheses, brackets and braces</a>.
1175 If necessary, you can add an extra pair of parentheses around an
1176 expression.
1177 </p>
1178
1179
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1180 <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]1181 emphasis=None, highlight=0)
1182
1183 if (width == 0 and height == 0 and
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1184 color == 'red' and emphasis == 'strong'):</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1185
1186
1187 <p>
1188 When a literal string won't fit on a single line, use parentheses for
1189 implicit line joining.
1190 </p>
1191
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1192 <DIV class=""><PRE>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1193<span class="external"></span>x = ('This will build a very long long '
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1194<span class="external"></span> 'long long long long long long string')</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1195
1196 <p>
1197 Make note of the indentation of the elements in the line
1198 continuation examples above; see the
1199 <a HREF="#indentation">indentation</a>
1200 section for explanation.
1201 </p>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1202 </DIV></DIV>
1203 </DIV>
1204 <DIV class="">
1205<H3><A name="Parentheses" id="Parentheses">Parentheses</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]1206<SPAN class="link_button" id="link-Parentheses__button" name="link-Parentheses__button"><A href="?showone=Parentheses#Parentheses">
1207 link
1208 </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]1209 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1210 Use parentheses sparingly.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1211 </DIV>
1212 <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]1213 <p>
1214 Do not use them in return statements or conditional statements unless
1215 using parentheses for implied line continuation. (See above.)
1216 It is however fine to use parentheses around tuples.
1217 </p>
1218
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1219<DIV class=""><PRE>Yes: <span class="external"></span>if foo:
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1220 <span class="external"> </span>bar()
1221 <span class="external"></span>while x:
1222 <span class="external"> </span>x = bar()
1223 <span class="external"></span>if x and y:
1224 <span class="external"> </span>bar()
1225 <span class="external"></span>if not x:
1226 <span class="external"> </span>bar()
1227 <span class="external"></span>return foo
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1228 <span class="external"></span>for (x, y) in dict.items(): ...</PRE></DIV>
1229<DIV class=""><PRE class="badcode">No: <span class="external"></span>if (x):
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1230 <span class="external"> </span>bar()
1231 <span class="external"></span>if not(x):
1232 <span class="external"> </span>bar()
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1233 <span class="external"></span>return (foo)</PRE></DIV>
1234 </DIV></DIV>
1235 </DIV>
1236 <DIV class="">
1237<H3><A name="Indentation" id="Indentation">Indentation</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]1238<SPAN class="link_button" id="link-Indentation__button" name="link-Indentation__button"><A href="?showone=Indentation#Indentation">
1239 link
1240 </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]1241 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1242 Indent your code blocks with <em>4 spaces</em>.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1243 </DIV>
1244 <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]1245 <p>
1246 Never use tabs or mix tabs and spaces.
1247 In cases of implied line continuation, you should align wrapped elements
1248 either vertically, as per the examples in the
1249 <a HREF="#Line_length">line length</a> section; or using a hanging
1250 indent of 4 spaces, in which case there should be no argument on
1251 the first line.
1252 </p>
1253
1254
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1255<DIV class=""><PRE>Yes: # Aligned with opening delimiter
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1256 foo = long_function_name(var_one, var_two,
1257 var_three, var_four)
1258
1259 # 4-space hanging indent; nothing on first line
1260 foo = long_function_name(
1261 var_one, var_two, var_three,
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1262 var_four)</PRE></DIV>
1263<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]1264 <span class="external"></span>foo = long_function_name(var_one, var_two,
1265 <span class="external"></span> var_three, var_four)
1266
1267 <span class="external"></span># 2-space hanging indent forbidden
1268 <span class="external"></span>foo = long_function_name(
1269 <span class="external"></span> var_one, var_two, var_three,
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1270 <span class="external"></span> var_four)</PRE></DIV>
1271 </DIV></DIV>
1272 </DIV>
1273 <DIV class="">
1274<H3><A name="Blank_Lines" id="Blank_Lines">Blank Lines</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]1275<SPAN class="link_button" id="link-Blank_Lines__button" name="link-Blank_Lines__button"><A href="?showone=Blank_Lines#Blank_Lines">
1276 link
1277 </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]1278 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1279 Two blank lines between top-level definitions, one blank line
1280 between method definitions.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1281 </DIV>
1282 <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]1283 <p>
1284 Two blank lines between top-level definitions, be they function
1285 or class definitions. One blank line between method definitions
1286 and between the <code>class</code> line and the first method.
1287 Use single blank lines as you judge appropriate within functions or
1288 methods.
1289 </p>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1290 </DIV></DIV>
1291 </DIV>
1292 <DIV class="">
1293<H3><A name="Whitespace" id="Whitespace">Whitespace</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]1294<SPAN class="link_button" id="link-Whitespace__button" name="link-Whitespace__button"><A href="?showone=Whitespace#Whitespace">
1295 link
1296 </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]1297 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1298 Follow standard typographic rules for the use of spaces around
1299 punctuation.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1300 </DIV>
1301 <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]1302 <p>
1303 No whitespace inside parentheses, brackets or braces.
1304 </p>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1305<DIV class=""><PRE>Yes: <span class="external"></span>spam(ham[1], {eggs: 2}, [])</PRE></DIV>
1306<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]1307 <p>
1308 No whitespace before a comma, semicolon, or colon. Do use
1309 whitespace after a comma, semicolon, or colon except at the end
1310 of the line.
1311 </p>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1312<DIV class=""><PRE>Yes: <span class="external"></span>if x == 4:
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1313 <span class="external"> </span>print x, y
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1314 <span class="external"></span>x, y = y, x</PRE></DIV>
1315<DIV class=""><PRE class="badcode">No: <span class="external"></span>if x == 4 :
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1316 <span class="external"> </span>print x , y
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1317 <span class="external"></span>x , y = y , x</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1318 <p>
1319 No whitespace before the open paren/bracket that starts an argument list,
1320 indexing or slicing.
1321 </p>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1322 <DIV class=""><PRE>Yes: <span class="external"></span>spam(1)</PRE></DIV>
1323<DIV class=""><PRE class="badcode">No: <span class="external"></span>spam (1)</PRE></DIV>
1324<DIV class=""><PRE>Yes: <span class="external"></span>dict['key'] = list[index]</PRE></DIV>
1325<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]1326
1327 <p>
1328 Surround binary operators with a single space on either side for
1329 assignment (<code>=</code>), comparisons (<code>==, &lt;, &gt;, !=,
1330 &lt;&gt;, &lt;=, &gt;=, in, not in, is, is not</code>), and Booleans
1331 (<code>and, or, not</code>). Use your better judgment for the
1332 insertion of spaces around arithmetic operators but always be
1333 consistent about whitespace on either side of a binary operator.
1334 </p>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1335<DIV class=""><PRE>Yes: <span class="external"></span>x == 1</PRE></DIV>
1336<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]1337 <p>
1338 Don't use spaces around the '=' sign when used to indicate a
1339 keyword argument or a default parameter value.
1340 </p>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1341<DIV class=""><PRE>Yes: <span class="external"></span>def complex(real, imag=0.0): return magic(r=real, i=imag)</PRE></DIV>
1342<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]1343
1344 <p>
1345 Don't use spaces to vertically align tokens on consecutive lines, since it
1346 becomes a maintenance burden (applies to <code>:</code>, <code>#</code>,
1347 <code>=</code>, etc.):
1348 </p>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1349<DIV class=""><PRE>Yes:
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1350 foo = 1000 # comment
1351 long_name = 2 # comment that should not be aligned
1352
1353 dictionary = {
1354 "foo": 1,
1355 "long_name": 2,
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1356 }</PRE></DIV>
1357<DIV class=""><PRE class="badcode">No:
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1358 foo = 1000 # comment
1359 long_name = 2 # comment that should not be aligned
1360
1361 dictionary = {
1362 "foo" : 1,
1363 "long_name": 2,
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1364 }</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1365
1366
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1367 </DIV></DIV>
1368 </DIV>
1369 <DIV class="">
1370<H3><A name="Python_Interpreter" id="Python_Interpreter">Python Interpreter</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]1371<SPAN class="link_button" id="link-Python_Interpreter__button" name="link-Python_Interpreter__button"><A href="?showone=Python_Interpreter#Python_Interpreter">
1372 link
1373 </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]1374 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1375 Modules should begin with
1376
1377 <code>#!/usr/bin/env python&lt;version&gt;</code>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1378 </DIV>
1379 <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]1380 <p>
1381 Modules should begin with a "shebang" line specifying the Python
1382 interpreter used to execute the program:
1383 </p>
1384
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1385<DIV class=""><PRE>
1386<span class="external"></span>#!/usr/bin/env python2.4</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1387
1388 <p>
1389 Always use the most specific version you can use, e.g.,
1390 <code>/usr/bin/python2.4</code>, not
1391 <code>/usr/bin/python2</code>. This makes it easier to find
mmentovaidb989ec2010-11-23 18:02:36 +0000[diff] [blame]1392 dependencies when upgrading to a different Python version
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1393 and also avoids confusion and breakage during use. E.g., Does
mmentovaidb989ec2010-11-23 18:02:36 +0000[diff] [blame]1394 <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]1395 </p>
1396
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1397 </DIV></DIV>
1398 </DIV>
1399 <DIV class="">
1400<H3><A name="Comments" id="Comments">Comments</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]1401<SPAN class="link_button" id="link-Comments__button" name="link-Comments__button"><A href="?showone=Comments#Comments">
1402 link
1403 </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]1404 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1405 Be sure to use the right style for module, function, method and in-line
1406 comments.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1407 </DIV>
1408 <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]1409
1410 <P class="">
1411<SPAN class="stylepoint_subsection">Doc Strings</SPAN>
1412
1413 <p>
1414 Python has a unique commenting style using doc strings. A doc
1415 string is a string that is the first statement in a package,
1416 module, class or function. These strings can be extracted
1417 automatically through the <code>__doc__</code> member of the
1418 object and are used by <code>pydoc</code>. (Try running
1419 <code>pydoc</code> on your module to see how it looks.) Our
1420 convention for doc strings is to use the three double-quote
1421 format for strings. A doc string should be organized as a
1422 summary line (one physical line) terminated by a period,
1423 question mark, or exclamation point, followed by a blank line,
1424 followed by the rest of the doc string starting at the same
1425 cursor position as the first quote of the first line. There are
1426 more formatting guidelines for doc strings below.
1427 </p>
1428
1429 </P>
1430 <P class="">
1431<SPAN class="stylepoint_subsection">Modules</SPAN>
1432
1433
1434
1435 <p>
1436 Every file should contain the following items, in order:
1437 <ul>
1438 <li>a copyright statement (for example,
1439 <code>Copyright 2008 Google Inc.</code>)</li>
1440 <li>a license boilerplate. Choose the appropriate boilerplate
1441 for the license used by the project (for example, Apache 2.0, BSD,
1442 LGPL, GPL)</li>
1443 <li>an author line to identify the original author of the file</li>
1444 </ul>
1445 </p>
1446 </P>
1447 <P class="">
1448<SPAN class="stylepoint_subsection">Functions and Methods</SPAN>
1449
1450 <p>
1451 Any function or method which is not both obvious and very short
1452 needs a doc string. Additionally, any externally accessible
1453 function or method regardless of length or simplicity needs a
1454 doc string. The doc string should include what the function does
1455 and have detailed descriptions of the input and output. It
1456 should not, generally, describe how it does it unless it's some
1457 complicated algorithm. For tricky code block/inline comments
1458 within the code are more appropriate. The doc string should give
1459 enough information to write a call to the function without
1460 looking at a single line of the function's code. Args should be
1461 individually documented, an explanation following after a colon,
1462 and should use a uniform hanging indent of 2 or 4 spaces. The
1463 doc string should specify the expected types where specific types
1464 are required. A "Raises:" section should list all exceptions
1465 that can be raised by the function. The doc string for generator
1466 functions should use "Yields:" rather than "Returns:".
1467 </p>
1468
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1469 <DIV class=""><PRE>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1470<span class="external"></span>def fetch_bigtable_rows(big_table, keys, other_silly_variable=None):
1471 <span class="external"> </span>"""Fetches rows from a Bigtable.
1472
1473 <span class="external"> </span>Retrieves rows pertaining to the given keys from the Table instance
1474 <span class="external"> </span>represented by big_table. Silly things may happen if
1475 <span class="external"> </span>other_silly_variable is not None.
1476
1477 <span class="external"> </span>Args:
1478 <span class="external"> </span>big_table: An open Bigtable Table instance.
1479 <span class="external"> </span>keys: A sequence of strings representing the key of each table row
1480 <span class="external"> </span> to fetch.
1481 <span class="external"> </span>other_silly_variable: Another optional variable, that has a much
1482 <span class="external"> </span> longer name than the other args, and which does nothing.
1483
1484 <span class="external"> </span>Returns:
1485 <span class="external"> </span>A dict mapping keys to the corresponding table row data
1486 <span class="external"> </span>fetched. Each row is represented as a tuple of strings. For
1487 <span class="external"> </span>example:
1488
1489 <span class="external"> </span>{'Serak': ('Rigel VII', 'Preparer'),
1490 <span class="external"> </span> 'Zim': ('Irk', 'Invader'),
1491 <span class="external"> </span> 'Lrrr': ('Omicron Persei 8', 'Emperor')}
1492
1493 <span class="external"> </span>If a key from the keys argument is missing from the dictionary,
1494 <span class="external"> </span>then that row was not found in the table.
1495
1496 <span class="external"> </span>Raises:
1497 <span class="external"> </span>IOError: An error occurred accessing the bigtable.Table object.
1498 <span class="external"> </span>"""
1499 <span class="external"> </span>pass
1500<span class="external"></span>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1501</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1502 </P>
1503 <P class="">
1504<SPAN class="stylepoint_subsection">Classes</SPAN>
1505
1506 <p>
1507 Classes should have a doc string below the class definition describing
1508 the class. If your class has public attributes, they should be documented
1509 here in an Attributes section and follow the same formatting as a
1510 function's Args section.
1511 </p>
1512
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1513 <DIV class=""><PRE>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1514<span class="external"></span>class SampleClass(object):
1515 <span class="external"> </span>"""Summary of class here.
1516
1517 <span class="external"> </span>Longer class information....
1518 <span class="external"> </span>Longer class information....
1519
1520 <span class="external"> </span>Attributes:
1521 <span class="external"> </span>likes_spam: A boolean indicating if we like SPAM or not.
1522 <span class="external"> </span>eggs: An integer count of the eggs we have laid.
1523 <span class="external"> </span>"""
1524
1525 <span class="external"> </span>def __init__(self, likes_spam=False):
1526 <span class="external"> </span>"""Inits SampleClass with blah."""
1527 <span class="external"> </span>self.likes_spam = likes_spam
1528 <span class="external"> </span>self.eggs = 0
1529
1530 <span class="external"> </span>def public_method(self):
1531 <span class="external"> </span>"""Performs operation blah."""
1532<span class="external"></span>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1533</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1534
1535 </P>
1536 <P class="">
1537<SPAN class="stylepoint_subsection">Block and Inline Comments</SPAN>
1538
1539 <p>
1540 The final place to have comments is in tricky parts of the
1541 code. If you're going to have to explain it at the next
1542 <a HREF="http://en.wikipedia.org/wiki/Code_review">code review</a>,
1543 you should comment it now. Complicated operations get a few lines of
1544 comments before the operations
1545 commence. Non-obvious ones get comments at the end of the line.
1546 </p>
1547
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1548 <DIV class=""><PRE>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1549<span class="external"></span># We use a weighted dictionary search to find out where i is in
1550<span class="external"></span># the array. We extrapolate position based on the largest num
1551<span class="external"></span># in the array and the array size and then do binary search to
1552<span class="external"></span># get the exact number.
1553
1554<span class="external"></span>if i &amp; (i-1) == 0: # true iff i is a power of 2
1555<span class="external"></span>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1556</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1557
1558 <p>
1559 To improve legibility, these comments should be at least 2 spaces away
1560 from the code.
1561 </p>
1562
1563 <p>
1564 On the other hand, never describe the code. Assume the person
1565 reading the code knows Python (though not what you're trying to
1566 do) better than you do.
1567 </p>
1568
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1569 <DIV class=""><PRE class="badcode">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1570<span class="external"></span># BAD COMMENT: Now go through the b array and make sure whenever i occurs
1571<span class="external"></span># the next element is i+1
1572<span class="external"></span>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1573</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1574
1575 </P>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1576 </DIV></DIV>
1577 </DIV>
1578 <DIV class="">
1579<H3><A name="Classes" id="Classes">Classes</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]1580<SPAN class="link_button" id="link-Classes__button" name="link-Classes__button"><A href="?showone=Classes#Classes">
1581 link
1582 </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]1583 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1584 If a class inherits from no other base classes, explicitly inherit
1585 from <code>object</code>. This also applies to nested classes.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1586 </DIV>
1587 <DIV class=""><DIV class="stylepoint_body" name="Classes__body" id="Classes__body" style="display: none">
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1588 <DIV class=""><PRE class="badcode">No: <span class="external"></span>class SampleClass:
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1589 <span class="external"> </span>pass
1590
1591
1592 <span class="external"></span>class OuterClass:
1593
1594 <span class="external"> </span>class InnerClass:
1595 <span class="external"> </span>pass
1596<span class="external"></span>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1597</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1598
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1599 <DIV class=""><PRE>Yes: <span class="external"></span>class SampleClass(object):
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1600 <span class="external"> </span>pass
1601
1602
1603 <span class="external"></span>class OuterClass(object):
1604
1605 <span class="external"> </span>class InnerClass(object):
1606 <span class="external"> </span>pass
1607
1608
1609 <span class="external"></span>class ChildClass(ParentClass):
1610 <span class="external"> </span>"""Explicitly inherits from another class already."""
1611<span class="external"></span>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1612</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1613
1614 <p>Inheriting from <code>object</code> is needed to make properties work
1615 properly, and it will protect your code from one particular potential
1616 incompatibility with Python 3000. It also defines
1617 special methods that implement the default semantics of objects including
1618 <code>__new__</code>, <code>__init__</code>, <code>__delattr__</code>,
1619 <code>__getattribute__</code>, <code>__setattr__</code>,
1620 <code>__hash__</code>, <code>__repr__</code>, and <code>__str__</code>.
1621 </p>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1622 </DIV></DIV>
1623 </DIV>
1624 <DIV class="">
1625<H3><A name="Strings" id="Strings">Strings</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]1626<SPAN class="link_button" id="link-Strings__button" name="link-Strings__button"><A href="?showone=Strings#Strings">
1627 link
1628 </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]1629 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1630 Use the <code>%</code> operator for formatting strings,
1631 even when the parameters are all strings. Use your best judgement
1632 to decide between <code>+</code> and <code>%</code> though.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1633 </DIV>
1634 <DIV class=""><DIV class="stylepoint_body" name="Strings__body" id="Strings__body" style="display: none">
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1635<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]1636 <span class="external"></span>x = imperative + ', ' + expletive + '!'
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1637 <span class="external"></span>x = 'name: ' + name + '; score: ' + str(n)</PRE></DIV>
1638<DIV class=""><PRE>Yes: <span class="external"></span>x = a + b
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1639 <span class="external"></span>x = '%s, %s!' % (imperative, expletive)
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1640 <span class="external"></span>x = 'name: %s; score: %d' % (name, n)</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1641
1642 <p>
1643 Avoid using the <code>+</code> and <code>+=</code> operators to
1644 accumulate a string within a loop. Since strings are immutable, this
1645 creates unnecessary temporary objects and results in quadratic rather
1646 than linear running time. Instead, add each substring to a list and
1647 <code>''.join</code> the list after the loop terminates (or, write each
1648 substring to a <code>cStringIO.StringIO</code> buffer).
1649 </p>
1650
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1651<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]1652 <span class="external"></span>for last_name, first_name in employee_list:
1653 <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]1654 <span class="external"></span>employee_table += '&lt;/table&gt;'</PRE></DIV>
1655<DIV class=""><PRE>Yes: <span class="external"></span>items = ['&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>items.append('&lt;tr&gt;&lt;td&gt;%s, %s&lt;/td&gt;&lt;/tr&gt;' % (last_name, first_name))
1658 <span class="external"></span>items.append('&lt;/table&gt;')
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1659 <span class="external"></span>employee_table = ''.join(items)</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1660
1661 <p>
1662 Use <code>"""</code> for multi-line strings rather than
1663 <code>'''</code>. Note, however, that it is often cleaner to
1664 use implicit line joining since multi-line strings do
1665 not flow with the indentation of the rest of the program:
1666 </p>
1667
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1668 <DIV class=""><PRE class="badcode"> No<span class="external"></span>:
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1669 <span class="external"></span>print """This is pretty ugly.
1670Don'<span class="external"></span>t do this.
1671"""<span class="external"></span>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1672</PRE></DIV>
1673<DIV class=""><PRE>Ye<span class="external"></span>s:
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1674 <span class="external"></span>print ("This is much nicer.\n"
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1675 <span class="external"></span> "Do it this way.\n")</PRE></DIV>
1676 </DIV></DIV>
1677 </DIV>
1678 <DIV class="">
1679<H3><A name="TODO_Comments" id="TODO_Comments">TODO Comments</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]1680<SPAN class="link_button" id="link-TODO_Comments__button" name="link-TODO_Comments__button"><A href="?showone=TODO_Comments#TODO_Comments">
1681 link
1682 </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]1683 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1684 Use <code>TODO</code> comments for code that is temporary, a
1685 short-term solution, or good-enough but not perfect.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1686 </DIV>
1687 <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]1688 <p>
1689 <code>TODO</code>s should include the string <code>TODO</code> in
1690 all caps, followed by your
1691
1692 name, e-mail address, or other
1693 identifier
1694 in parentheses. A colon is optional. A comment explaining what there
1695 is to do is required. The main purpose is to have
1696 a consistent <code>TODO</code> format searchable by the person
1697 adding the comment (who can provide more details upon request). A
1698 <code>TODO</code> is not a commitment to provide the fix yourself.
1699 </p>
1700
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1701 <DIV class=""><PRE># TODO(kl@gmail.com): Drop the use of "has_key".
1702# TODO(Zeke) change this to use relations.</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1703 <p>
1704 If your <code>TODO</code> is of the form "At a future date do
1705 something" make sure that you either include a very specific
1706 date ("Fix by November 2009") or a very specific event
1707 ("Remove this code when all clients can handle XML responses.").
1708 </p>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1709 </DIV></DIV>
1710 </DIV>
1711 <DIV class="">
1712<H3><A name="Imports_formatting" id="Imports_formatting">Imports formatting</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]1713<SPAN class="link_button" id="link-Imports_formatting__button" name="link-Imports_formatting__button"><A href="?showone=Imports_formatting#Imports_formatting">
1714 link
1715 </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]1716 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1717 Imports should be on separate lines.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1718 </DIV>
1719 <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]1720 <p>
1721 E.g.:
1722 </p>
1723
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1724<DIV class=""><PRE>Yes: <span class="external"></span>import os
1725 <span class="external"></span>import sys</PRE></DIV>
1726<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]1727 <p>
1728 Imports are always put at the top of the file, just after any
1729 module comments and doc strings and before module globals and
1730 constants. Imports should be grouped with the order being most generic
1731 to least generic:
1732 </p>
1733 <ul>
1734 <li>standard library imports</li>
1735 <li>third-party imports</li>
1736
1737 <li>application-specific imports</li>
1738 </ul>
1739 <p>
1740 Within each grouping, imports should be sorted lexicographically,
1741 ignoring case, according to each module's full package path.
1742 </p>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1743 <DIV class=""><PRE>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1744<span class="external"></span>import foo
1745<span class="external"></span>from foo import bar
1746<span class="external"></span>from foo.bar import baz
1747<span class="external"></span>from foo.bar import Quux
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1748<span class="external"></span>from Foob import ar</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1749
1750
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1751
1752 </DIV></DIV>
1753 </DIV>
1754 <DIV class="">
1755<H3><A name="Statements" id="Statements">Statements</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]1756<SPAN class="link_button" id="link-Statements__button" name="link-Statements__button"><A href="?showone=Statements#Statements">
1757 link
1758 </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]1759 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1760 Generally only one statement per line.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1761 </DIV>
1762 <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]1763 <p>
1764 However, you may put the
1765 result of a test on the same line as the test only if the entire
1766 statement fits on one line. In particular, you can never do so
1767 with <code>try</code>/<code>except</code> since the
1768 <code>try</code> and <code>except</code> can't both fit on the
1769 same line, and you can only do so with an <code>if</code> if
1770 there is no <code>else</code>.
1771 </p>
1772
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1773 <DIV class=""><PRE>Ye<span class="external"></span>s:
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1774
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1775 <span class="external"></span>if foo: bar(foo)</PRE></DIV>
1776<DIV class=""><PRE class="badcode">No<span class="external"></span>:
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1777
1778 <span class="external"></span>if foo: bar(foo)
1779 <span class="external"></span>else: baz(foo)
1780
1781 <span class="external"></span>try: bar(foo)
1782 <span class="external"></span>except ValueError: baz(foo)
1783
1784 <span class="external"></span>try:
1785 <span class="external"> </span>bar(foo)
1786 <span class="external"></span>except ValueError: baz(foo)
1787<span class="external"></span>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1788</PRE></DIV>
1789 </DIV></DIV>
1790 </DIV>
1791 <DIV class="">
1792<H3><A name="Access_Control" id="Access_Control">Access Control</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]1793<SPAN class="link_button" id="link-Access_Control__button" name="link-Access_Control__button"><A href="?showone=Access_Control#Access_Control">
1794 link
1795 </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]1796 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1797 If an accessor function would be trivial you should use public variables
1798 instead of accessor functions to avoid the extra cost of function
1799 calls in Python. When more functionality is added you can use
1800 <code>property</code> to keep the syntax consistent.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1801 </DIV>
1802 <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]1803 <p>
1804 On the other hand, if access is more complex, or the cost of accessing
1805 the variable is significant, you should use function calls (following the
1806 <a HREF="#naming">Naming</a> guidelines) such as <code>get_foo()</code>
1807 and <code>set_foo()</code>. If the past behavior allowed access through a
1808 property, do not bind the new accessor functions to the property. Any
1809 code still attempting to access the variable by the old method should
1810 break visibly so they are made aware of the change in complexity.
1811 </p>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1812 </DIV></DIV>
1813 </DIV>
1814 <DIV class="">
1815<H3><A name="Naming" id="Naming">Naming</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]1816<SPAN class="link_button" id="link-Naming__button" name="link-Naming__button"><A href="?showone=Naming#Naming">
1817 link
1818 </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]1819 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1820 <code>module_name, package_name, ClassName, method_name, ExceptionName,
1821 function_name, GLOBAL_VAR_NAME, instance_var_name,
1822 function_parameter_name, local_var_name.</code>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1823 </DIV>
1824 <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]1825 <P class="">
1826<SPAN class="stylepoint_subsection">Names to Avoid</SPAN>
1827
1828 <ul>
1829 <li>single character names except for counters or iterators</li>
1830 <li>dashes (<code>-</code>) in any package/module name</li>
1831 <li>
1832<code>__double_leading_and_trailing_underscore__</code> names
1833 (reserved by Python)</li>
1834 </ul>
1835
1836 </P>
1837 <P class="">
1838<SPAN class="stylepoint_subsection">Naming Convention</SPAN>
1839
1840 <ul>
1841 <li>
1842 "Internal" means internal to a module or protected
1843 or private within a class.</li>
1844 <li>
1845 Prepending a single underscore (<code>_</code>) has some
1846 support for protecting module variables and functions (not included
1847 with <code>import * from</code>). Prepending a double underscore
1848 (<code>__</code>) to an instance variable or method
1849 effectively serves to make the variable or method private to its class
1850 (using name mangling).</li>
1851 <li>
1852 Place related classes and top-level functions together in a
1853 module. Unlike Java,
1854 there is no need to limit yourself to one class per module.</li>
1855 <li>
1856 Use CapWords for class names, but lower_with_under.py for module names.
1857 Although there are many existing modules named CapWords.py, this is now
1858 discouraged because it's confusing when the module happens to be
1859 named after a class. ("wait -- did I write
1860 <code>import StringIO</code> or <code>from StringIO import
1861 StringIO</code>?")</li>
1862 </ul>
1863
1864 </P>
1865 <P class="">
1866<SPAN class="stylepoint_subsection">Guidelines derived from Guido's Recommendations</SPAN>
1867
1868 <table rules="all" border="1" cellspacing="2" cellpadding="2">
1869
1870 <tr>
1871 <th>Type</th>
1872 <th>Public</th>
1873 <th>Internal</th>
1874 </tr>
1875
1876
1877
1878 <tr>
1879 <td>Packages</td>
1880 <td><code>lower_with_under</code></td>
1881 <td></td>
1882 </tr>
1883
1884 <tr>
1885 <td>Modules</td>
1886 <td><code>lower_with_under</code></td>
1887 <td><code>_lower_with_under</code></td>
1888 </tr>
1889
1890 <tr>
1891 <td>Classes</td>
1892 <td><code>CapWords</code></td>
1893 <td><code>_CapWords</code></td>
1894 </tr>
1895
1896 <tr>
1897 <td>Exceptions</td>
1898 <td><code>CapWords</code></td>
1899 <td></td>
1900 </tr>
1901
1902
1903
1904 <tr>
1905 <td>Functions</td>
1906 <td><code>lower_with_under()</code></td>
1907 <td><code>_lower_with_under()</code></td>
1908 </tr>
1909
1910 <tr>
1911 <td>Global/Class Constants</td>
1912 <td><code>CAPS_WITH_UNDER</code></td>
1913 <td><code>_CAPS_WITH_UNDER</code></td>
1914 </tr>
1915
1916 <tr>
1917 <td>Global/Class Variables</td>
1918 <td><code>lower_with_under</code></td>
1919 <td><code>_lower_with_under</code></td>
1920 </tr>
1921
1922 <tr>
1923 <td>Instance Variables</td>
1924 <td><code>lower_with_under</code></td>
1925 <td><code>_lower_with_under (protected) or __lower_with_under (private)</code></td>
1926 </tr>
1927
1928
1929
1930 <tr>
1931 <td>Method Names</td>
1932 <td><code>lower_with_under()</code></td>
1933 <td><code>_lower_with_under() (protected) or __lower_with_under() (private)</code></td>
1934 </tr>
1935
1936 <tr>
1937 <td>Function/Method Parameters</td>
1938 <td><code>lower_with_under</code></td>
1939 <td></td>
1940 </tr>
1941
1942 <tr>
1943 <td>Local Variables</td>
1944 <td><code>lower_with_under</code></td>
1945 <td></td>
1946 </tr>
1947
1948
1949 </table>
1950
1951
1952 </P>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1953 </DIV></DIV>
1954 </DIV>
1955 <DIV class="">
1956<H3><A name="Main" id="Main">Main</A></H3>
mmentovai8411f0c2010-08-04 17:46:16 +0000[diff] [blame]1957<SPAN class="link_button" id="link-Main__button" name="link-Main__button"><A href="?showone=Main#Main">
1958 link
1959 </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]1960 <DIV style="display:inline;" class="">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1961 Even a file meant to be used as a script should be importable and a
1962 mere import should not have the side effect of executing the script's
1963 main functionality. The main functionality should be in a main()
1964 function.
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1965 </DIV>
1966 <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]1967 <p>
1968 In Python,
1969 <code>pychecker</code>, <code>pydoc</code>, and unit tests
1970 require modules to be importable. Your code should always check
1971 <code>if __name__ == '__main__'</code> before executing your
1972 main program so that the main program is not executed when the
1973 module is imported.
1974
1975 </p>
1976
1977
1978
1979
1980
1981
1982
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1983 <DIV class=""><PRE>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1984<span class="external"></span>def main():
1985 <span class="external"> </span>...
1986
1987<span class="external"></span>if __name__ == '__main__':
1988 <span class="external"> </span>main()
1989<span class="external"></span>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1990</PRE></DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1991
1992 <p>
1993 All code at the top level will be executed when the module is
1994 imported. Be careful not to call functions, create objects, or
1995 perform other operations that should not be executed when the
1996 file is being <code>pycheck</code>ed or <code>pydoc</code>ed.
1997 </p>
mmentovai9ec7bd62009-12-03 22:25:38 +0000[diff] [blame]1998 </DIV></DIV>
1999 </DIV>
2000 </DIV>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]2001
2002<H2>Parting Words</H2>
2003 <p>
2004 <em>BE CONSISTENT</em>.
2005 </p>
2006
2007 <p>
2008 If you're editing code, take a few minutes to look at the code
2009 around you and determine its style. If they use spaces around
2010 all their arithmetic operators, you should too. If their
2011 comments have little boxes of hash marks around them, make your
2012 comments have little boxes of hash marks around them too.
2013 </p>
2014
2015 <p>
2016 The point of having style guidelines is to have a common vocabulary
2017 of coding so people can concentrate on what you're saying rather
2018 than on how you're saying it. We present global style rules here so
2019 people know the vocabulary, but local style is also important. If
2020 code you add to a file looks drastically different from the existing
2021 code around it, it throws readers out of their rhythm when they go to
2022 read it. Avoid this.
2023 </p>
2024
2025
2026
2027<p align="right">
mmentovaidb989ec2010-11-23 18:02:36 +0000[diff] [blame]2028Revision 2.18
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]2029</p>
2030
2031
2032<address>
2033 Amit Patel<br>
2034 Antoine Picard<br>
2035 Eugene Jhong<br>
2036 Jeremy Hylton<br>
2037 Matt Smart<br>
2038 Mike Shields<br>
2039</address>
2040</BODY>
2041</HTML>