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94<BODY>
95<H1>Google Python Style Guide</H1>
96 <p align="right">
97
98 Revision 2.12
99 </p>
100
101 <address>
102 Amit Patel<br>
103 Antoine Picard<br>
104 Eugene Jhong<br>
105 Jeremy Hylton<br>
106 Matt Smart<br>
107 Mike Shields<br>
108 </address>
109 <DIV style="margin-left: 50%; font-size: 75%;">
110<P>
111 Each style point has a summary for which additional information is available
112 by toggling the accompanying arrow button that looks this way:
113 <SPAN class="showhide_button" style="margin-left: 0;">▶</SPAN>.
114 You may toggle all summaries with the big arrow button:
115 </P>
116<DIV style=" font-size: larger; margin-left: +2em;">
117<SPAN class="showhide_button" style="font-size: 180%;" onclick="javascript:ShowHideAll()" name="show_hide_all_button" id="show_hide_all_button">▶</SPAN>
118 Toggle all summaries
119 </DIV>
120</DIV>
121<DIV class="toc">
122<DIV class="toc_title">Table of Contents</DIV>
123<TABLE>
124<TR valign="top" class="">
125<TD><DIV class="toc_category"><A href="#Python_Language_Rules">Python Language Rules</A></DIV></TD>
126<TD><DIV class="toc_stylepoint">
127<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="#Using_apply,_filter,_map,_reduce">Using apply, filter, map, reduce</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="#String_Methods">String Methods</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>
128</TR>
129<TR valign="top" class="">
130<TD><DIV class="toc_category"><A href="#Python_Style_Rules">Python Style Rules</A></DIV></TD>
131<TD><DIV class="toc_stylepoint">
132<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>
133</TR>
134</TABLE>
135</DIV>
136<H2>Overview</H2>
137 <SPAN class=""><H2 name="Important_Note" id="Important_Note">Important Note</H2>
138 <SPAN class=""><H3><A name="Displaying_Hidden_Details_in_this_Guide" id="Displaying_Hidden_Details_in_this_Guide">Displaying Hidden Details in this Guide</A></H3>
139<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>
140 <SPAN class="">
141 This style guide contains many details that are initially
142 hidden from view. They are marked by the triangle icon, which you
143 see here on your left. Click it now.
144 You should see "Hooray" appear below.
145 </SPAN>
146 <SPAN class=""><BR><SPAN class="stylepoint_body" name="Displaying_Hidden_Details_in_this_Guide__body" id="Displaying_Hidden_Details_in_this_Guide__body" style="display: none"><SPAN class="link_button"><A href="?showone=Displaying_Hidden_Details_in_this_Guide#Displaying_Hidden_Details_in_this_Guide">
147 link
148 </A></SPAN>
149 <p>
150 Hooray! Now you know you can expand points to get more
151 details. Alternatively, there's a "toggle all" at the
152 top of this document.
153 </p>
154 </SPAN></SPAN>
155 </SPAN>
156 </SPAN>
157 <SPAN class=""><H2 name="Background" id="Background">Background</H2>
158 <p>
159 Python is the main scripting language used at Google. This
160 style guide is a list of <em>do</em>s and <em>don't</em>s for Python
161 programs.
162 </p>
163
164
165
166 </SPAN>
167
168 <SPAN class=""><H2 name="Python_Language_Rules" id="Python_Language_Rules">Python Language Rules</H2>
169
170 <SPAN class=""><H3><A name="pychecker" id="pychecker">pychecker</A></H3>
171<SPAN class="showhide_button" onclick="javascript:ShowHideByName('pychecker__body','pychecker__button')" name="pychecker__button" id="pychecker__button">▶</SPAN>
172 <SPAN class="">
173 Run <code>pychecker</code> over your code.
174 </SPAN>
175 <SPAN class=""><BR><SPAN class="stylepoint_body" name="pychecker__body" id="pychecker__body" style="display: none"><SPAN class="link_button"><A href="?showone=pychecker#pychecker">
176 link
177 </A></SPAN>
178 <P class="">
179<SPAN class="stylepoint_section">Definition: </SPAN>
180 PyChecker is a tool for finding bugs in Python source code. It finds
181 problems that are typically caught by a compiler for less dynamic
182 languages like C and C++. It is similar to lint. Because of the
183 dynamic nature of Python, some warnings may be incorrect; however,
184 spurious warnings should be fairly infrequent.
185 </P>
186 <P class="">
187<SPAN class="stylepoint_section">Pros: </SPAN>
188 Catches easy-to-miss errors like typos, use-vars-before-assignment, etc.
189 </P>
190 <P class="">
191<SPAN class="stylepoint_section">Cons: </SPAN>
192 <code>pychecker</code> isn't perfect. To take
193 advantage of it, we'll need to sometimes: a) Write around it b)
194 Suppress its warnings c) Improve it or d) Ignore it.
195 </P>
196 <P class="">
197<SPAN class="stylepoint_section">Decision: </SPAN>
198 Make sure you run <code>pychecker</code> on your code.
199 </P>
200
201 <p>
202 For information on how to run <code>pychecker</code>, see the
203 <a HREF="http://pychecker.sourceforge.net">pychecker
204 homepage</a>
205 </p>
206 <p>
207 To suppress warnings, you can set a module-level variable named
208 <code>__pychecker__</code> to suppress appropriate warnings.
209 For example:
210 </p>
211 <SPAN class=""><PRE>
212<span class="external"></span>__pychecker__ = 'no-callinit no-classattr'</PRE></SPAN>
213 <p>
214 Suppressing in this way has the advantage that we can easily search
215 for suppressions and revisit them.
216 </p>
217 <p>
218 You can get a list of pychecker warnings by doing
219 <code>pychecker --help</code>.
220 </p>
221 <p>
222 Unused argument warnings can be suppressed by using `_' as the
223 identifier for the unused argument or prefixing the argument name with
224 `unused_'. In situations where changing the argument names is
225 infeasible, you can mention them at the beginning of the function.
226 For example:
227 </p>
228 <SPAN class=""><PRE>
229<span class="external"></span>def foo(a, unused_b, unused_c, d=None, e=None):
230 <span class="external"> </span>(d, e) = (d, e) # Silence pychecker
231 <span class="external"> </span>return a
232<span class="external"></span>
233</PRE></SPAN>
234 <p>
235 Ideally, pychecker would be extended to ensure that such `unused
236 declarations' were true.
237 </p>
238
239 </SPAN></SPAN>
240 </SPAN>
241 <SPAN class=""><H3><A name="Imports" id="Imports">Imports</A></H3>
242<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Imports__body','Imports__button')" name="Imports__button" id="Imports__button">▶</SPAN>
243 <SPAN class="">
244 Use <code>import</code>s for packages and modules only.
245 </SPAN>
246 <SPAN class=""><BR><SPAN class="stylepoint_body" name="Imports__body" id="Imports__body" style="display: none"><SPAN class="link_button"><A href="?showone=Imports#Imports">
247 link
248 </A></SPAN>
249 <P class="">
250<SPAN class="stylepoint_section">Definition: </SPAN>
251 Reusability mechanism for sharing code from one module to another.
252 </P>
253 <P class="">
254<SPAN class="stylepoint_section">Pros: </SPAN>
255 Simplest and most commonly used way of sharing things.
256 </P>
257 <P class="">
258<SPAN class="stylepoint_section">Cons: </SPAN> <code>from foo import *</code> or
259 <code>from foo import Bar</code> is
260 very nasty and can lead to serious maintenance issues because
261 it makes it hard to find module dependencies.
262 </P>
263 <P class="">
264<SPAN class="stylepoint_section">Decision: </SPAN>
265 Use <code>import x</code> for importing packages and modules.
266 Use <code>from x import y</code> only when <code>x</code> is a
267 package and <code>y</code> is a module. This allows the
268 importer to refer to the module without specifying the full
269 package prefix. For example the module
270 <code>sound.effects.echo</code> may be imported as follows:
271 </P>
272 <SPAN class=""><PRE>
273<span class="external"></span>from sound.effects import echo
274<span class="external"></span>...
275<span class="external"></span>echo.echofilter(input, output, delay=0.7, atten=4)
276<span class="external"></span>
277</PRE></SPAN>
278 <p>
279 Even if the module is in the same package, do not directly import
280 the module without the full package name. This might cause the
281 package to be imported twice (with unintended side effects) when the
282 "main" module that is used to start an application lives inside a
283 package (and uses modules from that same package).
284 </p>
285 </SPAN></SPAN>
286 </SPAN>
287 <SPAN class=""><H3><A name="Packages" id="Packages">Packages</A></H3>
288<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Packages__body','Packages__button')" name="Packages__button" id="Packages__button">▶</SPAN>
289 <SPAN class="">
290 Import and refer to each module using the full pathname location of
291 that module.
292 </SPAN>
293 <SPAN class=""><BR><SPAN class="stylepoint_body" name="Packages__body" id="Packages__body" style="display: none"><SPAN class="link_button"><A href="?showone=Packages#Packages">
294 link
295 </A></SPAN>
296 <P class="">
297<SPAN class="stylepoint_section">Pros: </SPAN>
298 Avoids conflicts in module names. Makes it easier to find modules.
299 </P>
300 <P class="">
301<SPAN class="stylepoint_section">Cons: </SPAN>
302 Makes it harder to deploy code because you have to replicate the
303 package hierarchy.
304 </P>
305 <P class="">
306<SPAN class="stylepoint_section">Decision: </SPAN>
307 All new code should refer to modules based on their package
308 name.
309 </P>
310 <p>
311 Imports should be as follows:
312 </p>
313
314
315 <SPAN class=""><PRE># Reference in code with complete name.
316import sound.effects.echo
317
318# Reference in code with just module name.
319from sound.effects import echo
320</PRE></SPAN>
321
322 </SPAN></SPAN>
323 </SPAN>
324 <SPAN class=""><H3><A name="Exceptions" id="Exceptions">Exceptions</A></H3>
325<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Exceptions__body','Exceptions__button')" name="Exceptions__button" id="Exceptions__button">▶</SPAN>
326 <SPAN class="">
327 Exceptions are allowed but must be used carefully.
328 </SPAN>
329 <SPAN class=""><BR><SPAN class="stylepoint_body" name="Exceptions__body" id="Exceptions__body" style="display: none"><SPAN class="link_button"><A href="?showone=Exceptions#Exceptions">
330 link
331 </A></SPAN>
332 <P class="">
333<SPAN class="stylepoint_section">Definition: </SPAN>
334 Exceptions are a means of breaking out of the normal flow of control
335 of a code block to handle errors or other exceptional conditions.
336 </P>
337 <P class="">
338<SPAN class="stylepoint_section">Pros: </SPAN>
339 The control flow of normal operation code is not cluttered by
340 error-handling code. It also allows the control flow to skip multiple
341 frames when a certain condition occurs, e.g., returning from N
342 nested functions in one step instead of having to carry-through
343 error codes.
344 </P>
345 <P class="">
346<SPAN class="stylepoint_section">Cons: </SPAN>
347 May cause the control flow to be confusing. Easy to miss error
348 cases when making library calls.
349 </P>
350 <P class="">
351<SPAN class="stylepoint_section">Decision: </SPAN>
352
353
354 Exceptions must follow certain conditions:
355
356 <ul>
357 <li>Raise exceptions like this: <code>raise MyException("Error
358 message")</code> or <code>raise MyException</code>. Do not
359 use the two-argument form (<code>raise MyException, "Error
360 message"</code>) or deprecated string-based exceptions
361 (<code>raise "Error message"</code>).</li>
362 <li>Modules or packages should define their own domain-specific
363 base exception class, which should inherit from the built-in
364 Exception class. The base exception for a module should be called
365 <code>Error</code>.
366 <SPAN class=""><PRE>
367<span class="external"></span>class Error(Exception):
368 <span class="external"> </span>pass</PRE></SPAN>
369</li>
370 <li>Never use catch-all <code>except:</code> statements, or
371 catch <code>Exception</code> or <code>StandardError</code>,
372 unless you are re-raising the exception or in the outermost
373 block in your thread (and printing an error message). Python
374 is very tolerant in this regard and <code>except:</code> will
375 really catch everything including Python syntax errors. It is
376 easy to hide real bugs using <code>except:</code>.</li>
377 <li>Minimize the amount of code in a
378 <code>try</code>/<code>except</code> block. The larger the
379 body of the <code>try</code>, the more likely that an
380 exception will be raised by a line of code that you didn't
381 expect to raise an exception. In those cases,
382 the <code>try</code>/<code>except</code> block hides a real
383 error.</li>
384 <li>Use the <code>finally</code> clause to execute code whether
385 or not an exception is raised in the <code>try</code> block.
386 This is often useful for cleanup, i.e., closing a file.</li>
387 </ul>
388 </P>
389 </SPAN></SPAN>
390 </SPAN>
391 <SPAN class=""><H3><A name="Global_variables" id="Global_variables">Global variables</A></H3>
392<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Global_variables__body','Global_variables__button')" name="Global_variables__button" id="Global_variables__button">▶</SPAN>
393 <SPAN class="">
394 Avoid global variables.
395 </SPAN>
396 <SPAN class=""><BR><SPAN class="stylepoint_body" name="Global_variables__body" id="Global_variables__body" style="display: none"><SPAN class="link_button"><A href="?showone=Global_variables#Global_variables">
397 link
398 </A></SPAN>
399 <P class="">
400<SPAN class="stylepoint_section">Definition: </SPAN>
401 Variables that are declared at the module level.
402 </P>
403 <P class="">
404<SPAN class="stylepoint_section">Pros: </SPAN>
405 Occasionally useful.
406 </P>
407 <P class="">
408<SPAN class="stylepoint_section">Cons: </SPAN>
409 Has the potential to change module behavior during the import,
410 because assignments to module-level variables are done when the
411 module is imported.
412 </P>
413 <P class="">
414<SPAN class="stylepoint_section">Decision: </SPAN>
415 Avoid global variables in favor of class variables. Some
416 exceptions are:
417 <ul>
418 <li>Default options for scripts.</li>
419 <li>Module-level constants. For example: <code>PI = 3.14159</code>.
420 Constants should be named using all caps with underscores;
421 see <a HREF="#Naming">Naming</a> below.</li>
422 <li>It is sometimes useful for globals to cache values needed
423 or returned by functions.</li>
424 <li>If needed, globals should be made internal to the module
425 and accessed through public module level functions;
426 see <a HREF="#Naming">Naming</a> below.</li>
427 </ul>
428 </P>
429 </SPAN></SPAN>
430 </SPAN>
431 <SPAN class=""><H3><A name="Nested/Local/Inner_Classes_and_Functions" id="Nested/Local/Inner_Classes_and_Functions">Nested/Local/Inner Classes and Functions</A></H3>
432<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>
433 <SPAN class="">
434 Nested/local/inner classes and functions are fine.
435 </SPAN>
436 <SPAN class=""><BR><SPAN class="stylepoint_body" name="Nested/Local/Inner_Classes_and_Functions__body" id="Nested/Local/Inner_Classes_and_Functions__body" style="display: none"><SPAN class="link_button"><A href="?showone=Nested/Local/Inner_Classes_and_Functions#Nested/Local/Inner_Classes_and_Functions">
437 link
438 </A></SPAN>
439 <P class="">
440<SPAN class="stylepoint_section">Definition: </SPAN>
441 A class can be defined inside of a function or class. A function
442 can be defined inside a function. Nested functions have read-only
443 access to variables defined in enclosing scopes.
444 </P>
445 <P class="">
446<SPAN class="stylepoint_section">Pros: </SPAN>
447 Allows definition of utility classes and functions that are only
448 used inside of a very limited scope. Very <a HREF="http://en.wikipedia.org/wiki/Abstract_data_type">ADT</a>-y.
449 </P>
450 <P class="">
451<SPAN class="stylepoint_section">Cons: </SPAN>
452 Instances of nested or local classes cannot be pickled.
453 </P>
454 <P class="">
455<SPAN class="stylepoint_section">Decision: </SPAN>
456 They are fine.
457 </P>
458 </SPAN></SPAN>
459 </SPAN>
460 <SPAN class=""><H3><A name="List_Comprehensions" id="List_Comprehensions">List Comprehensions</A></H3>
461<SPAN class="showhide_button" onclick="javascript:ShowHideByName('List_Comprehensions__body','List_Comprehensions__button')" name="List_Comprehensions__button" id="List_Comprehensions__button">▶</SPAN>
462 <SPAN class="">
463 Okay to use for simple cases.
464 </SPAN>
465 <SPAN class=""><BR><SPAN class="stylepoint_body" name="List_Comprehensions__body" id="List_Comprehensions__body" style="display: none"><SPAN class="link_button"><A href="?showone=List_Comprehensions#List_Comprehensions">
466 link
467 </A></SPAN>
468 <P class="">
469<SPAN class="stylepoint_section">Definition: </SPAN>
470 List comprehensions and generator expressions provide a concise
471 and efficient way to create lists and iterators without
472 resorting to the use of <code>map()</code>,
473 <code>filter()</code>, or <code>lambda</code>.
474 </P>
475 <P class="">
476<SPAN class="stylepoint_section">Pros: </SPAN>
477 Simple list comprehensions can be clearer and simpler than
478 other list creation techniques. Generator expressions can be
479 very efficient, since they avoid the creation of a list
480 entirely.
481 </P>
482 <P class="">
483<SPAN class="stylepoint_section">Cons: </SPAN>
484 Complicated list comprehensions or generator expressions can be
485 hard to read.
486 </P>
487 <P class="">
488<SPAN class="stylepoint_section">Decision: </SPAN>
489 Okay to use for simple cases. Each portion must fit on one line:
490 mapping expression, <code>for</code> clause, filter expression.
491 Multiple <code>for</code> clauses or filter expressions are not
492 permitted. Use loops instead when things get more complicated.
493 </P>
494
495<SPAN class=""><PRE class="badcode">No<span class="external"></span>:
496 <span class="external"></span>result = [(x, y) for x in range(10) for y in range(5) if x * y &gt; 10]
497
498 <span class="external"></span>return ((x, y, z)
499 <span class="external"></span> for x in xrange(5)
500 <span class="external"></span> for y in xrange(5)
501 <span class="external"></span> if x != y
502 <span class="external"></span> for z in xrange(5)
503 <span class="external"></span> if y != z)</PRE></SPAN>
504<SPAN class=""><PRE>Ye<span class="external"></span>s:
505 <span class="external"></span>result = []
506 <span class="external"></span>for x in range(10):
507 <span class="external"> </span>for y in range(5):
508 <span class="external"> </span>if x * y &gt; 10:
509 <span class="external"> </span>result.append((x, y))
510
511 <span class="external"></span>for x in xrange(5):
512 <span class="external"> </span>for y in xrange(5):
513 <span class="external"> </span>if x != y:
514 <span class="external"> </span>for z in xrange(5):
515 <span class="external"> </span>if y != z:
516 <span class="external"> </span>yield (x, y, z)
517
518 <span class="external"></span>return ((x, complicated_transform(x))
519 <span class="external"></span> for x in long_generator_function(parameter)
520 <span class="external"></span> if x is not None)
521
522 <span class="external"></span>squares = [x * x for x in range(10)]
523
524 <span class="external"></span>eat(jelly_bean for jelly_bean in jelly_beans
525 <span class="external"></span> if jelly_bean.color == 'black')</PRE></SPAN>
526 </SPAN></SPAN>
527 </SPAN>
528 <SPAN class=""><H3><A name="Default_Iterators_and_Operators" id="Default_Iterators_and_Operators">Default Iterators and Operators</A></H3>
529<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>
530 <SPAN class="">
531 Use default iterators and operators for types that support them,
532 like lists, dictionaries, and files.
533 </SPAN>
534 <SPAN class=""><BR><SPAN class="stylepoint_body" name="Default_Iterators_and_Operators__body" id="Default_Iterators_and_Operators__body" style="display: none"><SPAN class="link_button"><A href="?showone=Default_Iterators_and_Operators#Default_Iterators_and_Operators">
535 link
536 </A></SPAN>
537 <P class="">
538<SPAN class="stylepoint_section">Definition: </SPAN>
539 Container types, like dictionaries and lists, define default
540 iterators and membership test operators ("in" and "not in").
541 </P>
542 <P class="">
543<SPAN class="stylepoint_section">Pros: </SPAN>
544 The default iterators and operators are simple and efficient.
545 They express the operation directly, without extra method calls.
546 A function that uses default operators is generic. It can be
547 used with any type that supports the operation.
548 </P>
549 <P class="">
550<SPAN class="stylepoint_section">Cons: </SPAN>
551 You can't tell the type of objects by reading the method names
552 (e.g. has_key() means a dictionary). This is also an advantage.
553 </P>
554 <P class="">
555<SPAN class="stylepoint_section">Decision: </SPAN> Use default iterators and operators for types
556 that support them, like lists, dictionaries, and files. The
557 built-in types define iterator methods, too. Prefer these
558 methods to methods that return lists, except that you should not
559 mutate a container while iterating over it.
560
561<SPAN class=""><PRE>Yes: <span class="external"></span>for key in adict: ...
562 <span class="external"></span>if key not in adict: ...
563 <span class="external"></span>if obj in alist: ...
564 <span class="external"></span>for line in afile: ...
565 <span class="external"></span>for k, v in dict.iteritems(): ...</PRE></SPAN>
566<SPAN class=""><PRE class="badcode">No: <span class="external"></span>for key in adict.keys(): ...
567 <span class="external"></span>if not adict.has_key(key): ...
568 <span class="external"></span>for line in afile.readlines(): ...</PRE></SPAN>
569 </P>
570 </SPAN></SPAN>
571 </SPAN>
572 <SPAN class=""><H3><A name="Generators" id="Generators">Generators</A></H3>
573<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Generators__body','Generators__button')" name="Generators__button" id="Generators__button">▶</SPAN>
574 <SPAN class="">
575 Use generators as needed.
576 </SPAN>
577 <SPAN class=""><BR><SPAN class="stylepoint_body" name="Generators__body" id="Generators__body" style="display: none"><SPAN class="link_button"><A href="?showone=Generators#Generators">
578 link
579 </A></SPAN>
580 <P class="">
581<SPAN class="stylepoint_section">Definition: </SPAN>
582 A generator function returns an iterator that yields a value each
583 time it executes a yield statement. After it yields a value, the
584 runtime state of the generator function is suspended until the
585 next value is needed.
586 </P>
587 <P class="">
588<SPAN class="stylepoint_section">Pros: </SPAN>
589 Simpler code, because the state of local variables and control flow
590 are preserved for each call. A generator uses less memory than a
591 function that creates an entire list of values at once.
592 </P>
593 <P class="">
594<SPAN class="stylepoint_section">Cons: </SPAN>
595 None.
596 </P>
597 <P class="">
598<SPAN class="stylepoint_section">Decision: </SPAN>
599 Fine. Use "Yields:" rather than "Returns:" in the
600 doc string for generator functions.
601 </P>
602 </SPAN></SPAN>
603 </SPAN>
604 <SPAN class=""><H3><A name="Using_apply,_filter,_map,_reduce" id="Using_apply,_filter,_map,_reduce">Using apply, filter, map, reduce</A></H3>
605<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Using_apply,_filter,_map,_reduce__body','Using_apply,_filter,_map,_reduce__button')" name="Using_apply,_filter,_map,_reduce__button" id="Using_apply,_filter,_map,_reduce__button">▶</SPAN>
606 <SPAN class="">
607 Avoid in favor of list comprehensions and for-loops.
608 </SPAN>
609 <SPAN class=""><BR><SPAN class="stylepoint_body" name="Using_apply,_filter,_map,_reduce__body" id="Using_apply,_filter,_map,_reduce__body" style="display: none"><SPAN class="link_button"><A href="?showone=Using_apply,_filter,_map,_reduce#Using_apply,_filter,_map,_reduce">
610 link
611 </A></SPAN>
612 <P class="">
613<SPAN class="stylepoint_section">Definition: </SPAN> Built-in functions useful for manipulating
614 lists. Commonly used in conjunction with <code>lambda</code>
615 functions.
616 </P>
617 <P class="">
618<SPAN class="stylepoint_section">Pros: </SPAN>
619 Code is compact.
620 </P>
621 <P class="">
622<SPAN class="stylepoint_section">Cons: </SPAN>
623 Higher-order functional programming tends to be harder to understand.
624 </P>
625 <P class="">
626<SPAN class="stylepoint_section">Decision: </SPAN> Use list comprehensions when possible and limit
627 use to simple code and one-liners. In general, if such code
628 grows longer than 60–80 chars or if it uses
629 multi-level function calls (e.g., <code>map(lambda x: x[1],
630 filter(…))</code>), that's a signal that you are better
631 off writing a regular loop instead. Compare:
632<SPAN class=""><PRE>Yes: <span class="external"></span>list comprehensions: [x[1] for x in my_list if x[2] == 5]</PRE></SPAN>
633<SPAN class=""><PRE class="badcode">No: <span class="external"></span>map/filter: map(lambda x: x[1], filter(lambda x: x[2] == 5, my_list))</PRE></SPAN>
634 </P>
635 </SPAN></SPAN>
636 </SPAN>
637 <SPAN class=""><H3><A name="Lambda_Functions" id="Lambda_Functions">Lambda Functions</A></H3>
638<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Lambda_Functions__body','Lambda_Functions__button')" name="Lambda_Functions__button" id="Lambda_Functions__button">▶</SPAN>
639 <SPAN class="">
640 Okay for one-liners.
641 </SPAN>
642 <SPAN class=""><BR><SPAN class="stylepoint_body" name="Lambda_Functions__body" id="Lambda_Functions__body" style="display: none"><SPAN class="link_button"><A href="?showone=Lambda_Functions#Lambda_Functions">
643 link
644 </A></SPAN>
645 <P class="">
646<SPAN class="stylepoint_section">Definition: </SPAN>
647 Lambdas define anonymous functions in an expression, as
648 opposed to a statement. They are often used to define callbacks or
649 operators for higher-order functions like <code>map()</code> and
650 <code>filter()</code>.
651 </P>
652 <P class="">
653<SPAN class="stylepoint_section">Pros: </SPAN>
654 Convenient.
655 </P>
656 <P class="">
657<SPAN class="stylepoint_section">Cons: </SPAN> Harder to read and debug than local functions. The
658 lack of names means stack traces are more difficult to
659 understand. Expressiveness is limited because the function may
660 only contain an expression.
661 </P>
662 <P class="">
663<SPAN class="stylepoint_section">Decision: </SPAN>
664 Okay to use them for one-liners. If the code inside the lambda
665 function is any longer than 60–80 chars, it's probably better to
666 define it as a regular (nested) function.
667 <p>
668 For common operations like multiplication, use the functions from the
669 <code>operator</code> module instead of lambda functions. For
670 example, prefer <code>operator.mul</code> to <code>lambda
671 x, y: x * y</code>.
672 </p>
673 </P>
674 </SPAN></SPAN>
675 </SPAN>
676 <SPAN class=""><H3><A name="Default_Argument_Values" id="Default_Argument_Values">Default Argument Values</A></H3>
677<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>
678 <SPAN class="">
679 Okay in most cases.
680 </SPAN>
681 <SPAN class=""><BR><SPAN class="stylepoint_body" name="Default_Argument_Values__body" id="Default_Argument_Values__body" style="display: none"><SPAN class="link_button"><A href="?showone=Default_Argument_Values#Default_Argument_Values">
682 link
683 </A></SPAN>
684 <P class="">
685<SPAN class="stylepoint_section">Definition: </SPAN>
686 You can specify values for variables at the end of a function's
687 parameter list, e.g., <code>def foo(a, b=0):</code>. If
688 <code>foo</code> is called with only one argument,
689 <code>b</code> is set to 0. If it is called with two arguments,
690 <code>b</code> has the value of the second argument.
691 </P>
692 <P class="">
693<SPAN class="stylepoint_section">Pros: </SPAN>
694 Often you have a function that uses lots of default values,
695 but—rarely—you want to override the
696 defaults. Default argument values provide an easy way to do this,
697 without having to define lots of functions for the rare
698 exceptions. Also, Python does not support overloaded
699 methods/functions and default arguments are an easy way of
700 "faking" the overloading behavior.
701 </P>
702 <P class="">
703<SPAN class="stylepoint_section">Cons: </SPAN>
704 Default arguments are evaluated once at module load
705 time. This may cause problems if the argument is a mutable
706 object such as a list or a dictionary. If the function modifies
707 the object (e.g., by appending an item to a list), the default
708 value is modified.
709 </P>
710 <P class="">
711<SPAN class="stylepoint_section">Decision: </SPAN>
712 Okay to use with the following caveats:
713 <p>
714 Do not use mutable objects as default values in the function or method
715 definition.
716 </p>
717<SPAN class=""><PRE>Yes: <span class="external"></span>def foo(a, b=None):
718 <span class="external"> </span>if b is None:
719 <span class="external"> </span>b = []</PRE></SPAN>
720<SPAN class=""><PRE class="badcode">No: <span class="external"></span>def foo(a, b=[]):
721 <span class="external"> </span>...</PRE></SPAN>
722 <p>
723 Calling code must use named values for the default args. This
724 helps document the code somewhat and helps prevent and detect
725 interface breakage when more arguments are added.
726 </p>
727<SPAN class=""><PRE>
728<span class="external"></span>def foo(a, b=1):
729 <span class="external"> </span>...</PRE></SPAN>
730<SPAN class=""><PRE>Yes: <span class="external"></span>foo(1)
731 <span class="external"></span>foo(1, b=2)</PRE></SPAN>
732<SPAN class=""><PRE class="badcode">No: <span class="external"></span>foo(1, 2)</PRE></SPAN>
733 </P>
734 </SPAN></SPAN>
735 </SPAN>
736 <SPAN class=""><H3><A name="Properties" id="Properties">Properties</A></H3>
737<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Properties__body','Properties__button')" name="Properties__button" id="Properties__button">▶</SPAN>
738 <SPAN class="">
739 Use properties for accessing or setting data where you would
740 normally have used simple, lightweight accessor or setter methods.
741 </SPAN>
742 <SPAN class=""><BR><SPAN class="stylepoint_body" name="Properties__body" id="Properties__body" style="display: none"><SPAN class="link_button"><A href="?showone=Properties#Properties">
743 link
744 </A></SPAN>
745 <P class="">
746<SPAN class="stylepoint_section">Definition: </SPAN> A way to wrap method calls for getting and
747 setting an attribute as a standard attribute access when the
748 computation is lightweight.
749 </P>
750 <P class="">
751<SPAN class="stylepoint_section">Pros: </SPAN> Readability is increased by eliminating explicit
752 get and set method calls for simple attribute access. Allows
753 calculations to be lazy. Considered the Pythonic way to
754 maintain the interface of a class. In terms of performance,
755 allowing properties bypasses needing trivial accessor methods
756 when a direct variable access is reasonable. This also allows
757 accessor methods to be added in the future without breaking the
758 interface.
759 </P>
760 <P class="">
761<SPAN class="stylepoint_section">Cons: </SPAN> Properties are specified after the getter and
762 setter methods are declared, requiring one to notice they are
763 used for properties farther down in the code (except for readonly
764 properties created with the <code>@property</code> decorator - see
765 below). Must inherit from
766 <code>object</code>. Can hide side-effects much like operator
767 overloading. Can be confusing for subclasses.
768 </P>
769 <P class="">
770<SPAN class="stylepoint_section">Decision: </SPAN> Use properties in new code to access or
771 set data where you would normally have used simple, lightweight
772 accessor or setter methods. Read-only properties should be created
773 with the <code>@property</code>
774 <a HREF="#Function_and_Method_Decorators">decorator</a>.
775
776 <p><a id="properties-template-dp">
777 Inheritance with properties can be non-obvious if the property itself is
778 not overridden. Thus one must make sure that accessor methods are
779 called indirectly to ensure methods overridden in subclasses are called
780 by the property (using the Template Method DP).
781 </a></p>
782
783 <SPAN class=""><PRE>Yes: <span class="external"></span>import math
784
785 <span class="external"></span>class Square(object):
786 <span class="external"> </span>"""A square with two properties: a writable area and a read-only perimeter.
787
788 <span class="external"> </span>To use:
789 <span class="external"> </span>&gt;&gt;&gt; sq = Square(3)
790 <span class="external"> </span>&gt;&gt;&gt; sq.area
791 <span class="external"> </span>9
792 <span class="external"> </span>&gt;&gt;&gt; sq.perimeter
793 <span class="external"> </span>12
794 <span class="external"> </span>&gt;&gt;&gt; sq.area = 16
795 <span class="external"> </span>&gt;&gt;&gt; sq.side
796 <span class="external"> </span>4
797 <span class="external"> </span>&gt;&gt;&gt; sq.perimeter
798 <span class="external"> </span>16
799 <span class="external"> </span>"""
800
801 <span class="external"> </span>def __init__(self, side):
802 <span class="external"> </span>self.side = side
803
804 <span class="external"> </span>def __get_area(self):
805 <span class="external"> </span>"""Calculates the 'area' property."""
806 <span class="external"> </span>return self.side ** 2
807
808 <span class="external"> </span>def ___get_area(self):
809 <span class="external"> </span>"""Indirect accessor for 'area' property."""
810 <span class="external"> </span>return self.__get_area()
811
812 <span class="external"> </span>def __set_area(self, area):
813 <span class="external"> </span>"""Sets the 'area' property."""
814 <span class="external"> </span>self.side = math.sqrt(area)
815
816 <span class="external"> </span>def ___set_area(self, area):
817 <span class="external"> </span>"""Indirect setter for 'area' property."""
818 <span class="external"> </span>self._SetArea(area)
819
820 <span class="external"> </span>area = property(___get_area, ___set_area,
821 <span class="external"> </span> doc="""Gets or sets the area of the square.""")
822
823 <span class="external"> </span>@property
824 <span class="external"> </span>def perimeter(self):
825 <span class="external"> </span>return self.side * 4
826<span class="external"></span>
827</PRE></SPAN>
828 </P>
829 </SPAN></SPAN>
830 </SPAN>
831 <SPAN class=""><H3><A name="True/False_evaluations" id="True/False_evaluations">True/False evaluations</A></H3>
832<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>
833 <SPAN class="">
834 Use the "implicit" false if at all possible.
835 </SPAN>
836 <SPAN class=""><BR><SPAN class="stylepoint_body" name="True/False_evaluations__body" id="True/False_evaluations__body" style="display: none"><SPAN class="link_button"><A href="?showone=True/False_evaluations#True/False_evaluations">
837 link
838 </A></SPAN>
839 <P class="">
840<SPAN class="stylepoint_section">Definition: </SPAN> Python evaluates certain values as <code>false</code>
841 when in a boolean context. A quick "rule of thumb" is that all
842 "empty" values are considered <code>false</code> so <code>0, None, [], {},
843 ""</code> all evaluate as <code>false</code> in a boolean context.
844 </P>
845 <P class="">
846<SPAN class="stylepoint_section">Pros: </SPAN> Conditions using Python booleans are easier to read
847 and less error-prone. In most cases, they're also faster.
848 </P>
849 <P class="">
850<SPAN class="stylepoint_section">Cons: </SPAN>
851 May look strange to C/C++ developers.
852 </P>
853 <P class="">
854<SPAN class="stylepoint_section">Decision: </SPAN>
855 Use the "implicit" false if at all possible, e.g., <code>if
856 foo:</code> rather than <code>if foo != []:</code>. There are a
857 few caveats that you should keep in mind though:
858 <ul>
859 <li>
860 Never use <code>==</code> or <code>!=</code> to compare
861 singletons like <code>None</code>. Use <code>is</code>
862 or <code>is not</code>.</li>
863
864 <li>Beware of writing <code>if x:</code> when you really mean
865 <code>if x is not None:</code>—e.g., when testing whether
866 a variable or argument that defaults to <code>None</code> was
867 set to some other value. The other value might be a value
868 that's false in a boolean context!</li>
869
870 <li>
871 Never compare a boolean variable to <code>False</code> using
872 <code>==</code>. Use <code>if not x:</code> instead. If
873 you need to distinguish <code>False</code> from
874 <code>None</code> then chain the expressions,
875 such as <code>if not x and x is not None:</code>.
876 </li>
877
878 <li>
879 For sequences (strings, lists, tuples), use the fact that
880 empty sequences are false, so <code>if not seq:</code> or
881 <code>if seq:</code> is preferable to <code>if
882 len(seq):</code> or <code>if not
883 len(seq):</code>.</li>
884
885 <li>
886 When handling integers, implicit false may involve more risk than
887 benefit (i.e., accidentally handling <code>None</code> as 0). You may
888 compare a value which is known to be an integer (and is not the
889 result of <code>len()</code>) against the integer 0.
890<SPAN class=""><PRE>Yes: <span class="external"></span>if not users:
891 <span class="external"> </span>print 'no users'
892
893 <span class="external"></span>if foo == 0:
894 <span class="external"> </span>self.handle_zero()
895
896 <span class="external"></span>if i % 10 == 0:
897 <span class="external"> </span>self.handle_multiple_of_ten()</PRE></SPAN>
898<SPAN class=""><PRE class="badcode">No: <span class="external"></span>if len(users) == 0:
899 <span class="external"> </span>print 'no users'
900
901 <span class="external"></span>if foo is not None and not foo:
902 <span class="external"> </span>self.handle_zero()
903
904 <span class="external"></span>if not i % 10:
905 <span class="external"> </span>self.handle_multiple_of_ten()</PRE></SPAN>
906</li>
907
908 <li>
909 Note that <code>'0'</code> (i.e., <code>0</code> as string)
910 evaluates to true.</li>
911 </ul>
912 </P>
913 </SPAN></SPAN>
914 </SPAN>
915 <SPAN class=""><H3><A name="String_Methods" id="String_Methods">String Methods</A></H3>
916<SPAN class="showhide_button" onclick="javascript:ShowHideByName('String_Methods__body','String_Methods__button')" name="String_Methods__button" id="String_Methods__button">▶</SPAN>
917 <SPAN class="">
918 Use string methods instead of the <code>string</code> module where
919 possible.
920 </SPAN>
921 <SPAN class=""><BR><SPAN class="stylepoint_body" name="String_Methods__body" id="String_Methods__body" style="display: none"><SPAN class="link_button"><A href="?showone=String_Methods#String_Methods">
922 link
923 </A></SPAN>
924 <P class="">
925<SPAN class="stylepoint_section">Definition: </SPAN> String objects include methods for most
926 functions in the <code>string</code> module.
927 </P>
928 <P class="">
929<SPAN class="stylepoint_section">Pros: </SPAN> No need to import the <code>string</code> module;
930 methods work with both regular byte-strings and Unicode-strings.
931 </P>
932 <P class="">
933<SPAN class="stylepoint_section">Cons: </SPAN>
934 None.
935 </P>
936 <P class="">
937<SPAN class="stylepoint_section">Decision: </SPAN> Use string object methods. The <code>string</code> module is
938 deprecated in favor of string methods.
939<SPAN class=""><PRE class="badcode">No: <span class="external"></span>words = string.split(foo, ':')</PRE></SPAN>
940<SPAN class=""><PRE>Yes: <span class="external"></span>words = foo.split(':')</PRE></SPAN>
941 </P>
942 </SPAN></SPAN>
943 </SPAN>
944 <SPAN class=""><H3><A name="Lexical_Scoping" id="Lexical_Scoping">Lexical Scoping</A></H3>
945<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Lexical_Scoping__body','Lexical_Scoping__button')" name="Lexical_Scoping__button" id="Lexical_Scoping__button">▶</SPAN>
946 <SPAN class="">
947 Okay to use.
948 </SPAN>
949 <SPAN class=""><BR><SPAN class="stylepoint_body" name="Lexical_Scoping__body" id="Lexical_Scoping__body" style="display: none"><SPAN class="link_button"><A href="?showone=Lexical_Scoping#Lexical_Scoping">
950 link
951 </A></SPAN>
952 <P class="">
953<SPAN class="stylepoint_section">Definition: </SPAN>
954 A nested Python function can refer to variables defined in
955 enclosing functions, but can not assign to them. Variable
956 bindings are resolved using lexical scoping, that is, based on
957 the static program text. Any assignment to a name in a block
958 will cause Python to treat all references to that name as a
959 local variable, even if the use precedes the assignment. If a
960 global declaration occurs, the name is treated as a global
961 variable.
962
963 <p>
964 An example of the use of this feature is:
965 </p>
966
967 <SPAN class=""><PRE>
968<span class="external"></span>def get_adder(summand1):
969 <span class="external"> </span>"""Returns a function that adds numbers to a given number."""
970 <span class="external"> </span>def adder(summand2):
971 <span class="external"> </span>return summand1 + summand2
972
973 <span class="external"> </span>return adder
974<span class="external"></span>
975</PRE></SPAN>
976 </P>
977 <P class="">
978<SPAN class="stylepoint_section">Pros: </SPAN>
979 Often results in clearer, more elegant code. Especially comforting
980 to experienced Lisp and Scheme (and Haskell and ML and …)
981 programmers.
982 </P>
983 <P class="">
984<SPAN class="stylepoint_section">Cons: </SPAN>
985 Can lead to confusing bugs. Such as this example based on
986 <a HREF="http://www.python.org/dev/peps/pep-0227/">PEP-0227</a>:
987<SPAN class=""><PRE class="badcode">
988<span class="external"></span>i = 4
989<span class="external"></span>def foo(x):
990 <span class="external"> </span>def bar():
991 <span class="external"> </span>print i,
992 <span class="external"> </span># ...
993 <span class="external"> </span># A bunch of code here
994 <span class="external"> </span># ...
995 <span class="external"> </span>for i in x: # Ah, i *is* local to Foo, so this is what Bar sees
996 <span class="external"> </span>print i,
997 <span class="external"> </span>bar()</PRE></SPAN>
998 <p>
999 So <code>foo([1, 2, 3])</code> will print <code>1 2 3 3</code>, not
1000 <code>1 2 3 4</code>.
1001 </p>
1002 </P>
1003 <P class="">
1004<SPAN class="stylepoint_section">Decision: </SPAN>
1005 Okay to use.
1006 </P>
1007 </SPAN></SPAN>
1008 </SPAN>
1009 <SPAN class=""><H3><A name="Function_and_Method_Decorators" id="Function_and_Method_Decorators">Function and Method Decorators</A></H3>
1010<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>
1011 <SPAN class="">
1012 Use decorators judiciously when there is a clear advantage.
1013 </SPAN>
1014 <SPAN class=""><BR><SPAN class="stylepoint_body" name="Function_and_Method_Decorators__body" id="Function_and_Method_Decorators__body" style="display: none"><SPAN class="link_button"><A href="?showone=Function_and_Method_Decorators#Function_and_Method_Decorators">
1015 link
1016 </A></SPAN>
1017 <P class="">
1018<SPAN class="stylepoint_section">Definition: </SPAN>
1019
1020 <a HREF="http://www.python.org/doc/2.4.3/whatsnew/node6.html">Decorators
1021 for Functions and Methods</a>
1022 (a.k.a "the <code>@</code> notation").
1023 The most common decorators are <code>@classmethod</code> and
1024 <code>@staticmethod</code>, for converting ordinary methods to class or
1025 static methods. However, the decorator syntax allows for
1026 user-defined decorators as well. Specifically, for some function
1027 <code>my_decorator</code>, this:
1028 <SPAN class=""><PRE>
1029<span class="external"></span>class C(object):
1030 <span class="external"> </span>@my_decorator
1031 <span class="external"> </span>def method(self):
1032 <span class="external"> </span># method body ...
1033<span class="external"></span>
1034</PRE></SPAN>
1035
1036 is equivalent to:
1037 <SPAN class=""><PRE>
1038<span class="external"></span>class C(object):
1039 <span class="external"> </span>def method(self):
1040 <span class="external"> </span># method body ...
1041 <span class="external"> </span>method = my_decorator(method)
1042<span class="external"></span>
1043</PRE></SPAN>
1044 </P>
1045 <P class="">
1046<SPAN class="stylepoint_section">Pros: </SPAN> Elegantly specifies some transformation on a method; the
1047 transformation might eliminate some repetitive code, enforce invariants,
1048 etc.
1049 </P>
1050 <P class="">
1051<SPAN class="stylepoint_section">Cons: </SPAN> Decorators can perform arbitrary operations on a
1052 function's arguments or return values, resulting in surprising
1053 implicit behavior.
1054 Additionally, decorators execute at import time. Failures in decorator
1055 code are pretty much impossible to recover from.
1056 </P>
1057 <P class="">
1058<SPAN class="stylepoint_section">Decision: </SPAN> Use decorators judiciously when there is a clear
1059 advantage. Decorators should follow the same import and naming
1060 guidelines as functions. Decorator pydoc should clearly state that the
1061 function is a decorator. Write unit tests for decorators.
1062
1063 <p>
1064 Avoid external dependencies in the decorator itself (e.g. don't rely on
1065 files, sockets, database connections, etc.), since they might not be
1066 available when the decorator runs (at import time, perhaps from
1067 <code>pychecker</code> or other tools). A decorator that is
1068 called with valid parameters should (as much as possible) be guaranteed
1069 to succeed in all cases.
1070 </p>
1071 <p>
1072 Decorators are a special case of "top level code" - see
1073 <a HREF="#Main">main</a> for more discussion.
1074 </p>
1075 </P>
1076 </SPAN></SPAN>
1077 </SPAN>
1078 <SPAN class=""><H3><A name="Threading" id="Threading">Threading</A></H3>
1079<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Threading__body','Threading__button')" name="Threading__button" id="Threading__button">▶</SPAN>
1080 <SPAN class="">
1081 Do not rely on the atomicity of built-in types.
1082 </SPAN>
1083 <SPAN class=""><BR><SPAN class="stylepoint_body" name="Threading__body" id="Threading__body" style="display: none"><SPAN class="link_button"><A href="?showone=Threading#Threading">
1084 link
1085 </A></SPAN>
1086 <p>
1087 While Python's built-in data types such as dictionaries appear
1088 to have atomic operations, there are corner cases where they
1089 aren't atomic (e.g. if <code>__hash__</code> or
1090 <code>__eq__</code> are implemented as Python methods) and their
1091 atomicity should not be relied upon. Neither should you rely on
1092 atomic variable assignment (since this in turn depends on
1093 dictionaries).
1094 </p>
1095
1096 <p>
1097 Use the Queue module's <code>Queue</code> data type as the preferred
1098 way to
1099 communicate data between threads. Otherwise, use the threading
1100 module and its locking primitives. Learn about the proper use
1101 of condition variables so you can use
1102 <code>threading.Condition</code> instead of using lower-level
1103 locks.
1104 </p>
1105 </SPAN></SPAN>
1106 </SPAN>
1107 <SPAN class=""><H3><A name="Power_Features" id="Power_Features">Power Features</A></H3>
1108<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Power_Features__body','Power_Features__button')" name="Power_Features__button" id="Power_Features__button">▶</SPAN>
1109 <SPAN class="">
1110 Avoid these features.
1111 </SPAN>
1112 <SPAN class=""><BR><SPAN class="stylepoint_body" name="Power_Features__body" id="Power_Features__body" style="display: none"><SPAN class="link_button"><A href="?showone=Power_Features#Power_Features">
1113 link
1114 </A></SPAN>
1115 <P class="">
1116<SPAN class="stylepoint_section">Definition: </SPAN> Python is an extremely flexible language and
1117 gives you many fancy features such as metaclasses, access to bytecode,
1118 on-the-fly compilation, dynamic inheritance, object reparenting,
1119 import hacks, reflection, modification of system internals,
1120 etc.
1121 </P>
1122 <P class="">
1123<SPAN class="stylepoint_section">Pros: </SPAN> These are powerful language features. They can
1124 make your code more compact.
1125 </P>
1126 <P class="">
1127<SPAN class="stylepoint_section">Cons: </SPAN> It's very tempting to use these "cool" features
1128 when they're not absolutely necessary. It's harder to read,
1129 understand, and debug code that's using unusual features
1130 underneath. It doesn't seem that way at first (to the original
1131 author), but when revisiting the code, it tends to be more
1132 difficult than code that is longer but is straightforward.
1133 </P>
1134 <P class="">
1135<SPAN class="stylepoint_section">Decision: </SPAN>
1136 Avoid these features in
1137 your code.
1138 </P>
1139 </SPAN></SPAN>
1140 </SPAN>
1141 </SPAN>
1142 <SPAN class=""><H2 name="Python_Style_Rules" id="Python_Style_Rules">Python Style Rules</H2>
1143 <SPAN class=""><H3><A name="Semicolons" id="Semicolons">Semicolons</A></H3>
1144<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Semicolons__body','Semicolons__button')" name="Semicolons__button" id="Semicolons__button">▶</SPAN>
1145 <SPAN class="">
1146 Do not terminate your lines with semi-colons and do not use
1147 semi-colons to put two commands on the same line.
1148 </SPAN>
1149 <SPAN class=""><BR><SPAN class="stylepoint_body" name="Semicolons__body" id="Semicolons__body" style="display: none"><SPAN class="link_button"><A href="?showone=Semicolons#Semicolons">
1150 link
1151 </A></SPAN>
1152 </SPAN></SPAN>
1153 </SPAN>
1154 <SPAN class=""><H3><A name="Line_length" id="Line_length">Line length</A></H3>
1155<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Line_length__body','Line_length__button')" name="Line_length__button" id="Line_length__button">▶</SPAN>
1156 <SPAN class="">
1157 Maximum line length is <em>80 characters</em>.
1158 </SPAN>
1159 <SPAN class=""><BR><SPAN class="stylepoint_body" name="Line_length__body" id="Line_length__body" style="display: none"><SPAN class="link_button"><A href="?showone=Line_length#Line_length">
1160 link
1161 </A></SPAN>
1162 <p>
1163 Exception: lines importing modules may end up longer than 80
1164 characters only if using Python 2.4 or
1165 earlier.
1166 </p>
1167
1168 <p>
1169 Make use of Python's
1170
1171 <a HREF="http://www.python.org/doc/ref/implicit-joining.html">implicit
1172 line joining inside parentheses, brackets and braces</a>.
1173 If necessary, you can add an extra pair of parentheses around an
1174 expression.
1175 </p>
1176
1177
1178 <SPAN class=""><PRE>Yes: foo_bar(self, width, height, color='black', design=None, x='foo',
1179 emphasis=None, highlight=0)
1180
1181 if (width == 0 and height == 0 and
1182 color == 'red' and emphasis == 'strong'):</PRE></SPAN>
1183
1184
1185 <p>
1186 When a literal string won't fit on a single line, use parentheses for
1187 implicit line joining.
1188 </p>
1189
1190 <SPAN class=""><PRE>
1191<span class="external"></span>x = ('This will build a very long long '
1192<span class="external"></span> 'long long long long long long string')</PRE></SPAN>
1193
1194 <p>
1195 Make note of the indentation of the elements in the line
1196 continuation examples above; see the
1197 <a HREF="#indentation">indentation</a>
1198 section for explanation.
1199 </p>
1200 </SPAN></SPAN>
1201 </SPAN>
1202 <SPAN class=""><H3><A name="Parentheses" id="Parentheses">Parentheses</A></H3>
1203<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Parentheses__body','Parentheses__button')" name="Parentheses__button" id="Parentheses__button">▶</SPAN>
1204 <SPAN class="">
1205 Use parentheses sparingly.
1206 </SPAN>
1207 <SPAN class=""><BR><SPAN class="stylepoint_body" name="Parentheses__body" id="Parentheses__body" style="display: none"><SPAN class="link_button"><A href="?showone=Parentheses#Parentheses">
1208 link
1209 </A></SPAN>
1210 <p>
1211 Do not use them in return statements or conditional statements unless
1212 using parentheses for implied line continuation. (See above.)
1213 It is however fine to use parentheses around tuples.
1214 </p>
1215
1216<SPAN class=""><PRE>Yes: <span class="external"></span>if foo:
1217 <span class="external"> </span>bar()
1218 <span class="external"></span>while x:
1219 <span class="external"> </span>x = bar()
1220 <span class="external"></span>if x and y:
1221 <span class="external"> </span>bar()
1222 <span class="external"></span>if not x:
1223 <span class="external"> </span>bar()
1224 <span class="external"></span>return foo
1225 <span class="external"></span>for (x, y) in dict.items(): ...</PRE></SPAN>
1226<SPAN class=""><PRE class="badcode">No: <span class="external"></span>if (x):
1227 <span class="external"> </span>bar()
1228 <span class="external"></span>if not(x):
1229 <span class="external"> </span>bar()
1230 <span class="external"></span>return (foo)</PRE></SPAN>
1231 </SPAN></SPAN>
1232 </SPAN>
1233 <SPAN class=""><H3><A name="Indentation" id="Indentation">Indentation</A></H3>
1234<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Indentation__body','Indentation__button')" name="Indentation__button" id="Indentation__button">▶</SPAN>
1235 <SPAN class="">
1236 Indent your code blocks with <em>4 spaces</em>.
1237 </SPAN>
1238 <SPAN class=""><BR><SPAN class="stylepoint_body" name="Indentation__body" id="Indentation__body" style="display: none"><SPAN class="link_button"><A href="?showone=Indentation#Indentation">
1239 link
1240 </A></SPAN>
1241 <p>
1242 Never use tabs or mix tabs and spaces.
1243 In cases of implied line continuation, you should align wrapped elements
1244 either vertically, as per the examples in the
1245 <a HREF="#Line_length">line length</a> section; or using a hanging
1246 indent of 4 spaces, in which case there should be no argument on
1247 the first line.
1248 </p>
1249
1250
1251<SPAN class=""><PRE>Yes: # Aligned with opening delimiter
1252 foo = long_function_name(var_one, var_two,
1253 var_three, var_four)
1254
1255 # 4-space hanging indent; nothing on first line
1256 foo = long_function_name(
1257 var_one, var_two, var_three,
1258 var_four)</PRE></SPAN>
1259<SPAN class=""><PRE class="badcode">No: <span class="external"></span># Stuff on first line forbidden
1260 <span class="external"></span>foo = long_function_name(var_one, var_two,
1261 <span class="external"></span> var_three, var_four)
1262
1263 <span class="external"></span># 2-space hanging indent forbidden
1264 <span class="external"></span>foo = long_function_name(
1265 <span class="external"></span> var_one, var_two, var_three,
1266 <span class="external"></span> var_four)</PRE></SPAN>
1267 </SPAN></SPAN>
1268 </SPAN>
1269 <SPAN class=""><H3><A name="Blank_Lines" id="Blank_Lines">Blank Lines</A></H3>
1270<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Blank_Lines__body','Blank_Lines__button')" name="Blank_Lines__button" id="Blank_Lines__button">▶</SPAN>
1271 <SPAN class="">
1272 Two blank lines between top-level definitions, one blank line
1273 between method definitions.
1274 </SPAN>
1275 <SPAN class=""><BR><SPAN class="stylepoint_body" name="Blank_Lines__body" id="Blank_Lines__body" style="display: none"><SPAN class="link_button"><A href="?showone=Blank_Lines#Blank_Lines">
1276 link
1277 </A></SPAN>
1278 <p>
1279 Two blank lines between top-level definitions, be they function
1280 or class definitions. One blank line between method definitions
1281 and between the <code>class</code> line and the first method.
1282 Use single blank lines as you judge appropriate within functions or
1283 methods.
1284 </p>
1285 </SPAN></SPAN>
1286 </SPAN>
1287 <SPAN class=""><H3><A name="Whitespace" id="Whitespace">Whitespace</A></H3>
1288<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Whitespace__body','Whitespace__button')" name="Whitespace__button" id="Whitespace__button">▶</SPAN>
1289 <SPAN class="">
1290 Follow standard typographic rules for the use of spaces around
1291 punctuation.
1292 </SPAN>
1293 <SPAN class=""><BR><SPAN class="stylepoint_body" name="Whitespace__body" id="Whitespace__body" style="display: none"><SPAN class="link_button"><A href="?showone=Whitespace#Whitespace">
1294 link
1295 </A></SPAN>
1296 <p>
1297 No whitespace inside parentheses, brackets or braces.
1298 </p>
1299<SPAN class=""><PRE>Yes: <span class="external"></span>spam(ham[1], {eggs: 2}, [])</PRE></SPAN>
1300<SPAN class=""><PRE class="badcode">No: <span class="external"></span>spam( ham[ 1 ], { eggs: 2 }, [ ] )</PRE></SPAN>
1301 <p>
1302 No whitespace before a comma, semicolon, or colon. Do use
1303 whitespace after a comma, semicolon, or colon except at the end
1304 of the line.
1305 </p>
1306<SPAN class=""><PRE>Yes: <span class="external"></span>if x == 4:
1307 <span class="external"> </span>print x, y
1308 <span class="external"></span>x, y = y, x</PRE></SPAN>
1309<SPAN class=""><PRE class="badcode">No: <span class="external"></span>if x == 4 :
1310 <span class="external"> </span>print x , y
1311 <span class="external"></span>x , y = y , x</PRE></SPAN>
1312 <p>
1313 No whitespace before the open paren/bracket that starts an argument list,
1314 indexing or slicing.
1315 </p>
1316 <SPAN class=""><PRE>Yes: <span class="external"></span>spam(1)</PRE></SPAN>
1317<SPAN class=""><PRE class="badcode">No: <span class="external"></span>spam (1)</PRE></SPAN>
1318<SPAN class=""><PRE>Yes: <span class="external"></span>dict['key'] = list[index]</PRE></SPAN>
1319<SPAN class=""><PRE class="badcode">No: <span class="external"></span>dict ['key'] = list [index]</PRE></SPAN>
1320
1321 <p>
1322 Surround binary operators with a single space on either side for
1323 assignment (<code>=</code>), comparisons (<code>==, &lt;, &gt;, !=,
1324 &lt;&gt;, &lt;=, &gt;=, in, not in, is, is not</code>), and Booleans
1325 (<code>and, or, not</code>). Use your better judgment for the
1326 insertion of spaces around arithmetic operators but always be
1327 consistent about whitespace on either side of a binary operator.
1328 </p>
1329<SPAN class=""><PRE>Yes: <span class="external"></span>x == 1</PRE></SPAN>
1330<SPAN class=""><PRE class="badcode">No: <span class="external"></span>x&lt;1</PRE></SPAN>
1331 <p>
1332 Don't use spaces around the '=' sign when used to indicate a
1333 keyword argument or a default parameter value.
1334 </p>
1335<SPAN class=""><PRE>Yes: <span class="external"></span>def complex(real, imag=0.0): return magic(r=real, i=imag)</PRE></SPAN>
1336<SPAN class=""><PRE class="badcode">No: <span class="external"></span>def complex(real, imag = 0.0): return magic(r = real, i = imag)</PRE></SPAN>
1337
1338 <p>
1339 Don't use spaces to vertically align tokens on consecutive lines, since it
1340 becomes a maintenance burden (applies to <code>:</code>, <code>#</code>,
1341 <code>=</code>, etc.):
1342 </p>
1343<SPAN class=""><PRE>Yes:
1344 foo = 1000 # comment
1345 long_name = 2 # comment that should not be aligned
1346
1347 dictionary = {
1348 "foo": 1,
1349 "long_name": 2,
1350 }</PRE></SPAN>
1351<SPAN class=""><PRE class="badcode">No:
1352 foo = 1000 # comment
1353 long_name = 2 # comment that should not be aligned
1354
1355 dictionary = {
1356 "foo" : 1,
1357 "long_name": 2,
1358 }</PRE></SPAN>
1359
1360
1361 </SPAN></SPAN>
1362 </SPAN>
1363 <SPAN class=""><H3><A name="Python_Interpreter" id="Python_Interpreter">Python Interpreter</A></H3>
1364<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Python_Interpreter__body','Python_Interpreter__button')" name="Python_Interpreter__button" id="Python_Interpreter__button">▶</SPAN>
1365 <SPAN class="">
1366 Modules should begin with
1367
1368 <code>#!/usr/bin/env python&lt;version&gt;</code>
1369 </SPAN>
1370 <SPAN class=""><BR><SPAN class="stylepoint_body" name="Python_Interpreter__body" id="Python_Interpreter__body" style="display: none"><SPAN class="link_button"><A href="?showone=Python_Interpreter#Python_Interpreter">
1371 link
1372 </A></SPAN>
1373 <p>
1374 Modules should begin with a "shebang" line specifying the Python
1375 interpreter used to execute the program:
1376 </p>
1377
1378<SPAN class=""><PRE>
1379<span class="external"></span>#!/usr/bin/env python2.4</PRE></SPAN>
1380
1381 <p>
1382 Always use the most specific version you can use, e.g.,
1383 <code>/usr/bin/python2.4</code>, not
1384 <code>/usr/bin/python2</code>. This makes it easier to find
1385 dependencies when
1386
1387 upgrading to a different Python version
1388 and also avoids confusion and breakage during use. E.g., Does
1389 <code>/usr/bin/python2</code> mean Python 2.0.1 or Python
1390 2.3.0?
1391 </p>
1392
1393 </SPAN></SPAN>
1394 </SPAN>
1395 <SPAN class=""><H3><A name="Comments" id="Comments">Comments</A></H3>
1396<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Comments__body','Comments__button')" name="Comments__button" id="Comments__button">▶</SPAN>
1397 <SPAN class="">
1398 Be sure to use the right style for module, function, method and in-line
1399 comments.
1400 </SPAN>
1401 <SPAN class=""><BR><SPAN class="stylepoint_body" name="Comments__body" id="Comments__body" style="display: none"><SPAN class="link_button"><A href="?showone=Comments#Comments">
1402 link
1403 </A></SPAN>
1404
1405 <P class="">
1406<SPAN class="stylepoint_subsection">Doc Strings</SPAN>
1407
1408 <p>
1409 Python has a unique commenting style using doc strings. A doc
1410 string is a string that is the first statement in a package,
1411 module, class or function. These strings can be extracted
1412 automatically through the <code>__doc__</code> member of the
1413 object and are used by <code>pydoc</code>. (Try running
1414 <code>pydoc</code> on your module to see how it looks.) Our
1415 convention for doc strings is to use the three double-quote
1416 format for strings. A doc string should be organized as a
1417 summary line (one physical line) terminated by a period,
1418 question mark, or exclamation point, followed by a blank line,
1419 followed by the rest of the doc string starting at the same
1420 cursor position as the first quote of the first line. There are
1421 more formatting guidelines for doc strings below.
1422 </p>
1423
1424 </P>
1425 <P class="">
1426<SPAN class="stylepoint_subsection">Modules</SPAN>
1427
1428
1429
1430 <p>
1431 Every file should contain the following items, in order:
1432 <ul>
1433 <li>a copyright statement (for example,
1434 <code>Copyright 2008 Google Inc.</code>)</li>
1435 <li>a license boilerplate. Choose the appropriate boilerplate
1436 for the license used by the project (for example, Apache 2.0, BSD,
1437 LGPL, GPL)</li>
1438 <li>an author line to identify the original author of the file</li>
1439 </ul>
1440 </p>
1441 </P>
1442 <P class="">
1443<SPAN class="stylepoint_subsection">Functions and Methods</SPAN>
1444
1445 <p>
1446 Any function or method which is not both obvious and very short
1447 needs a doc string. Additionally, any externally accessible
1448 function or method regardless of length or simplicity needs a
1449 doc string. The doc string should include what the function does
1450 and have detailed descriptions of the input and output. It
1451 should not, generally, describe how it does it unless it's some
1452 complicated algorithm. For tricky code block/inline comments
1453 within the code are more appropriate. The doc string should give
1454 enough information to write a call to the function without
1455 looking at a single line of the function's code. Args should be
1456 individually documented, an explanation following after a colon,
1457 and should use a uniform hanging indent of 2 or 4 spaces. The
1458 doc string should specify the expected types where specific types
1459 are required. A "Raises:" section should list all exceptions
1460 that can be raised by the function. The doc string for generator
1461 functions should use "Yields:" rather than "Returns:".
1462 </p>
1463
1464 <SPAN class=""><PRE>
1465<span class="external"></span>def fetch_bigtable_rows(big_table, keys, other_silly_variable=None):
1466 <span class="external"> </span>"""Fetches rows from a Bigtable.
1467
1468 <span class="external"> </span>Retrieves rows pertaining to the given keys from the Table instance
1469 <span class="external"> </span>represented by big_table. Silly things may happen if
1470 <span class="external"> </span>other_silly_variable is not None.
1471
1472 <span class="external"> </span>Args:
1473 <span class="external"> </span>big_table: An open Bigtable Table instance.
1474 <span class="external"> </span>keys: A sequence of strings representing the key of each table row
1475 <span class="external"> </span> to fetch.
1476 <span class="external"> </span>other_silly_variable: Another optional variable, that has a much
1477 <span class="external"> </span> longer name than the other args, and which does nothing.
1478
1479 <span class="external"> </span>Returns:
1480 <span class="external"> </span>A dict mapping keys to the corresponding table row data
1481 <span class="external"> </span>fetched. Each row is represented as a tuple of strings. For
1482 <span class="external"> </span>example:
1483
1484 <span class="external"> </span>{'Serak': ('Rigel VII', 'Preparer'),
1485 <span class="external"> </span> 'Zim': ('Irk', 'Invader'),
1486 <span class="external"> </span> 'Lrrr': ('Omicron Persei 8', 'Emperor')}
1487
1488 <span class="external"> </span>If a key from the keys argument is missing from the dictionary,
1489 <span class="external"> </span>then that row was not found in the table.
1490
1491 <span class="external"> </span>Raises:
1492 <span class="external"> </span>IOError: An error occurred accessing the bigtable.Table object.
1493 <span class="external"> </span>"""
1494 <span class="external"> </span>pass
1495<span class="external"></span>
1496</PRE></SPAN>
1497 </P>
1498 <P class="">
1499<SPAN class="stylepoint_subsection">Classes</SPAN>
1500
1501 <p>
1502 Classes should have a doc string below the class definition describing
1503 the class. If your class has public attributes, they should be documented
1504 here in an Attributes section and follow the same formatting as a
1505 function's Args section.
1506 </p>
1507
1508 <SPAN class=""><PRE>
1509<span class="external"></span>class SampleClass(object):
1510 <span class="external"> </span>"""Summary of class here.
1511
1512 <span class="external"> </span>Longer class information....
1513 <span class="external"> </span>Longer class information....
1514
1515 <span class="external"> </span>Attributes:
1516 <span class="external"> </span>likes_spam: A boolean indicating if we like SPAM or not.
1517 <span class="external"> </span>eggs: An integer count of the eggs we have laid.
1518 <span class="external"> </span>"""
1519
1520 <span class="external"> </span>def __init__(self, likes_spam=False):
1521 <span class="external"> </span>"""Inits SampleClass with blah."""
1522 <span class="external"> </span>self.likes_spam = likes_spam
1523 <span class="external"> </span>self.eggs = 0
1524
1525 <span class="external"> </span>def public_method(self):
1526 <span class="external"> </span>"""Performs operation blah."""
1527<span class="external"></span>
1528</PRE></SPAN>
1529
1530 </P>
1531 <P class="">
1532<SPAN class="stylepoint_subsection">Block and Inline Comments</SPAN>
1533
1534 <p>
1535 The final place to have comments is in tricky parts of the
1536 code. If you're going to have to explain it at the next
1537 <a HREF="http://en.wikipedia.org/wiki/Code_review">code review</a>,
1538 you should comment it now. Complicated operations get a few lines of
1539 comments before the operations
1540 commence. Non-obvious ones get comments at the end of the line.
1541 </p>
1542
1543 <SPAN class=""><PRE>
1544<span class="external"></span># We use a weighted dictionary search to find out where i is in
1545<span class="external"></span># the array. We extrapolate position based on the largest num
1546<span class="external"></span># in the array and the array size and then do binary search to
1547<span class="external"></span># get the exact number.
1548
1549<span class="external"></span>if i &amp; (i-1) == 0: # true iff i is a power of 2
1550<span class="external"></span>
1551</PRE></SPAN>
1552
1553 <p>
1554 To improve legibility, these comments should be at least 2 spaces away
1555 from the code.
1556 </p>
1557
1558 <p>
1559 On the other hand, never describe the code. Assume the person
1560 reading the code knows Python (though not what you're trying to
1561 do) better than you do.
1562 </p>
1563
1564 <SPAN class=""><PRE class="badcode">
1565<span class="external"></span># BAD COMMENT: Now go through the b array and make sure whenever i occurs
1566<span class="external"></span># the next element is i+1
1567<span class="external"></span>
1568</PRE></SPAN>
1569
1570 </P>
1571 </SPAN></SPAN>
1572 </SPAN>
1573 <SPAN class=""><H3><A name="Classes" id="Classes">Classes</A></H3>
1574<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Classes__body','Classes__button')" name="Classes__button" id="Classes__button">▶</SPAN>
1575 <SPAN class="">
1576 If a class inherits from no other base classes, explicitly inherit
1577 from <code>object</code>. This also applies to nested classes.
1578 </SPAN>
1579 <SPAN class=""><BR><SPAN class="stylepoint_body" name="Classes__body" id="Classes__body" style="display: none"><SPAN class="link_button"><A href="?showone=Classes#Classes">
1580 link
1581 </A></SPAN>
1582 <SPAN class=""><PRE class="badcode">No: <span class="external"></span>class SampleClass:
1583 <span class="external"> </span>pass
1584
1585
1586 <span class="external"></span>class OuterClass:
1587
1588 <span class="external"> </span>class InnerClass:
1589 <span class="external"> </span>pass
1590<span class="external"></span>
1591</PRE></SPAN>
1592
1593 <SPAN class=""><PRE>Yes: <span class="external"></span>class SampleClass(object):
1594 <span class="external"> </span>pass
1595
1596
1597 <span class="external"></span>class OuterClass(object):
1598
1599 <span class="external"> </span>class InnerClass(object):
1600 <span class="external"> </span>pass
1601
1602
1603 <span class="external"></span>class ChildClass(ParentClass):
1604 <span class="external"> </span>"""Explicitly inherits from another class already."""
1605<span class="external"></span>
1606</PRE></SPAN>
1607
1608 <p>Inheriting from <code>object</code> is needed to make properties work
1609 properly, and it will protect your code from one particular potential
1610 incompatibility with Python 3000. It also defines
1611 special methods that implement the default semantics of objects including
1612 <code>__new__</code>, <code>__init__</code>, <code>__delattr__</code>,
1613 <code>__getattribute__</code>, <code>__setattr__</code>,
1614 <code>__hash__</code>, <code>__repr__</code>, and <code>__str__</code>.
1615 </p>
1616 </SPAN></SPAN>
1617 </SPAN>
1618 <SPAN class=""><H3><A name="Strings" id="Strings">Strings</A></H3>
1619<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Strings__body','Strings__button')" name="Strings__button" id="Strings__button">▶</SPAN>
1620 <SPAN class="">
1621 Use the <code>%</code> operator for formatting strings,
1622 even when the parameters are all strings. Use your best judgement
1623 to decide between <code>+</code> and <code>%</code> though.
1624 </SPAN>
1625 <SPAN class=""><BR><SPAN class="stylepoint_body" name="Strings__body" id="Strings__body" style="display: none"><SPAN class="link_button"><A href="?showone=Strings#Strings">
1626 link
1627 </A></SPAN>
1628<SPAN class=""><PRE class="badcode">No: <span class="external"></span>x = '%s%s' % (a, b) # use + in this case
1629 <span class="external"></span>x = imperative + ', ' + expletive + '!'
1630 <span class="external"></span>x = 'name: ' + name + '; score: ' + str(n)</PRE></SPAN>
1631<SPAN class=""><PRE>Yes: <span class="external"></span>x = a + b
1632 <span class="external"></span>x = '%s, %s!' % (imperative, expletive)
1633 <span class="external"></span>x = 'name: %s; score: %d' % (name, n)</PRE></SPAN>
1634
1635 <p>
1636 Avoid using the <code>+</code> and <code>+=</code> operators to
1637 accumulate a string within a loop. Since strings are immutable, this
1638 creates unnecessary temporary objects and results in quadratic rather
1639 than linear running time. Instead, add each substring to a list and
1640 <code>''.join</code> the list after the loop terminates (or, write each
1641 substring to a <code>cStringIO.StringIO</code> buffer).
1642 </p>
1643
1644<SPAN class=""><PRE class="badcode">No: <span class="external"></span>employee_table = '&lt;table&gt;'
1645 <span class="external"></span>for last_name, first_name in employee_list:
1646 <span class="external"> </span>employee_table += '&lt;tr&gt;&lt;td&gt;%s, %s&lt;/td&gt;&lt;/tr&gt;' % (last_name, first_name)
1647 <span class="external"></span>employee_table += '&lt;/table&gt;'</PRE></SPAN>
1648<SPAN class=""><PRE>Yes: <span class="external"></span>items = ['&lt;table&gt;']
1649 <span class="external"></span>for last_name, first_name in employee_list:
1650 <span class="external"> </span>items.append('&lt;tr&gt;&lt;td&gt;%s, %s&lt;/td&gt;&lt;/tr&gt;' % (last_name, first_name))
1651 <span class="external"></span>items.append('&lt;/table&gt;')
1652 <span class="external"></span>employee_table = ''.join(items)</PRE></SPAN>
1653
1654 <p>
1655 Use <code>"""</code> for multi-line strings rather than
1656 <code>'''</code>. Note, however, that it is often cleaner to
1657 use implicit line joining since multi-line strings do
1658 not flow with the indentation of the rest of the program:
1659 </p>
1660
1661 <SPAN class=""><PRE class="badcode"> No<span class="external"></span>:
1662 <span class="external"></span>print """This is pretty ugly.
1663Don'<span class="external"></span>t do this.
1664"""<span class="external"></span>
1665</PRE></SPAN>
1666<SPAN class=""><PRE>Ye<span class="external"></span>s:
1667 <span class="external"></span>print ("This is much nicer.\n"
1668 <span class="external"></span> "Do it this way.\n")</PRE></SPAN>
1669 </SPAN></SPAN>
1670 </SPAN>
1671 <SPAN class=""><H3><A name="TODO_Comments" id="TODO_Comments">TODO Comments</A></H3>
1672<SPAN class="showhide_button" onclick="javascript:ShowHideByName('TODO_Comments__body','TODO_Comments__button')" name="TODO_Comments__button" id="TODO_Comments__button">▶</SPAN>
1673 <SPAN class="">
1674 Use <code>TODO</code> comments for code that is temporary, a
1675 short-term solution, or good-enough but not perfect.
1676 </SPAN>
1677 <SPAN class=""><BR><SPAN class="stylepoint_body" name="TODO_Comments__body" id="TODO_Comments__body" style="display: none"><SPAN class="link_button"><A href="?showone=TODO_Comments#TODO_Comments">
1678 link
1679 </A></SPAN>
1680 <p>
1681 <code>TODO</code>s should include the string <code>TODO</code> in
1682 all caps, followed by your
1683
1684 name, e-mail address, or other
1685 identifier
1686 in parentheses. A colon is optional. A comment explaining what there
1687 is to do is required. The main purpose is to have
1688 a consistent <code>TODO</code> format searchable by the person
1689 adding the comment (who can provide more details upon request). A
1690 <code>TODO</code> is not a commitment to provide the fix yourself.
1691 </p>
1692
1693 <SPAN class=""><PRE># TODO(kl@gmail.com): Drop the use of "has_key".
1694# TODO(Zeke) change this to use relations.</PRE></SPAN>
1695 <p>
1696 If your <code>TODO</code> is of the form "At a future date do
1697 something" make sure that you either include a very specific
1698 date ("Fix by November 2009") or a very specific event
1699 ("Remove this code when all clients can handle XML responses.").
1700 </p>
1701 </SPAN></SPAN>
1702 </SPAN>
1703 <SPAN class=""><H3><A name="Imports_formatting" id="Imports_formatting">Imports formatting</A></H3>
1704<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Imports_formatting__body','Imports_formatting__button')" name="Imports_formatting__button" id="Imports_formatting__button">▶</SPAN>
1705 <SPAN class="">
1706 Imports should be on separate lines.
1707 </SPAN>
1708 <SPAN class=""><BR><SPAN class="stylepoint_body" name="Imports_formatting__body" id="Imports_formatting__body" style="display: none"><SPAN class="link_button"><A href="?showone=Imports_formatting#Imports_formatting">
1709 link
1710 </A></SPAN>
1711 <p>
1712 E.g.:
1713 </p>
1714
1715<SPAN class=""><PRE>Yes: <span class="external"></span>import os
1716 <span class="external"></span>import sys</PRE></SPAN>
1717<SPAN class=""><PRE class="badcode">No: <span class="external"></span>import os, sys</PRE></SPAN>
1718 <p>
1719 Imports are always put at the top of the file, just after any
1720 module comments and doc strings and before module globals and
1721 constants. Imports should be grouped with the order being most generic
1722 to least generic:
1723 </p>
1724 <ul>
1725 <li>standard library imports</li>
1726 <li>third-party imports</li>
1727
1728 <li>application-specific imports</li>
1729 </ul>
1730 <p>
1731 Within each grouping, imports should be sorted lexicographically,
1732 ignoring case, according to each module's full package path.
1733 </p>
1734 <SPAN class=""><PRE>
1735<span class="external"></span>import foo
1736<span class="external"></span>from foo import bar
1737<span class="external"></span>from foo.bar import baz
1738<span class="external"></span>from foo.bar import Quux
1739<span class="external"></span>from Foob import ar</PRE></SPAN>
1740
1741
1742 </SPAN></SPAN>
1743 </SPAN>
1744 <SPAN class=""><H3><A name="Statements" id="Statements">Statements</A></H3>
1745<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Statements__body','Statements__button')" name="Statements__button" id="Statements__button">▶</SPAN>
1746 <SPAN class="">
1747 Generally only one statement per line.
1748 </SPAN>
1749 <SPAN class=""><BR><SPAN class="stylepoint_body" name="Statements__body" id="Statements__body" style="display: none"><SPAN class="link_button"><A href="?showone=Statements#Statements">
1750 link
1751 </A></SPAN>
1752 <p>
1753 However, you may put the
1754 result of a test on the same line as the test only if the entire
1755 statement fits on one line. In particular, you can never do so
1756 with <code>try</code>/<code>except</code> since the
1757 <code>try</code> and <code>except</code> can't both fit on the
1758 same line, and you can only do so with an <code>if</code> if
1759 there is no <code>else</code>.
1760 </p>
1761
1762 <SPAN class=""><PRE>Ye<span class="external"></span>s:
1763
1764 <span class="external"></span>if foo: bar(foo)</PRE></SPAN>
1765<SPAN class=""><PRE class="badcode">No<span class="external"></span>:
1766
1767 <span class="external"></span>if foo: bar(foo)
1768 <span class="external"></span>else: baz(foo)
1769
1770 <span class="external"></span>try: bar(foo)
1771 <span class="external"></span>except ValueError: baz(foo)
1772
1773 <span class="external"></span>try:
1774 <span class="external"> </span>bar(foo)
1775 <span class="external"></span>except ValueError: baz(foo)
1776<span class="external"></span>
1777</PRE></SPAN>
1778 </SPAN></SPAN>
1779 </SPAN>
1780 <SPAN class=""><H3><A name="Access_Control" id="Access_Control">Access Control</A></H3>
1781<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Access_Control__body','Access_Control__button')" name="Access_Control__button" id="Access_Control__button">▶</SPAN>
1782 <SPAN class="">
1783 If an accessor function would be trivial you should use public variables
1784 instead of accessor functions to avoid the extra cost of function
1785 calls in Python. When more functionality is added you can use
1786 <code>property</code> to keep the syntax consistent.
1787 </SPAN>
1788 <SPAN class=""><BR><SPAN class="stylepoint_body" name="Access_Control__body" id="Access_Control__body" style="display: none"><SPAN class="link_button"><A href="?showone=Access_Control#Access_Control">
1789 link
1790 </A></SPAN>
1791 <p>
1792 On the other hand, if access is more complex, or the cost of accessing
1793 the variable is significant, you should use function calls (following the
1794 <a HREF="#naming">Naming</a> guidelines) such as <code>get_foo()</code>
1795 and <code>set_foo()</code>. If the past behavior allowed access through a
1796 property, do not bind the new accessor functions to the property. Any
1797 code still attempting to access the variable by the old method should
1798 break visibly so they are made aware of the change in complexity.
1799 </p>
1800 </SPAN></SPAN>
1801 </SPAN>
1802 <SPAN class=""><H3><A name="Naming" id="Naming">Naming</A></H3>
1803<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Naming__body','Naming__button')" name="Naming__button" id="Naming__button">▶</SPAN>
1804 <SPAN class="">
1805 <code>module_name, package_name, ClassName, method_name, ExceptionName,
1806 function_name, GLOBAL_VAR_NAME, instance_var_name,
1807 function_parameter_name, local_var_name.</code>
1808 </SPAN>
1809 <SPAN class=""><BR><SPAN class="stylepoint_body" name="Naming__body" id="Naming__body" style="display: none"><SPAN class="link_button"><A href="?showone=Naming#Naming">
1810 link
1811 </A></SPAN>
1812 <P class="">
1813<SPAN class="stylepoint_subsection">Names to Avoid</SPAN>
1814
1815 <ul>
1816 <li>single character names except for counters or iterators</li>
1817 <li>dashes (<code>-</code>) in any package/module name</li>
1818 <li>
1819<code>__double_leading_and_trailing_underscore__</code> names
1820 (reserved by Python)</li>
1821 </ul>
1822
1823 </P>
1824 <P class="">
1825<SPAN class="stylepoint_subsection">Naming Convention</SPAN>
1826
1827 <ul>
1828 <li>
1829 "Internal" means internal to a module or protected
1830 or private within a class.</li>
1831 <li>
1832 Prepending a single underscore (<code>_</code>) has some
1833 support for protecting module variables and functions (not included
1834 with <code>import * from</code>). Prepending a double underscore
1835 (<code>__</code>) to an instance variable or method
1836 effectively serves to make the variable or method private to its class
1837 (using name mangling).</li>
1838 <li>
1839 Place related classes and top-level functions together in a
1840 module. Unlike Java,
1841 there is no need to limit yourself to one class per module.</li>
1842 <li>
1843 Use CapWords for class names, but lower_with_under.py for module names.
1844 Although there are many existing modules named CapWords.py, this is now
1845 discouraged because it's confusing when the module happens to be
1846 named after a class. ("wait -- did I write
1847 <code>import StringIO</code> or <code>from StringIO import
1848 StringIO</code>?")</li>
1849 </ul>
1850
1851 </P>
1852 <P class="">
1853<SPAN class="stylepoint_subsection">Guidelines derived from Guido's Recommendations</SPAN>
1854
1855 <table rules="all" border="1" cellspacing="2" cellpadding="2">
1856
1857 <tr>
1858 <th>Type</th>
1859 <th>Public</th>
1860 <th>Internal</th>
1861 </tr>
1862
1863
1864
1865 <tr>
1866 <td>Packages</td>
1867 <td><code>lower_with_under</code></td>
1868 <td></td>
1869 </tr>
1870
1871 <tr>
1872 <td>Modules</td>
1873 <td><code>lower_with_under</code></td>
1874 <td><code>_lower_with_under</code></td>
1875 </tr>
1876
1877 <tr>
1878 <td>Classes</td>
1879 <td><code>CapWords</code></td>
1880 <td><code>_CapWords</code></td>
1881 </tr>
1882
1883 <tr>
1884 <td>Exceptions</td>
1885 <td><code>CapWords</code></td>
1886 <td></td>
1887 </tr>
1888
1889
1890
1891 <tr>
1892 <td>Functions</td>
1893 <td><code>lower_with_under()</code></td>
1894 <td><code>_lower_with_under()</code></td>
1895 </tr>
1896
1897 <tr>
1898 <td>Global/Class Constants</td>
1899 <td><code>CAPS_WITH_UNDER</code></td>
1900 <td><code>_CAPS_WITH_UNDER</code></td>
1901 </tr>
1902
1903 <tr>
1904 <td>Global/Class Variables</td>
1905 <td><code>lower_with_under</code></td>
1906 <td><code>_lower_with_under</code></td>
1907 </tr>
1908
1909 <tr>
1910 <td>Instance Variables</td>
1911 <td><code>lower_with_under</code></td>
1912 <td><code>_lower_with_under (protected) or __lower_with_under (private)</code></td>
1913 </tr>
1914
1915
1916
1917 <tr>
1918 <td>Method Names</td>
1919 <td><code>lower_with_under()</code></td>
1920 <td><code>_lower_with_under() (protected) or __lower_with_under() (private)</code></td>
1921 </tr>
1922
1923 <tr>
1924 <td>Function/Method Parameters</td>
1925 <td><code>lower_with_under</code></td>
1926 <td></td>
1927 </tr>
1928
1929 <tr>
1930 <td>Local Variables</td>
1931 <td><code>lower_with_under</code></td>
1932 <td></td>
1933 </tr>
1934
1935
1936 </table>
1937
1938
1939 </P>
1940 </SPAN></SPAN>
1941 </SPAN>
1942 <SPAN class=""><H3><A name="Main" id="Main">Main</A></H3>
1943<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Main__body','Main__button')" name="Main__button" id="Main__button">▶</SPAN>
1944 <SPAN class="">
1945 Even a file meant to be used as a script should be importable and a
1946 mere import should not have the side effect of executing the script's
1947 main functionality. The main functionality should be in a main()
1948 function.
1949 </SPAN>
1950 <SPAN class=""><BR><SPAN class="stylepoint_body" name="Main__body" id="Main__body" style="display: none"><SPAN class="link_button"><A href="?showone=Main#Main">
1951 link
1952 </A></SPAN>
1953 <p>
1954 In Python,
1955 <code>pychecker</code>, <code>pydoc</code>, and unit tests
1956 require modules to be importable. Your code should always check
1957 <code>if __name__ == '__main__'</code> before executing your
1958 main program so that the main program is not executed when the
1959 module is imported.
1960
1961 </p>
1962
1963
1964
1965
1966
1967
1968
1969 <SPAN class=""><PRE>
1970<span class="external"></span>def main():
1971 <span class="external"> </span>...
1972
1973<span class="external"></span>if __name__ == '__main__':
1974 <span class="external"> </span>main()
1975<span class="external"></span>
1976</PRE></SPAN>
1977
1978 <p>
1979 All code at the top level will be executed when the module is
1980 imported. Be careful not to call functions, create objects, or
1981 perform other operations that should not be executed when the
1982 file is being <code>pycheck</code>ed or <code>pydoc</code>ed.
1983 </p>
1984 </SPAN></SPAN>
1985 </SPAN>
1986 </SPAN>
1987
1988<H2>Parting Words</H2>
1989 <p>
1990 <em>BE CONSISTENT</em>.
1991 </p>
1992
1993 <p>
1994 If you're editing code, take a few minutes to look at the code
1995 around you and determine its style. If they use spaces around
1996 all their arithmetic operators, you should too. If their
1997 comments have little boxes of hash marks around them, make your
1998 comments have little boxes of hash marks around them too.
1999 </p>
2000
2001 <p>
2002 The point of having style guidelines is to have a common vocabulary
2003 of coding so people can concentrate on what you're saying rather
2004 than on how you're saying it. We present global style rules here so
2005 people know the vocabulary, but local style is also important. If
2006 code you add to a file looks drastically different from the existing
2007 code around it, it throws readers out of their rhythm when they go to
2008 read it. Avoid this.
2009 </p>
2010
2011
2012
2013<p align="right">
2014Revision 2.12
2015</p>
2016
2017
2018<address>
2019 Amit Patel<br>
2020 Antoine Picard<br>
2021 Eugene Jhong<br>
2022 Jeremy Hylton<br>
2023 Matt Smart<br>
2024 Mike Shields<br>
2025</address>
2026</BODY>
2027</HTML>