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93<BODY>
94<H1>Google Python Style Guide</H1>
95 <p align="right">
96
mmentovaif7facf92009-10-23 21:01:49 +0000[diff] [blame^]97 Revision 2.14
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]98 </p>
99
100 <address>
101 Amit Patel<br>
102 Antoine Picard<br>
103 Eugene Jhong<br>
104 Jeremy Hylton<br>
105 Matt Smart<br>
106 Mike Shields<br>
107 </address>
108 <DIV style="margin-left: 50%; font-size: 75%;">
109<P>
110 Each style point has a summary for which additional information is available
111 by toggling the accompanying arrow button that looks this way:
112 <SPAN class="showhide_button" style="margin-left: 0;">▶</SPAN>.
113 You may toggle all summaries with the big arrow button:
114 </P>
115<DIV style=" font-size: larger; margin-left: +2em;">
116<SPAN class="showhide_button" style="font-size: 180%;" onclick="javascript:ShowHideAll()" name="show_hide_all_button" id="show_hide_all_button">▶</SPAN>
117 Toggle all summaries
118 </DIV>
119</DIV>
120<DIV class="toc">
121<DIV class="toc_title">Table of Contents</DIV>
122<TABLE>
123<TR valign="top" class="">
124<TD><DIV class="toc_category"><A href="#Python_Language_Rules">Python Language Rules</A></DIV></TD>
125<TD><DIV class="toc_stylepoint">
mmentovaif7facf92009-10-23 21:01:49 +0000[diff] [blame^]126<SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#pychecker">pychecker</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Imports">Imports</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Packages">Packages</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Exceptions">Exceptions</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Global_variables">Global variables</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Nested/Local/Inner_Classes_and_Functions">Nested/Local/Inner Classes and Functions</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#List_Comprehensions">List Comprehensions</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Default_Iterators_and_Operators">Default Iterators and Operators</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Generators">Generators</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Lambda_Functions">Lambda Functions</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Default_Argument_Values">Default Argument Values</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Properties">Properties</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#True/False_evaluations">True/False evaluations</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Deprecated_Language_Features">Deprecated Language Features</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Lexical_Scoping">Lexical Scoping</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Function_and_Method_Decorators">Function and Method Decorators</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Threading">Threading</A></SPAN> <SPAN style="padding-right: 1em; white-space:nowrap;" class=""><A href="#Power_Features">Power Features</A></SPAN> </DIV></TD>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]127</TR>
128<TR valign="top" class="">
129<TD><DIV class="toc_category"><A href="#Python_Style_Rules">Python Style Rules</A></DIV></TD>
130<TD><DIV class="toc_stylepoint">
131<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>
132</TR>
133</TABLE>
134</DIV>
135<H2>Overview</H2>
136 <SPAN class=""><H2 name="Important_Note" id="Important_Note">Important Note</H2>
137 <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>
138<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>
139 <SPAN class="">
140 This style guide contains many details that are initially
141 hidden from view. They are marked by the triangle icon, which you
142 see here on your left. Click it now.
143 You should see "Hooray" appear below.
144 </SPAN>
145 <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">
146 link
147 </A></SPAN>
148 <p>
149 Hooray! Now you know you can expand points to get more
150 details. Alternatively, there's a "toggle all" at the
151 top of this document.
152 </p>
153 </SPAN></SPAN>
154 </SPAN>
155 </SPAN>
156 <SPAN class=""><H2 name="Background" id="Background">Background</H2>
157 <p>
158 Python is the main scripting language used at Google. This
159 style guide is a list of <em>do</em>s and <em>don't</em>s for Python
160 programs.
161 </p>
162
163
164
165 </SPAN>
166
167 <SPAN class=""><H2 name="Python_Language_Rules" id="Python_Language_Rules">Python Language Rules</H2>
168
169 <SPAN class=""><H3><A name="pychecker" id="pychecker">pychecker</A></H3>
170<SPAN class="showhide_button" onclick="javascript:ShowHideByName('pychecker__body','pychecker__button')" name="pychecker__button" id="pychecker__button">▶</SPAN>
171 <SPAN class="">
172 Run <code>pychecker</code> over your code.
173 </SPAN>
174 <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">
175 link
176 </A></SPAN>
177 <P class="">
178<SPAN class="stylepoint_section">Definition: </SPAN>
179 PyChecker is a tool for finding bugs in Python source code. It finds
180 problems that are typically caught by a compiler for less dynamic
181 languages like C and C++. It is similar to lint. Because of the
182 dynamic nature of Python, some warnings may be incorrect; however,
183 spurious warnings should be fairly infrequent.
184 </P>
185 <P class="">
186<SPAN class="stylepoint_section">Pros: </SPAN>
187 Catches easy-to-miss errors like typos, use-vars-before-assignment, etc.
188 </P>
189 <P class="">
190<SPAN class="stylepoint_section">Cons: </SPAN>
191 <code>pychecker</code> isn't perfect. To take
192 advantage of it, we'll need to sometimes: a) Write around it b)
193 Suppress its warnings c) Improve it or d) Ignore it.
194 </P>
195 <P class="">
196<SPAN class="stylepoint_section">Decision: </SPAN>
197 Make sure you run <code>pychecker</code> on your code.
198 </P>
199
200 <p>
201 For information on how to run <code>pychecker</code>, see the
202 <a HREF="http://pychecker.sourceforge.net">pychecker
203 homepage</a>
204 </p>
205 <p>
206 To suppress warnings, you can set a module-level variable named
207 <code>__pychecker__</code> to suppress appropriate warnings.
208 For example:
209 </p>
210 <SPAN class=""><PRE>
211<span class="external"></span>__pychecker__ = 'no-callinit no-classattr'</PRE></SPAN>
212 <p>
213 Suppressing in this way has the advantage that we can easily search
214 for suppressions and revisit them.
215 </p>
216 <p>
217 You can get a list of pychecker warnings by doing
218 <code>pychecker --help</code>.
219 </p>
220 <p>
221 Unused argument warnings can be suppressed by using `_' as the
222 identifier for the unused argument or prefixing the argument name with
223 `unused_'. In situations where changing the argument names is
224 infeasible, you can mention them at the beginning of the function.
225 For example:
226 </p>
227 <SPAN class=""><PRE>
228<span class="external"></span>def foo(a, unused_b, unused_c, d=None, e=None):
229 <span class="external"> </span>(d, e) = (d, e) # Silence pychecker
230 <span class="external"> </span>return a
231<span class="external"></span>
232</PRE></SPAN>
233 <p>
234 Ideally, pychecker would be extended to ensure that such `unused
235 declarations' were true.
236 </p>
237
238 </SPAN></SPAN>
239 </SPAN>
240 <SPAN class=""><H3><A name="Imports" id="Imports">Imports</A></H3>
241<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Imports__body','Imports__button')" name="Imports__button" id="Imports__button">▶</SPAN>
242 <SPAN class="">
243 Use <code>import</code>s for packages and modules only.
244 </SPAN>
245 <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">
246 link
247 </A></SPAN>
248 <P class="">
249<SPAN class="stylepoint_section">Definition: </SPAN>
250 Reusability mechanism for sharing code from one module to another.
251 </P>
252 <P class="">
253<SPAN class="stylepoint_section">Pros: </SPAN>
254 Simplest and most commonly used way of sharing things.
255 </P>
256 <P class="">
257<SPAN class="stylepoint_section">Cons: </SPAN> <code>from foo import *</code> or
258 <code>from foo import Bar</code> is
259 very nasty and can lead to serious maintenance issues because
260 it makes it hard to find module dependencies.
261 </P>
262 <P class="">
263<SPAN class="stylepoint_section">Decision: </SPAN>
264 Use <code>import x</code> for importing packages and modules.
265 Use <code>from x import y</code> only when <code>x</code> is a
266 package and <code>y</code> is a module. This allows the
267 importer to refer to the module without specifying the full
268 package prefix. For example the module
269 <code>sound.effects.echo</code> may be imported as follows:
270 </P>
271 <SPAN class=""><PRE>
272<span class="external"></span>from sound.effects import echo
273<span class="external"></span>...
274<span class="external"></span>echo.echofilter(input, output, delay=0.7, atten=4)
275<span class="external"></span>
276</PRE></SPAN>
277 <p>
278 Even if the module is in the same package, do not directly import
279 the module without the full package name. This might cause the
280 package to be imported twice (with unintended side effects) when the
281 "main" module that is used to start an application lives inside a
282 package (and uses modules from that same package).
283 </p>
284 </SPAN></SPAN>
285 </SPAN>
286 <SPAN class=""><H3><A name="Packages" id="Packages">Packages</A></H3>
287<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Packages__body','Packages__button')" name="Packages__button" id="Packages__button">▶</SPAN>
288 <SPAN class="">
289 Import and refer to each module using the full pathname location of
290 that module.
291 </SPAN>
292 <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">
293 link
294 </A></SPAN>
295 <P class="">
296<SPAN class="stylepoint_section">Pros: </SPAN>
297 Avoids conflicts in module names. Makes it easier to find modules.
298 </P>
299 <P class="">
300<SPAN class="stylepoint_section">Cons: </SPAN>
301 Makes it harder to deploy code because you have to replicate the
302 package hierarchy.
303 </P>
304 <P class="">
305<SPAN class="stylepoint_section">Decision: </SPAN>
306 All new code should refer to modules based on their package
307 name.
308 </P>
309 <p>
310 Imports should be as follows:
311 </p>
312
313
314 <SPAN class=""><PRE># Reference in code with complete name.
315import sound.effects.echo
316
317# Reference in code with just module name.
318from sound.effects import echo
319</PRE></SPAN>
320
321 </SPAN></SPAN>
322 </SPAN>
323 <SPAN class=""><H3><A name="Exceptions" id="Exceptions">Exceptions</A></H3>
324<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Exceptions__body','Exceptions__button')" name="Exceptions__button" id="Exceptions__button">▶</SPAN>
325 <SPAN class="">
326 Exceptions are allowed but must be used carefully.
327 </SPAN>
328 <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">
329 link
330 </A></SPAN>
331 <P class="">
332<SPAN class="stylepoint_section">Definition: </SPAN>
333 Exceptions are a means of breaking out of the normal flow of control
334 of a code block to handle errors or other exceptional conditions.
335 </P>
336 <P class="">
337<SPAN class="stylepoint_section">Pros: </SPAN>
338 The control flow of normal operation code is not cluttered by
339 error-handling code. It also allows the control flow to skip multiple
340 frames when a certain condition occurs, e.g., returning from N
341 nested functions in one step instead of having to carry-through
342 error codes.
343 </P>
344 <P class="">
345<SPAN class="stylepoint_section">Cons: </SPAN>
346 May cause the control flow to be confusing. Easy to miss error
347 cases when making library calls.
348 </P>
349 <P class="">
350<SPAN class="stylepoint_section">Decision: </SPAN>
351
352
353 Exceptions must follow certain conditions:
354
355 <ul>
356 <li>Raise exceptions like this: <code>raise MyException("Error
357 message")</code> or <code>raise MyException</code>. Do not
358 use the two-argument form (<code>raise MyException, "Error
359 message"</code>) or deprecated string-based exceptions
360 (<code>raise "Error message"</code>).</li>
361 <li>Modules or packages should define their own domain-specific
362 base exception class, which should inherit from the built-in
363 Exception class. The base exception for a module should be called
364 <code>Error</code>.
365 <SPAN class=""><PRE>
366<span class="external"></span>class Error(Exception):
367 <span class="external"> </span>pass</PRE></SPAN>
368</li>
369 <li>Never use catch-all <code>except:</code> statements, or
370 catch <code>Exception</code> or <code>StandardError</code>,
371 unless you are re-raising the exception or in the outermost
372 block in your thread (and printing an error message). Python
373 is very tolerant in this regard and <code>except:</code> will
374 really catch everything including Python syntax errors. It is
375 easy to hide real bugs using <code>except:</code>.</li>
376 <li>Minimize the amount of code in a
377 <code>try</code>/<code>except</code> block. The larger the
378 body of the <code>try</code>, the more likely that an
379 exception will be raised by a line of code that you didn't
380 expect to raise an exception. In those cases,
381 the <code>try</code>/<code>except</code> block hides a real
382 error.</li>
383 <li>Use the <code>finally</code> clause to execute code whether
384 or not an exception is raised in the <code>try</code> block.
385 This is often useful for cleanup, i.e., closing a file.</li>
386 </ul>
387 </P>
388 </SPAN></SPAN>
389 </SPAN>
390 <SPAN class=""><H3><A name="Global_variables" id="Global_variables">Global variables</A></H3>
391<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Global_variables__body','Global_variables__button')" name="Global_variables__button" id="Global_variables__button">▶</SPAN>
392 <SPAN class="">
393 Avoid global variables.
394 </SPAN>
395 <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">
396 link
397 </A></SPAN>
398 <P class="">
399<SPAN class="stylepoint_section">Definition: </SPAN>
400 Variables that are declared at the module level.
401 </P>
402 <P class="">
403<SPAN class="stylepoint_section">Pros: </SPAN>
404 Occasionally useful.
405 </P>
406 <P class="">
407<SPAN class="stylepoint_section">Cons: </SPAN>
408 Has the potential to change module behavior during the import,
409 because assignments to module-level variables are done when the
410 module is imported.
411 </P>
412 <P class="">
413<SPAN class="stylepoint_section">Decision: </SPAN>
414 Avoid global variables in favor of class variables. Some
415 exceptions are:
416 <ul>
417 <li>Default options for scripts.</li>
418 <li>Module-level constants. For example: <code>PI = 3.14159</code>.
419 Constants should be named using all caps with underscores;
420 see <a HREF="#Naming">Naming</a> below.</li>
421 <li>It is sometimes useful for globals to cache values needed
422 or returned by functions.</li>
423 <li>If needed, globals should be made internal to the module
424 and accessed through public module level functions;
425 see <a HREF="#Naming">Naming</a> below.</li>
426 </ul>
427 </P>
428 </SPAN></SPAN>
429 </SPAN>
430 <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>
431<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>
432 <SPAN class="">
433 Nested/local/inner classes and functions are fine.
434 </SPAN>
435 <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">
436 link
437 </A></SPAN>
438 <P class="">
439<SPAN class="stylepoint_section">Definition: </SPAN>
mmentovaif7facf92009-10-23 21:01:49 +0000[diff] [blame^]440 A class can be defined inside of a method, function, or class. A
441 function can be defined inside a method or function. Nested functions
442 have read-only access to variables defined in enclosing scopes.
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]443 </P>
444 <P class="">
445<SPAN class="stylepoint_section">Pros: </SPAN>
446 Allows definition of utility classes and functions that are only
447 used inside of a very limited scope. Very <a HREF="http://en.wikipedia.org/wiki/Abstract_data_type">ADT</a>-y.
448 </P>
449 <P class="">
450<SPAN class="stylepoint_section">Cons: </SPAN>
451 Instances of nested or local classes cannot be pickled.
452 </P>
453 <P class="">
454<SPAN class="stylepoint_section">Decision: </SPAN>
455 They are fine.
456 </P>
457 </SPAN></SPAN>
458 </SPAN>
459 <SPAN class=""><H3><A name="List_Comprehensions" id="List_Comprehensions">List Comprehensions</A></H3>
460<SPAN class="showhide_button" onclick="javascript:ShowHideByName('List_Comprehensions__body','List_Comprehensions__button')" name="List_Comprehensions__button" id="List_Comprehensions__button">▶</SPAN>
461 <SPAN class="">
462 Okay to use for simple cases.
463 </SPAN>
464 <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">
465 link
466 </A></SPAN>
467 <P class="">
468<SPAN class="stylepoint_section">Definition: </SPAN>
469 List comprehensions and generator expressions provide a concise
470 and efficient way to create lists and iterators without
471 resorting to the use of <code>map()</code>,
472 <code>filter()</code>, or <code>lambda</code>.
473 </P>
474 <P class="">
475<SPAN class="stylepoint_section">Pros: </SPAN>
476 Simple list comprehensions can be clearer and simpler than
477 other list creation techniques. Generator expressions can be
478 very efficient, since they avoid the creation of a list
479 entirely.
480 </P>
481 <P class="">
482<SPAN class="stylepoint_section">Cons: </SPAN>
483 Complicated list comprehensions or generator expressions can be
484 hard to read.
485 </P>
486 <P class="">
487<SPAN class="stylepoint_section">Decision: </SPAN>
488 Okay to use for simple cases. Each portion must fit on one line:
489 mapping expression, <code>for</code> clause, filter expression.
490 Multiple <code>for</code> clauses or filter expressions are not
491 permitted. Use loops instead when things get more complicated.
492 </P>
493
494<SPAN class=""><PRE class="badcode">No<span class="external"></span>:
495 <span class="external"></span>result = [(x, y) for x in range(10) for y in range(5) if x * y &gt; 10]
496
497 <span class="external"></span>return ((x, y, z)
498 <span class="external"></span> for x in xrange(5)
499 <span class="external"></span> for y in xrange(5)
500 <span class="external"></span> if x != y
501 <span class="external"></span> for z in xrange(5)
502 <span class="external"></span> if y != z)</PRE></SPAN>
503<SPAN class=""><PRE>Ye<span class="external"></span>s:
504 <span class="external"></span>result = []
505 <span class="external"></span>for x in range(10):
506 <span class="external"> </span>for y in range(5):
507 <span class="external"> </span>if x * y &gt; 10:
508 <span class="external"> </span>result.append((x, y))
509
510 <span class="external"></span>for x in xrange(5):
511 <span class="external"> </span>for y in xrange(5):
512 <span class="external"> </span>if x != y:
513 <span class="external"> </span>for z in xrange(5):
514 <span class="external"> </span>if y != z:
515 <span class="external"> </span>yield (x, y, z)
516
517 <span class="external"></span>return ((x, complicated_transform(x))
518 <span class="external"></span> for x in long_generator_function(parameter)
519 <span class="external"></span> if x is not None)
520
521 <span class="external"></span>squares = [x * x for x in range(10)]
522
523 <span class="external"></span>eat(jelly_bean for jelly_bean in jelly_beans
524 <span class="external"></span> if jelly_bean.color == 'black')</PRE></SPAN>
525 </SPAN></SPAN>
526 </SPAN>
527 <SPAN class=""><H3><A name="Default_Iterators_and_Operators" id="Default_Iterators_and_Operators">Default Iterators and Operators</A></H3>
528<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>
529 <SPAN class="">
530 Use default iterators and operators for types that support them,
531 like lists, dictionaries, and files.
532 </SPAN>
533 <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">
534 link
535 </A></SPAN>
536 <P class="">
537<SPAN class="stylepoint_section">Definition: </SPAN>
538 Container types, like dictionaries and lists, define default
539 iterators and membership test operators ("in" and "not in").
540 </P>
541 <P class="">
542<SPAN class="stylepoint_section">Pros: </SPAN>
543 The default iterators and operators are simple and efficient.
544 They express the operation directly, without extra method calls.
545 A function that uses default operators is generic. It can be
546 used with any type that supports the operation.
547 </P>
548 <P class="">
549<SPAN class="stylepoint_section">Cons: </SPAN>
550 You can't tell the type of objects by reading the method names
551 (e.g. has_key() means a dictionary). This is also an advantage.
552 </P>
553 <P class="">
554<SPAN class="stylepoint_section">Decision: </SPAN> Use default iterators and operators for types
555 that support them, like lists, dictionaries, and files. The
556 built-in types define iterator methods, too. Prefer these
557 methods to methods that return lists, except that you should not
558 mutate a container while iterating over it.
559
560<SPAN class=""><PRE>Yes: <span class="external"></span>for key in adict: ...
561 <span class="external"></span>if key not in adict: ...
562 <span class="external"></span>if obj in alist: ...
563 <span class="external"></span>for line in afile: ...
564 <span class="external"></span>for k, v in dict.iteritems(): ...</PRE></SPAN>
565<SPAN class=""><PRE class="badcode">No: <span class="external"></span>for key in adict.keys(): ...
566 <span class="external"></span>if not adict.has_key(key): ...
567 <span class="external"></span>for line in afile.readlines(): ...</PRE></SPAN>
568 </P>
569 </SPAN></SPAN>
570 </SPAN>
571 <SPAN class=""><H3><A name="Generators" id="Generators">Generators</A></H3>
572<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Generators__body','Generators__button')" name="Generators__button" id="Generators__button">▶</SPAN>
573 <SPAN class="">
574 Use generators as needed.
575 </SPAN>
576 <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">
577 link
578 </A></SPAN>
579 <P class="">
580<SPAN class="stylepoint_section">Definition: </SPAN>
581 A generator function returns an iterator that yields a value each
582 time it executes a yield statement. After it yields a value, the
583 runtime state of the generator function is suspended until the
584 next value is needed.
585 </P>
586 <P class="">
587<SPAN class="stylepoint_section">Pros: </SPAN>
588 Simpler code, because the state of local variables and control flow
589 are preserved for each call. A generator uses less memory than a
590 function that creates an entire list of values at once.
591 </P>
592 <P class="">
593<SPAN class="stylepoint_section">Cons: </SPAN>
594 None.
595 </P>
596 <P class="">
597<SPAN class="stylepoint_section">Decision: </SPAN>
598 Fine. Use "Yields:" rather than "Returns:" in the
599 doc string for generator functions.
600 </P>
601 </SPAN></SPAN>
602 </SPAN>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]603 <SPAN class=""><H3><A name="Lambda_Functions" id="Lambda_Functions">Lambda Functions</A></H3>
604<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Lambda_Functions__body','Lambda_Functions__button')" name="Lambda_Functions__button" id="Lambda_Functions__button">▶</SPAN>
605 <SPAN class="">
606 Okay for one-liners.
607 </SPAN>
608 <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">
609 link
610 </A></SPAN>
611 <P class="">
612<SPAN class="stylepoint_section">Definition: </SPAN>
613 Lambdas define anonymous functions in an expression, as
614 opposed to a statement. They are often used to define callbacks or
615 operators for higher-order functions like <code>map()</code> and
616 <code>filter()</code>.
617 </P>
618 <P class="">
619<SPAN class="stylepoint_section">Pros: </SPAN>
620 Convenient.
621 </P>
622 <P class="">
623<SPAN class="stylepoint_section">Cons: </SPAN> Harder to read and debug than local functions. The
624 lack of names means stack traces are more difficult to
625 understand. Expressiveness is limited because the function may
626 only contain an expression.
627 </P>
628 <P class="">
629<SPAN class="stylepoint_section">Decision: </SPAN>
630 Okay to use them for one-liners. If the code inside the lambda
631 function is any longer than 60–80 chars, it's probably better to
632 define it as a regular (nested) function.
633 <p>
634 For common operations like multiplication, use the functions from the
635 <code>operator</code> module instead of lambda functions. For
636 example, prefer <code>operator.mul</code> to <code>lambda
637 x, y: x * y</code>.
638 </p>
639 </P>
640 </SPAN></SPAN>
641 </SPAN>
642 <SPAN class=""><H3><A name="Default_Argument_Values" id="Default_Argument_Values">Default Argument Values</A></H3>
643<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>
644 <SPAN class="">
645 Okay in most cases.
646 </SPAN>
647 <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">
648 link
649 </A></SPAN>
650 <P class="">
651<SPAN class="stylepoint_section">Definition: </SPAN>
652 You can specify values for variables at the end of a function's
653 parameter list, e.g., <code>def foo(a, b=0):</code>. If
654 <code>foo</code> is called with only one argument,
655 <code>b</code> is set to 0. If it is called with two arguments,
656 <code>b</code> has the value of the second argument.
657 </P>
658 <P class="">
659<SPAN class="stylepoint_section">Pros: </SPAN>
660 Often you have a function that uses lots of default values,
661 but—rarely—you want to override the
662 defaults. Default argument values provide an easy way to do this,
663 without having to define lots of functions for the rare
664 exceptions. Also, Python does not support overloaded
665 methods/functions and default arguments are an easy way of
666 "faking" the overloading behavior.
667 </P>
668 <P class="">
669<SPAN class="stylepoint_section">Cons: </SPAN>
670 Default arguments are evaluated once at module load
671 time. This may cause problems if the argument is a mutable
672 object such as a list or a dictionary. If the function modifies
673 the object (e.g., by appending an item to a list), the default
674 value is modified.
675 </P>
676 <P class="">
677<SPAN class="stylepoint_section">Decision: </SPAN>
678 Okay to use with the following caveats:
679 <p>
680 Do not use mutable objects as default values in the function or method
681 definition.
682 </p>
683<SPAN class=""><PRE>Yes: <span class="external"></span>def foo(a, b=None):
684 <span class="external"> </span>if b is None:
685 <span class="external"> </span>b = []</PRE></SPAN>
686<SPAN class=""><PRE class="badcode">No: <span class="external"></span>def foo(a, b=[]):
687 <span class="external"> </span>...</PRE></SPAN>
688 <p>
mmentovaif7facf92009-10-23 21:01:49 +0000[diff] [blame^]689 Calling code must use named values for arguments with a default value.
690 This helps document the code somewhat and helps prevent and detect
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]691 interface breakage when more arguments are added.
692 </p>
693<SPAN class=""><PRE>
694<span class="external"></span>def foo(a, b=1):
695 <span class="external"> </span>...</PRE></SPAN>
696<SPAN class=""><PRE>Yes: <span class="external"></span>foo(1)
697 <span class="external"></span>foo(1, b=2)</PRE></SPAN>
698<SPAN class=""><PRE class="badcode">No: <span class="external"></span>foo(1, 2)</PRE></SPAN>
699 </P>
700 </SPAN></SPAN>
701 </SPAN>
702 <SPAN class=""><H3><A name="Properties" id="Properties">Properties</A></H3>
703<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Properties__body','Properties__button')" name="Properties__button" id="Properties__button">▶</SPAN>
704 <SPAN class="">
705 Use properties for accessing or setting data where you would
706 normally have used simple, lightweight accessor or setter methods.
707 </SPAN>
708 <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">
709 link
710 </A></SPAN>
711 <P class="">
712<SPAN class="stylepoint_section">Definition: </SPAN> A way to wrap method calls for getting and
713 setting an attribute as a standard attribute access when the
714 computation is lightweight.
715 </P>
716 <P class="">
717<SPAN class="stylepoint_section">Pros: </SPAN> Readability is increased by eliminating explicit
718 get and set method calls for simple attribute access. Allows
719 calculations to be lazy. Considered the Pythonic way to
720 maintain the interface of a class. In terms of performance,
721 allowing properties bypasses needing trivial accessor methods
722 when a direct variable access is reasonable. This also allows
723 accessor methods to be added in the future without breaking the
724 interface.
725 </P>
726 <P class="">
727<SPAN class="stylepoint_section">Cons: </SPAN> Properties are specified after the getter and
728 setter methods are declared, requiring one to notice they are
729 used for properties farther down in the code (except for readonly
730 properties created with the <code>@property</code> decorator - see
731 below). Must inherit from
732 <code>object</code>. Can hide side-effects much like operator
733 overloading. Can be confusing for subclasses.
734 </P>
735 <P class="">
736<SPAN class="stylepoint_section">Decision: </SPAN> Use properties in new code to access or
737 set data where you would normally have used simple, lightweight
738 accessor or setter methods. Read-only properties should be created
739 with the <code>@property</code>
740 <a HREF="#Function_and_Method_Decorators">decorator</a>.
741
742 <p><a id="properties-template-dp">
743 Inheritance with properties can be non-obvious if the property itself is
744 not overridden. Thus one must make sure that accessor methods are
745 called indirectly to ensure methods overridden in subclasses are called
746 by the property (using the Template Method DP).
747 </a></p>
748
749 <SPAN class=""><PRE>Yes: <span class="external"></span>import math
750
751 <span class="external"></span>class Square(object):
752 <span class="external"> </span>"""A square with two properties: a writable area and a read-only perimeter.
753
754 <span class="external"> </span>To use:
755 <span class="external"> </span>&gt;&gt;&gt; sq = Square(3)
756 <span class="external"> </span>&gt;&gt;&gt; sq.area
757 <span class="external"> </span>9
758 <span class="external"> </span>&gt;&gt;&gt; sq.perimeter
759 <span class="external"> </span>12
760 <span class="external"> </span>&gt;&gt;&gt; sq.area = 16
761 <span class="external"> </span>&gt;&gt;&gt; sq.side
762 <span class="external"> </span>4
763 <span class="external"> </span>&gt;&gt;&gt; sq.perimeter
764 <span class="external"> </span>16
765 <span class="external"> </span>"""
766
767 <span class="external"> </span>def __init__(self, side):
768 <span class="external"> </span>self.side = side
769
770 <span class="external"> </span>def __get_area(self):
771 <span class="external"> </span>"""Calculates the 'area' property."""
772 <span class="external"> </span>return self.side ** 2
773
774 <span class="external"> </span>def ___get_area(self):
775 <span class="external"> </span>"""Indirect accessor for 'area' property."""
776 <span class="external"> </span>return self.__get_area()
777
778 <span class="external"> </span>def __set_area(self, area):
779 <span class="external"> </span>"""Sets the 'area' property."""
780 <span class="external"> </span>self.side = math.sqrt(area)
781
782 <span class="external"> </span>def ___set_area(self, area):
783 <span class="external"> </span>"""Indirect setter for 'area' property."""
784 <span class="external"> </span>self._SetArea(area)
785
786 <span class="external"> </span>area = property(___get_area, ___set_area,
787 <span class="external"> </span> doc="""Gets or sets the area of the square.""")
788
789 <span class="external"> </span>@property
790 <span class="external"> </span>def perimeter(self):
791 <span class="external"> </span>return self.side * 4
792<span class="external"></span>
793</PRE></SPAN>
794 </P>
795 </SPAN></SPAN>
796 </SPAN>
797 <SPAN class=""><H3><A name="True/False_evaluations" id="True/False_evaluations">True/False evaluations</A></H3>
798<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>
799 <SPAN class="">
800 Use the "implicit" false if at all possible.
801 </SPAN>
802 <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">
803 link
804 </A></SPAN>
805 <P class="">
806<SPAN class="stylepoint_section">Definition: </SPAN> Python evaluates certain values as <code>false</code>
807 when in a boolean context. A quick "rule of thumb" is that all
808 "empty" values are considered <code>false</code> so <code>0, None, [], {},
809 ""</code> all evaluate as <code>false</code> in a boolean context.
810 </P>
811 <P class="">
812<SPAN class="stylepoint_section">Pros: </SPAN> Conditions using Python booleans are easier to read
813 and less error-prone. In most cases, they're also faster.
814 </P>
815 <P class="">
816<SPAN class="stylepoint_section">Cons: </SPAN>
817 May look strange to C/C++ developers.
818 </P>
819 <P class="">
820<SPAN class="stylepoint_section">Decision: </SPAN>
821 Use the "implicit" false if at all possible, e.g., <code>if
822 foo:</code> rather than <code>if foo != []:</code>. There are a
823 few caveats that you should keep in mind though:
824 <ul>
825 <li>
826 Never use <code>==</code> or <code>!=</code> to compare
827 singletons like <code>None</code>. Use <code>is</code>
828 or <code>is not</code>.</li>
829
830 <li>Beware of writing <code>if x:</code> when you really mean
831 <code>if x is not None:</code>—e.g., when testing whether
832 a variable or argument that defaults to <code>None</code> was
833 set to some other value. The other value might be a value
834 that's false in a boolean context!</li>
835
836 <li>
837 Never compare a boolean variable to <code>False</code> using
838 <code>==</code>. Use <code>if not x:</code> instead. If
839 you need to distinguish <code>False</code> from
840 <code>None</code> then chain the expressions,
841 such as <code>if not x and x is not None:</code>.
842 </li>
843
844 <li>
845 For sequences (strings, lists, tuples), use the fact that
846 empty sequences are false, so <code>if not seq:</code> or
847 <code>if seq:</code> is preferable to <code>if
848 len(seq):</code> or <code>if not
849 len(seq):</code>.</li>
850
851 <li>
852 When handling integers, implicit false may involve more risk than
853 benefit (i.e., accidentally handling <code>None</code> as 0). You may
854 compare a value which is known to be an integer (and is not the
855 result of <code>len()</code>) against the integer 0.
856<SPAN class=""><PRE>Yes: <span class="external"></span>if not users:
857 <span class="external"> </span>print 'no users'
858
859 <span class="external"></span>if foo == 0:
860 <span class="external"> </span>self.handle_zero()
861
862 <span class="external"></span>if i % 10 == 0:
863 <span class="external"> </span>self.handle_multiple_of_ten()</PRE></SPAN>
864<SPAN class=""><PRE class="badcode">No: <span class="external"></span>if len(users) == 0:
865 <span class="external"> </span>print 'no users'
866
867 <span class="external"></span>if foo is not None and not foo:
868 <span class="external"> </span>self.handle_zero()
869
870 <span class="external"></span>if not i % 10:
871 <span class="external"> </span>self.handle_multiple_of_ten()</PRE></SPAN>
872</li>
873
874 <li>
875 Note that <code>'0'</code> (i.e., <code>0</code> as string)
876 evaluates to true.</li>
877 </ul>
878 </P>
879 </SPAN></SPAN>
880 </SPAN>
mmentovaif7facf92009-10-23 21:01:49 +0000[diff] [blame^]881 <SPAN class=""><H3><A name="Deprecated_Language_Features" id="Deprecated_Language_Features">Deprecated Language Features</A></H3>
882<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Deprecated_Language_Features__body','Deprecated_Language_Features__button')" name="Deprecated_Language_Features__button" id="Deprecated_Language_Features__button">▶</SPAN>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]883 <SPAN class="">
mmentovaif7facf92009-10-23 21:01:49 +0000[diff] [blame^]884 Use string methods instead of the <code>string</code> module
885 where possible. Use function call syntax instead
886 of <code>apply</code>. Use list comprehensions
887 and <code>for</code> loops instead of <code>filter</code>,
888 <code>map</code>, and <code>reduce</code>.
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]889 </SPAN>
mmentovaif7facf92009-10-23 21:01:49 +0000[diff] [blame^]890 <SPAN class=""><BR><SPAN class="stylepoint_body" name="Deprecated_Language_Features__body" id="Deprecated_Language_Features__body" style="display: none"><SPAN class="link_button"><A href="?showone=Deprecated_Language_Features#Deprecated_Language_Features">
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]891 link
892 </A></SPAN>
893 <P class="">
mmentovaif7facf92009-10-23 21:01:49 +0000[diff] [blame^]894<SPAN class="stylepoint_section">Definition: </SPAN>
895 Current versions of Python provide alternative constructs
896 that people find generally preferable.
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]897 </P>
898 <P class="">
mmentovaif7facf92009-10-23 21:01:49 +0000[diff] [blame^]899<SPAN class="stylepoint_section">Decision: </SPAN>
900 We do not use any Python version which does not support
901 these features, so there is no reason not to use the new
902 styles.
903<SPAN class=""><PRE class="badcode">No: <span class="external"></span>words = string.split(foo, ':')
904
905 <span class="external"></span>map(lambda x: x[1], filter(lambda x: x[2] == 5, my_list))
906
907 <span class="external"></span>apply(fn, args, kwargs)</PRE></SPAN>
908<SPAN class=""><PRE>Yes: <span class="external"></span>words = foo.split(':')
909
910 <span class="external"></span>[x[1] for x in my_list if x[2] == 5]
911
912 <span class="external"></span>fn(*args, **kwargs)</PRE></SPAN>
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]913 </P>
914 </SPAN></SPAN>
915 </SPAN>
916 <SPAN class=""><H3><A name="Lexical_Scoping" id="Lexical_Scoping">Lexical Scoping</A></H3>
917<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Lexical_Scoping__body','Lexical_Scoping__button')" name="Lexical_Scoping__button" id="Lexical_Scoping__button">▶</SPAN>
918 <SPAN class="">
919 Okay to use.
920 </SPAN>
921 <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">
922 link
923 </A></SPAN>
924 <P class="">
925<SPAN class="stylepoint_section">Definition: </SPAN>
926 A nested Python function can refer to variables defined in
927 enclosing functions, but can not assign to them. Variable
928 bindings are resolved using lexical scoping, that is, based on
929 the static program text. Any assignment to a name in a block
930 will cause Python to treat all references to that name as a
931 local variable, even if the use precedes the assignment. If a
932 global declaration occurs, the name is treated as a global
933 variable.
934
935 <p>
936 An example of the use of this feature is:
937 </p>
938
939 <SPAN class=""><PRE>
940<span class="external"></span>def get_adder(summand1):
941 <span class="external"> </span>"""Returns a function that adds numbers to a given number."""
942 <span class="external"> </span>def adder(summand2):
943 <span class="external"> </span>return summand1 + summand2
944
945 <span class="external"> </span>return adder
946<span class="external"></span>
947</PRE></SPAN>
948 </P>
949 <P class="">
950<SPAN class="stylepoint_section">Pros: </SPAN>
951 Often results in clearer, more elegant code. Especially comforting
952 to experienced Lisp and Scheme (and Haskell and ML and …)
953 programmers.
954 </P>
955 <P class="">
956<SPAN class="stylepoint_section">Cons: </SPAN>
957 Can lead to confusing bugs. Such as this example based on
958 <a HREF="http://www.python.org/dev/peps/pep-0227/">PEP-0227</a>:
959<SPAN class=""><PRE class="badcode">
960<span class="external"></span>i = 4
961<span class="external"></span>def foo(x):
962 <span class="external"> </span>def bar():
963 <span class="external"> </span>print i,
964 <span class="external"> </span># ...
965 <span class="external"> </span># A bunch of code here
966 <span class="external"> </span># ...
967 <span class="external"> </span>for i in x: # Ah, i *is* local to Foo, so this is what Bar sees
968 <span class="external"> </span>print i,
969 <span class="external"> </span>bar()</PRE></SPAN>
970 <p>
971 So <code>foo([1, 2, 3])</code> will print <code>1 2 3 3</code>, not
972 <code>1 2 3 4</code>.
973 </p>
974 </P>
975 <P class="">
976<SPAN class="stylepoint_section">Decision: </SPAN>
977 Okay to use.
978 </P>
979 </SPAN></SPAN>
980 </SPAN>
981 <SPAN class=""><H3><A name="Function_and_Method_Decorators" id="Function_and_Method_Decorators">Function and Method Decorators</A></H3>
982<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>
983 <SPAN class="">
984 Use decorators judiciously when there is a clear advantage.
985 </SPAN>
986 <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">
987 link
988 </A></SPAN>
989 <P class="">
990<SPAN class="stylepoint_section">Definition: </SPAN>
991
992 <a HREF="http://www.python.org/doc/2.4.3/whatsnew/node6.html">Decorators
993 for Functions and Methods</a>
994 (a.k.a "the <code>@</code> notation").
995 The most common decorators are <code>@classmethod</code> and
996 <code>@staticmethod</code>, for converting ordinary methods to class or
997 static methods. However, the decorator syntax allows for
998 user-defined decorators as well. Specifically, for some function
999 <code>my_decorator</code>, this:
1000 <SPAN class=""><PRE>
1001<span class="external"></span>class C(object):
1002 <span class="external"> </span>@my_decorator
1003 <span class="external"> </span>def method(self):
1004 <span class="external"> </span># method body ...
1005<span class="external"></span>
1006</PRE></SPAN>
1007
1008 is equivalent to:
1009 <SPAN class=""><PRE>
1010<span class="external"></span>class C(object):
1011 <span class="external"> </span>def method(self):
1012 <span class="external"> </span># method body ...
1013 <span class="external"> </span>method = my_decorator(method)
1014<span class="external"></span>
1015</PRE></SPAN>
1016 </P>
1017 <P class="">
1018<SPAN class="stylepoint_section">Pros: </SPAN> Elegantly specifies some transformation on a method; the
1019 transformation might eliminate some repetitive code, enforce invariants,
1020 etc.
1021 </P>
1022 <P class="">
1023<SPAN class="stylepoint_section">Cons: </SPAN> Decorators can perform arbitrary operations on a
1024 function's arguments or return values, resulting in surprising
1025 implicit behavior.
1026 Additionally, decorators execute at import time. Failures in decorator
1027 code are pretty much impossible to recover from.
1028 </P>
1029 <P class="">
1030<SPAN class="stylepoint_section">Decision: </SPAN> Use decorators judiciously when there is a clear
1031 advantage. Decorators should follow the same import and naming
1032 guidelines as functions. Decorator pydoc should clearly state that the
1033 function is a decorator. Write unit tests for decorators.
1034
1035 <p>
1036 Avoid external dependencies in the decorator itself (e.g. don't rely on
1037 files, sockets, database connections, etc.), since they might not be
1038 available when the decorator runs (at import time, perhaps from
1039 <code>pychecker</code> or other tools). A decorator that is
1040 called with valid parameters should (as much as possible) be guaranteed
1041 to succeed in all cases.
1042 </p>
1043 <p>
1044 Decorators are a special case of "top level code" - see
1045 <a HREF="#Main">main</a> for more discussion.
1046 </p>
1047 </P>
1048 </SPAN></SPAN>
1049 </SPAN>
1050 <SPAN class=""><H3><A name="Threading" id="Threading">Threading</A></H3>
1051<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Threading__body','Threading__button')" name="Threading__button" id="Threading__button">▶</SPAN>
1052 <SPAN class="">
1053 Do not rely on the atomicity of built-in types.
1054 </SPAN>
1055 <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">
1056 link
1057 </A></SPAN>
1058 <p>
1059 While Python's built-in data types such as dictionaries appear
1060 to have atomic operations, there are corner cases where they
1061 aren't atomic (e.g. if <code>__hash__</code> or
1062 <code>__eq__</code> are implemented as Python methods) and their
1063 atomicity should not be relied upon. Neither should you rely on
1064 atomic variable assignment (since this in turn depends on
1065 dictionaries).
1066 </p>
1067
1068 <p>
1069 Use the Queue module's <code>Queue</code> data type as the preferred
1070 way to
1071 communicate data between threads. Otherwise, use the threading
1072 module and its locking primitives. Learn about the proper use
1073 of condition variables so you can use
1074 <code>threading.Condition</code> instead of using lower-level
1075 locks.
1076 </p>
1077 </SPAN></SPAN>
1078 </SPAN>
1079 <SPAN class=""><H3><A name="Power_Features" id="Power_Features">Power Features</A></H3>
1080<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Power_Features__body','Power_Features__button')" name="Power_Features__button" id="Power_Features__button">▶</SPAN>
1081 <SPAN class="">
1082 Avoid these features.
1083 </SPAN>
1084 <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">
1085 link
1086 </A></SPAN>
1087 <P class="">
1088<SPAN class="stylepoint_section">Definition: </SPAN> Python is an extremely flexible language and
1089 gives you many fancy features such as metaclasses, access to bytecode,
1090 on-the-fly compilation, dynamic inheritance, object reparenting,
1091 import hacks, reflection, modification of system internals,
1092 etc.
1093 </P>
1094 <P class="">
1095<SPAN class="stylepoint_section">Pros: </SPAN> These are powerful language features. They can
1096 make your code more compact.
1097 </P>
1098 <P class="">
1099<SPAN class="stylepoint_section">Cons: </SPAN> It's very tempting to use these "cool" features
1100 when they're not absolutely necessary. It's harder to read,
1101 understand, and debug code that's using unusual features
1102 underneath. It doesn't seem that way at first (to the original
1103 author), but when revisiting the code, it tends to be more
1104 difficult than code that is longer but is straightforward.
1105 </P>
1106 <P class="">
1107<SPAN class="stylepoint_section">Decision: </SPAN>
1108 Avoid these features in
1109 your code.
1110 </P>
1111 </SPAN></SPAN>
1112 </SPAN>
1113 </SPAN>
1114 <SPAN class=""><H2 name="Python_Style_Rules" id="Python_Style_Rules">Python Style Rules</H2>
1115 <SPAN class=""><H3><A name="Semicolons" id="Semicolons">Semicolons</A></H3>
1116<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Semicolons__body','Semicolons__button')" name="Semicolons__button" id="Semicolons__button">▶</SPAN>
1117 <SPAN class="">
1118 Do not terminate your lines with semi-colons and do not use
1119 semi-colons to put two commands on the same line.
1120 </SPAN>
1121 <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">
1122 link
1123 </A></SPAN>
1124 </SPAN></SPAN>
1125 </SPAN>
1126 <SPAN class=""><H3><A name="Line_length" id="Line_length">Line length</A></H3>
1127<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Line_length__body','Line_length__button')" name="Line_length__button" id="Line_length__button">▶</SPAN>
1128 <SPAN class="">
1129 Maximum line length is <em>80 characters</em>.
1130 </SPAN>
1131 <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">
1132 link
1133 </A></SPAN>
1134 <p>
1135 Exception: lines importing modules may end up longer than 80
1136 characters only if using Python 2.4 or
1137 earlier.
1138 </p>
1139
1140 <p>
1141 Make use of Python's
1142
1143 <a HREF="http://www.python.org/doc/ref/implicit-joining.html">implicit
1144 line joining inside parentheses, brackets and braces</a>.
1145 If necessary, you can add an extra pair of parentheses around an
1146 expression.
1147 </p>
1148
1149
1150 <SPAN class=""><PRE>Yes: foo_bar(self, width, height, color='black', design=None, x='foo',
1151 emphasis=None, highlight=0)
1152
1153 if (width == 0 and height == 0 and
1154 color == 'red' and emphasis == 'strong'):</PRE></SPAN>
1155
1156
1157 <p>
1158 When a literal string won't fit on a single line, use parentheses for
1159 implicit line joining.
1160 </p>
1161
1162 <SPAN class=""><PRE>
1163<span class="external"></span>x = ('This will build a very long long '
1164<span class="external"></span> 'long long long long long long string')</PRE></SPAN>
1165
1166 <p>
1167 Make note of the indentation of the elements in the line
1168 continuation examples above; see the
1169 <a HREF="#indentation">indentation</a>
1170 section for explanation.
1171 </p>
1172 </SPAN></SPAN>
1173 </SPAN>
1174 <SPAN class=""><H3><A name="Parentheses" id="Parentheses">Parentheses</A></H3>
1175<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Parentheses__body','Parentheses__button')" name="Parentheses__button" id="Parentheses__button">▶</SPAN>
1176 <SPAN class="">
1177 Use parentheses sparingly.
1178 </SPAN>
1179 <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">
1180 link
1181 </A></SPAN>
1182 <p>
1183 Do not use them in return statements or conditional statements unless
1184 using parentheses for implied line continuation. (See above.)
1185 It is however fine to use parentheses around tuples.
1186 </p>
1187
1188<SPAN class=""><PRE>Yes: <span class="external"></span>if foo:
1189 <span class="external"> </span>bar()
1190 <span class="external"></span>while x:
1191 <span class="external"> </span>x = bar()
1192 <span class="external"></span>if x and y:
1193 <span class="external"> </span>bar()
1194 <span class="external"></span>if not x:
1195 <span class="external"> </span>bar()
1196 <span class="external"></span>return foo
1197 <span class="external"></span>for (x, y) in dict.items(): ...</PRE></SPAN>
1198<SPAN class=""><PRE class="badcode">No: <span class="external"></span>if (x):
1199 <span class="external"> </span>bar()
1200 <span class="external"></span>if not(x):
1201 <span class="external"> </span>bar()
1202 <span class="external"></span>return (foo)</PRE></SPAN>
1203 </SPAN></SPAN>
1204 </SPAN>
1205 <SPAN class=""><H3><A name="Indentation" id="Indentation">Indentation</A></H3>
1206<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Indentation__body','Indentation__button')" name="Indentation__button" id="Indentation__button">▶</SPAN>
1207 <SPAN class="">
1208 Indent your code blocks with <em>4 spaces</em>.
1209 </SPAN>
1210 <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">
1211 link
1212 </A></SPAN>
1213 <p>
1214 Never use tabs or mix tabs and spaces.
1215 In cases of implied line continuation, you should align wrapped elements
1216 either vertically, as per the examples in the
1217 <a HREF="#Line_length">line length</a> section; or using a hanging
1218 indent of 4 spaces, in which case there should be no argument on
1219 the first line.
1220 </p>
1221
1222
1223<SPAN class=""><PRE>Yes: # Aligned with opening delimiter
1224 foo = long_function_name(var_one, var_two,
1225 var_three, var_four)
1226
1227 # 4-space hanging indent; nothing on first line
1228 foo = long_function_name(
1229 var_one, var_two, var_three,
1230 var_four)</PRE></SPAN>
1231<SPAN class=""><PRE class="badcode">No: <span class="external"></span># Stuff on first line forbidden
1232 <span class="external"></span>foo = long_function_name(var_one, var_two,
1233 <span class="external"></span> var_three, var_four)
1234
1235 <span class="external"></span># 2-space hanging indent forbidden
1236 <span class="external"></span>foo = long_function_name(
1237 <span class="external"></span> var_one, var_two, var_three,
1238 <span class="external"></span> var_four)</PRE></SPAN>
1239 </SPAN></SPAN>
1240 </SPAN>
1241 <SPAN class=""><H3><A name="Blank_Lines" id="Blank_Lines">Blank Lines</A></H3>
1242<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Blank_Lines__body','Blank_Lines__button')" name="Blank_Lines__button" id="Blank_Lines__button">▶</SPAN>
1243 <SPAN class="">
1244 Two blank lines between top-level definitions, one blank line
1245 between method definitions.
1246 </SPAN>
1247 <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">
1248 link
1249 </A></SPAN>
1250 <p>
1251 Two blank lines between top-level definitions, be they function
1252 or class definitions. One blank line between method definitions
1253 and between the <code>class</code> line and the first method.
1254 Use single blank lines as you judge appropriate within functions or
1255 methods.
1256 </p>
1257 </SPAN></SPAN>
1258 </SPAN>
1259 <SPAN class=""><H3><A name="Whitespace" id="Whitespace">Whitespace</A></H3>
1260<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Whitespace__body','Whitespace__button')" name="Whitespace__button" id="Whitespace__button">▶</SPAN>
1261 <SPAN class="">
1262 Follow standard typographic rules for the use of spaces around
1263 punctuation.
1264 </SPAN>
1265 <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">
1266 link
1267 </A></SPAN>
1268 <p>
1269 No whitespace inside parentheses, brackets or braces.
1270 </p>
1271<SPAN class=""><PRE>Yes: <span class="external"></span>spam(ham[1], {eggs: 2}, [])</PRE></SPAN>
1272<SPAN class=""><PRE class="badcode">No: <span class="external"></span>spam( ham[ 1 ], { eggs: 2 }, [ ] )</PRE></SPAN>
1273 <p>
1274 No whitespace before a comma, semicolon, or colon. Do use
1275 whitespace after a comma, semicolon, or colon except at the end
1276 of the line.
1277 </p>
1278<SPAN class=""><PRE>Yes: <span class="external"></span>if x == 4:
1279 <span class="external"> </span>print x, y
1280 <span class="external"></span>x, y = y, x</PRE></SPAN>
1281<SPAN class=""><PRE class="badcode">No: <span class="external"></span>if x == 4 :
1282 <span class="external"> </span>print x , y
1283 <span class="external"></span>x , y = y , x</PRE></SPAN>
1284 <p>
1285 No whitespace before the open paren/bracket that starts an argument list,
1286 indexing or slicing.
1287 </p>
1288 <SPAN class=""><PRE>Yes: <span class="external"></span>spam(1)</PRE></SPAN>
1289<SPAN class=""><PRE class="badcode">No: <span class="external"></span>spam (1)</PRE></SPAN>
1290<SPAN class=""><PRE>Yes: <span class="external"></span>dict['key'] = list[index]</PRE></SPAN>
1291<SPAN class=""><PRE class="badcode">No: <span class="external"></span>dict ['key'] = list [index]</PRE></SPAN>
1292
1293 <p>
1294 Surround binary operators with a single space on either side for
1295 assignment (<code>=</code>), comparisons (<code>==, &lt;, &gt;, !=,
1296 &lt;&gt;, &lt;=, &gt;=, in, not in, is, is not</code>), and Booleans
1297 (<code>and, or, not</code>). Use your better judgment for the
1298 insertion of spaces around arithmetic operators but always be
1299 consistent about whitespace on either side of a binary operator.
1300 </p>
1301<SPAN class=""><PRE>Yes: <span class="external"></span>x == 1</PRE></SPAN>
1302<SPAN class=""><PRE class="badcode">No: <span class="external"></span>x&lt;1</PRE></SPAN>
1303 <p>
1304 Don't use spaces around the '=' sign when used to indicate a
1305 keyword argument or a default parameter value.
1306 </p>
1307<SPAN class=""><PRE>Yes: <span class="external"></span>def complex(real, imag=0.0): return magic(r=real, i=imag)</PRE></SPAN>
1308<SPAN class=""><PRE class="badcode">No: <span class="external"></span>def complex(real, imag = 0.0): return magic(r = real, i = imag)</PRE></SPAN>
1309
1310 <p>
1311 Don't use spaces to vertically align tokens on consecutive lines, since it
1312 becomes a maintenance burden (applies to <code>:</code>, <code>#</code>,
1313 <code>=</code>, etc.):
1314 </p>
1315<SPAN class=""><PRE>Yes:
1316 foo = 1000 # comment
1317 long_name = 2 # comment that should not be aligned
1318
1319 dictionary = {
1320 "foo": 1,
1321 "long_name": 2,
1322 }</PRE></SPAN>
1323<SPAN class=""><PRE class="badcode">No:
1324 foo = 1000 # comment
1325 long_name = 2 # comment that should not be aligned
1326
1327 dictionary = {
1328 "foo" : 1,
1329 "long_name": 2,
1330 }</PRE></SPAN>
1331
1332
1333 </SPAN></SPAN>
1334 </SPAN>
1335 <SPAN class=""><H3><A name="Python_Interpreter" id="Python_Interpreter">Python Interpreter</A></H3>
1336<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Python_Interpreter__body','Python_Interpreter__button')" name="Python_Interpreter__button" id="Python_Interpreter__button">▶</SPAN>
1337 <SPAN class="">
1338 Modules should begin with
1339
1340 <code>#!/usr/bin/env python&lt;version&gt;</code>
1341 </SPAN>
1342 <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">
1343 link
1344 </A></SPAN>
1345 <p>
1346 Modules should begin with a "shebang" line specifying the Python
1347 interpreter used to execute the program:
1348 </p>
1349
1350<SPAN class=""><PRE>
1351<span class="external"></span>#!/usr/bin/env python2.4</PRE></SPAN>
1352
1353 <p>
1354 Always use the most specific version you can use, e.g.,
1355 <code>/usr/bin/python2.4</code>, not
1356 <code>/usr/bin/python2</code>. This makes it easier to find
1357 dependencies when
1358
1359 upgrading to a different Python version
1360 and also avoids confusion and breakage during use. E.g., Does
1361 <code>/usr/bin/python2</code> mean Python 2.0.1 or Python
1362 2.3.0?
1363 </p>
1364
1365 </SPAN></SPAN>
1366 </SPAN>
1367 <SPAN class=""><H3><A name="Comments" id="Comments">Comments</A></H3>
1368<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Comments__body','Comments__button')" name="Comments__button" id="Comments__button">▶</SPAN>
1369 <SPAN class="">
1370 Be sure to use the right style for module, function, method and in-line
1371 comments.
1372 </SPAN>
1373 <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">
1374 link
1375 </A></SPAN>
1376
1377 <P class="">
1378<SPAN class="stylepoint_subsection">Doc Strings</SPAN>
1379
1380 <p>
1381 Python has a unique commenting style using doc strings. A doc
1382 string is a string that is the first statement in a package,
1383 module, class or function. These strings can be extracted
1384 automatically through the <code>__doc__</code> member of the
1385 object and are used by <code>pydoc</code>. (Try running
1386 <code>pydoc</code> on your module to see how it looks.) Our
1387 convention for doc strings is to use the three double-quote
1388 format for strings. A doc string should be organized as a
1389 summary line (one physical line) terminated by a period,
1390 question mark, or exclamation point, followed by a blank line,
1391 followed by the rest of the doc string starting at the same
1392 cursor position as the first quote of the first line. There are
1393 more formatting guidelines for doc strings below.
1394 </p>
1395
1396 </P>
1397 <P class="">
1398<SPAN class="stylepoint_subsection">Modules</SPAN>
1399
1400
1401
1402 <p>
1403 Every file should contain the following items, in order:
1404 <ul>
1405 <li>a copyright statement (for example,
1406 <code>Copyright 2008 Google Inc.</code>)</li>
1407 <li>a license boilerplate. Choose the appropriate boilerplate
1408 for the license used by the project (for example, Apache 2.0, BSD,
1409 LGPL, GPL)</li>
1410 <li>an author line to identify the original author of the file</li>
1411 </ul>
1412 </p>
1413 </P>
1414 <P class="">
1415<SPAN class="stylepoint_subsection">Functions and Methods</SPAN>
1416
1417 <p>
1418 Any function or method which is not both obvious and very short
1419 needs a doc string. Additionally, any externally accessible
1420 function or method regardless of length or simplicity needs a
1421 doc string. The doc string should include what the function does
1422 and have detailed descriptions of the input and output. It
1423 should not, generally, describe how it does it unless it's some
1424 complicated algorithm. For tricky code block/inline comments
1425 within the code are more appropriate. The doc string should give
1426 enough information to write a call to the function without
1427 looking at a single line of the function's code. Args should be
1428 individually documented, an explanation following after a colon,
1429 and should use a uniform hanging indent of 2 or 4 spaces. The
1430 doc string should specify the expected types where specific types
1431 are required. A "Raises:" section should list all exceptions
1432 that can be raised by the function. The doc string for generator
1433 functions should use "Yields:" rather than "Returns:".
1434 </p>
1435
1436 <SPAN class=""><PRE>
1437<span class="external"></span>def fetch_bigtable_rows(big_table, keys, other_silly_variable=None):
1438 <span class="external"> </span>"""Fetches rows from a Bigtable.
1439
1440 <span class="external"> </span>Retrieves rows pertaining to the given keys from the Table instance
1441 <span class="external"> </span>represented by big_table. Silly things may happen if
1442 <span class="external"> </span>other_silly_variable is not None.
1443
1444 <span class="external"> </span>Args:
1445 <span class="external"> </span>big_table: An open Bigtable Table instance.
1446 <span class="external"> </span>keys: A sequence of strings representing the key of each table row
1447 <span class="external"> </span> to fetch.
1448 <span class="external"> </span>other_silly_variable: Another optional variable, that has a much
1449 <span class="external"> </span> longer name than the other args, and which does nothing.
1450
1451 <span class="external"> </span>Returns:
1452 <span class="external"> </span>A dict mapping keys to the corresponding table row data
1453 <span class="external"> </span>fetched. Each row is represented as a tuple of strings. For
1454 <span class="external"> </span>example:
1455
1456 <span class="external"> </span>{'Serak': ('Rigel VII', 'Preparer'),
1457 <span class="external"> </span> 'Zim': ('Irk', 'Invader'),
1458 <span class="external"> </span> 'Lrrr': ('Omicron Persei 8', 'Emperor')}
1459
1460 <span class="external"> </span>If a key from the keys argument is missing from the dictionary,
1461 <span class="external"> </span>then that row was not found in the table.
1462
1463 <span class="external"> </span>Raises:
1464 <span class="external"> </span>IOError: An error occurred accessing the bigtable.Table object.
1465 <span class="external"> </span>"""
1466 <span class="external"> </span>pass
1467<span class="external"></span>
1468</PRE></SPAN>
1469 </P>
1470 <P class="">
1471<SPAN class="stylepoint_subsection">Classes</SPAN>
1472
1473 <p>
1474 Classes should have a doc string below the class definition describing
1475 the class. If your class has public attributes, they should be documented
1476 here in an Attributes section and follow the same formatting as a
1477 function's Args section.
1478 </p>
1479
1480 <SPAN class=""><PRE>
1481<span class="external"></span>class SampleClass(object):
1482 <span class="external"> </span>"""Summary of class here.
1483
1484 <span class="external"> </span>Longer class information....
1485 <span class="external"> </span>Longer class information....
1486
1487 <span class="external"> </span>Attributes:
1488 <span class="external"> </span>likes_spam: A boolean indicating if we like SPAM or not.
1489 <span class="external"> </span>eggs: An integer count of the eggs we have laid.
1490 <span class="external"> </span>"""
1491
1492 <span class="external"> </span>def __init__(self, likes_spam=False):
1493 <span class="external"> </span>"""Inits SampleClass with blah."""
1494 <span class="external"> </span>self.likes_spam = likes_spam
1495 <span class="external"> </span>self.eggs = 0
1496
1497 <span class="external"> </span>def public_method(self):
1498 <span class="external"> </span>"""Performs operation blah."""
1499<span class="external"></span>
1500</PRE></SPAN>
1501
1502 </P>
1503 <P class="">
1504<SPAN class="stylepoint_subsection">Block and Inline Comments</SPAN>
1505
1506 <p>
1507 The final place to have comments is in tricky parts of the
1508 code. If you're going to have to explain it at the next
1509 <a HREF="http://en.wikipedia.org/wiki/Code_review">code review</a>,
1510 you should comment it now. Complicated operations get a few lines of
1511 comments before the operations
1512 commence. Non-obvious ones get comments at the end of the line.
1513 </p>
1514
1515 <SPAN class=""><PRE>
1516<span class="external"></span># We use a weighted dictionary search to find out where i is in
1517<span class="external"></span># the array. We extrapolate position based on the largest num
1518<span class="external"></span># in the array and the array size and then do binary search to
1519<span class="external"></span># get the exact number.
1520
1521<span class="external"></span>if i &amp; (i-1) == 0: # true iff i is a power of 2
1522<span class="external"></span>
1523</PRE></SPAN>
1524
1525 <p>
1526 To improve legibility, these comments should be at least 2 spaces away
1527 from the code.
1528 </p>
1529
1530 <p>
1531 On the other hand, never describe the code. Assume the person
1532 reading the code knows Python (though not what you're trying to
1533 do) better than you do.
1534 </p>
1535
1536 <SPAN class=""><PRE class="badcode">
1537<span class="external"></span># BAD COMMENT: Now go through the b array and make sure whenever i occurs
1538<span class="external"></span># the next element is i+1
1539<span class="external"></span>
1540</PRE></SPAN>
1541
1542 </P>
1543 </SPAN></SPAN>
1544 </SPAN>
1545 <SPAN class=""><H3><A name="Classes" id="Classes">Classes</A></H3>
1546<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Classes__body','Classes__button')" name="Classes__button" id="Classes__button">▶</SPAN>
1547 <SPAN class="">
1548 If a class inherits from no other base classes, explicitly inherit
1549 from <code>object</code>. This also applies to nested classes.
1550 </SPAN>
1551 <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">
1552 link
1553 </A></SPAN>
1554 <SPAN class=""><PRE class="badcode">No: <span class="external"></span>class SampleClass:
1555 <span class="external"> </span>pass
1556
1557
1558 <span class="external"></span>class OuterClass:
1559
1560 <span class="external"> </span>class InnerClass:
1561 <span class="external"> </span>pass
1562<span class="external"></span>
1563</PRE></SPAN>
1564
1565 <SPAN class=""><PRE>Yes: <span class="external"></span>class SampleClass(object):
1566 <span class="external"> </span>pass
1567
1568
1569 <span class="external"></span>class OuterClass(object):
1570
1571 <span class="external"> </span>class InnerClass(object):
1572 <span class="external"> </span>pass
1573
1574
1575 <span class="external"></span>class ChildClass(ParentClass):
1576 <span class="external"> </span>"""Explicitly inherits from another class already."""
1577<span class="external"></span>
1578</PRE></SPAN>
1579
1580 <p>Inheriting from <code>object</code> is needed to make properties work
1581 properly, and it will protect your code from one particular potential
1582 incompatibility with Python 3000. It also defines
1583 special methods that implement the default semantics of objects including
1584 <code>__new__</code>, <code>__init__</code>, <code>__delattr__</code>,
1585 <code>__getattribute__</code>, <code>__setattr__</code>,
1586 <code>__hash__</code>, <code>__repr__</code>, and <code>__str__</code>.
1587 </p>
1588 </SPAN></SPAN>
1589 </SPAN>
1590 <SPAN class=""><H3><A name="Strings" id="Strings">Strings</A></H3>
1591<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Strings__body','Strings__button')" name="Strings__button" id="Strings__button">▶</SPAN>
1592 <SPAN class="">
1593 Use the <code>%</code> operator for formatting strings,
1594 even when the parameters are all strings. Use your best judgement
1595 to decide between <code>+</code> and <code>%</code> though.
1596 </SPAN>
1597 <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">
1598 link
1599 </A></SPAN>
1600<SPAN class=""><PRE class="badcode">No: <span class="external"></span>x = '%s%s' % (a, b) # use + in this case
1601 <span class="external"></span>x = imperative + ', ' + expletive + '!'
1602 <span class="external"></span>x = 'name: ' + name + '; score: ' + str(n)</PRE></SPAN>
1603<SPAN class=""><PRE>Yes: <span class="external"></span>x = a + b
1604 <span class="external"></span>x = '%s, %s!' % (imperative, expletive)
1605 <span class="external"></span>x = 'name: %s; score: %d' % (name, n)</PRE></SPAN>
1606
1607 <p>
1608 Avoid using the <code>+</code> and <code>+=</code> operators to
1609 accumulate a string within a loop. Since strings are immutable, this
1610 creates unnecessary temporary objects and results in quadratic rather
1611 than linear running time. Instead, add each substring to a list and
1612 <code>''.join</code> the list after the loop terminates (or, write each
1613 substring to a <code>cStringIO.StringIO</code> buffer).
1614 </p>
1615
1616<SPAN class=""><PRE class="badcode">No: <span class="external"></span>employee_table = '&lt;table&gt;'
1617 <span class="external"></span>for last_name, first_name in employee_list:
1618 <span class="external"> </span>employee_table += '&lt;tr&gt;&lt;td&gt;%s, %s&lt;/td&gt;&lt;/tr&gt;' % (last_name, first_name)
1619 <span class="external"></span>employee_table += '&lt;/table&gt;'</PRE></SPAN>
1620<SPAN class=""><PRE>Yes: <span class="external"></span>items = ['&lt;table&gt;']
1621 <span class="external"></span>for last_name, first_name in employee_list:
1622 <span class="external"> </span>items.append('&lt;tr&gt;&lt;td&gt;%s, %s&lt;/td&gt;&lt;/tr&gt;' % (last_name, first_name))
1623 <span class="external"></span>items.append('&lt;/table&gt;')
1624 <span class="external"></span>employee_table = ''.join(items)</PRE></SPAN>
1625
1626 <p>
1627 Use <code>"""</code> for multi-line strings rather than
1628 <code>'''</code>. Note, however, that it is often cleaner to
1629 use implicit line joining since multi-line strings do
1630 not flow with the indentation of the rest of the program:
1631 </p>
1632
1633 <SPAN class=""><PRE class="badcode"> No<span class="external"></span>:
1634 <span class="external"></span>print """This is pretty ugly.
1635Don'<span class="external"></span>t do this.
1636"""<span class="external"></span>
1637</PRE></SPAN>
1638<SPAN class=""><PRE>Ye<span class="external"></span>s:
1639 <span class="external"></span>print ("This is much nicer.\n"
1640 <span class="external"></span> "Do it this way.\n")</PRE></SPAN>
1641 </SPAN></SPAN>
1642 </SPAN>
1643 <SPAN class=""><H3><A name="TODO_Comments" id="TODO_Comments">TODO Comments</A></H3>
1644<SPAN class="showhide_button" onclick="javascript:ShowHideByName('TODO_Comments__body','TODO_Comments__button')" name="TODO_Comments__button" id="TODO_Comments__button">▶</SPAN>
1645 <SPAN class="">
1646 Use <code>TODO</code> comments for code that is temporary, a
1647 short-term solution, or good-enough but not perfect.
1648 </SPAN>
1649 <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">
1650 link
1651 </A></SPAN>
1652 <p>
1653 <code>TODO</code>s should include the string <code>TODO</code> in
1654 all caps, followed by your
1655
1656 name, e-mail address, or other
1657 identifier
1658 in parentheses. A colon is optional. A comment explaining what there
1659 is to do is required. The main purpose is to have
1660 a consistent <code>TODO</code> format searchable by the person
1661 adding the comment (who can provide more details upon request). A
1662 <code>TODO</code> is not a commitment to provide the fix yourself.
1663 </p>
1664
1665 <SPAN class=""><PRE># TODO(kl@gmail.com): Drop the use of "has_key".
1666# TODO(Zeke) change this to use relations.</PRE></SPAN>
1667 <p>
1668 If your <code>TODO</code> is of the form "At a future date do
1669 something" make sure that you either include a very specific
1670 date ("Fix by November 2009") or a very specific event
1671 ("Remove this code when all clients can handle XML responses.").
1672 </p>
1673 </SPAN></SPAN>
1674 </SPAN>
1675 <SPAN class=""><H3><A name="Imports_formatting" id="Imports_formatting">Imports formatting</A></H3>
1676<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Imports_formatting__body','Imports_formatting__button')" name="Imports_formatting__button" id="Imports_formatting__button">▶</SPAN>
1677 <SPAN class="">
1678 Imports should be on separate lines.
1679 </SPAN>
1680 <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">
1681 link
1682 </A></SPAN>
1683 <p>
1684 E.g.:
1685 </p>
1686
1687<SPAN class=""><PRE>Yes: <span class="external"></span>import os
1688 <span class="external"></span>import sys</PRE></SPAN>
1689<SPAN class=""><PRE class="badcode">No: <span class="external"></span>import os, sys</PRE></SPAN>
1690 <p>
1691 Imports are always put at the top of the file, just after any
1692 module comments and doc strings and before module globals and
1693 constants. Imports should be grouped with the order being most generic
1694 to least generic:
1695 </p>
1696 <ul>
1697 <li>standard library imports</li>
1698 <li>third-party imports</li>
1699
1700 <li>application-specific imports</li>
1701 </ul>
1702 <p>
1703 Within each grouping, imports should be sorted lexicographically,
1704 ignoring case, according to each module's full package path.
1705 </p>
1706 <SPAN class=""><PRE>
1707<span class="external"></span>import foo
1708<span class="external"></span>from foo import bar
1709<span class="external"></span>from foo.bar import baz
1710<span class="external"></span>from foo.bar import Quux
1711<span class="external"></span>from Foob import ar</PRE></SPAN>
1712
1713
1714 </SPAN></SPAN>
1715 </SPAN>
1716 <SPAN class=""><H3><A name="Statements" id="Statements">Statements</A></H3>
1717<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Statements__body','Statements__button')" name="Statements__button" id="Statements__button">▶</SPAN>
1718 <SPAN class="">
1719 Generally only one statement per line.
1720 </SPAN>
1721 <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">
1722 link
1723 </A></SPAN>
1724 <p>
1725 However, you may put the
1726 result of a test on the same line as the test only if the entire
1727 statement fits on one line. In particular, you can never do so
1728 with <code>try</code>/<code>except</code> since the
1729 <code>try</code> and <code>except</code> can't both fit on the
1730 same line, and you can only do so with an <code>if</code> if
1731 there is no <code>else</code>.
1732 </p>
1733
1734 <SPAN class=""><PRE>Ye<span class="external"></span>s:
1735
1736 <span class="external"></span>if foo: bar(foo)</PRE></SPAN>
1737<SPAN class=""><PRE class="badcode">No<span class="external"></span>:
1738
1739 <span class="external"></span>if foo: bar(foo)
1740 <span class="external"></span>else: baz(foo)
1741
1742 <span class="external"></span>try: bar(foo)
1743 <span class="external"></span>except ValueError: baz(foo)
1744
1745 <span class="external"></span>try:
1746 <span class="external"> </span>bar(foo)
1747 <span class="external"></span>except ValueError: baz(foo)
1748<span class="external"></span>
1749</PRE></SPAN>
1750 </SPAN></SPAN>
1751 </SPAN>
1752 <SPAN class=""><H3><A name="Access_Control" id="Access_Control">Access Control</A></H3>
1753<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Access_Control__body','Access_Control__button')" name="Access_Control__button" id="Access_Control__button">▶</SPAN>
1754 <SPAN class="">
1755 If an accessor function would be trivial you should use public variables
1756 instead of accessor functions to avoid the extra cost of function
1757 calls in Python. When more functionality is added you can use
1758 <code>property</code> to keep the syntax consistent.
1759 </SPAN>
1760 <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">
1761 link
1762 </A></SPAN>
1763 <p>
1764 On the other hand, if access is more complex, or the cost of accessing
1765 the variable is significant, you should use function calls (following the
1766 <a HREF="#naming">Naming</a> guidelines) such as <code>get_foo()</code>
1767 and <code>set_foo()</code>. If the past behavior allowed access through a
1768 property, do not bind the new accessor functions to the property. Any
1769 code still attempting to access the variable by the old method should
1770 break visibly so they are made aware of the change in complexity.
1771 </p>
1772 </SPAN></SPAN>
1773 </SPAN>
1774 <SPAN class=""><H3><A name="Naming" id="Naming">Naming</A></H3>
1775<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Naming__body','Naming__button')" name="Naming__button" id="Naming__button">▶</SPAN>
1776 <SPAN class="">
1777 <code>module_name, package_name, ClassName, method_name, ExceptionName,
1778 function_name, GLOBAL_VAR_NAME, instance_var_name,
1779 function_parameter_name, local_var_name.</code>
1780 </SPAN>
1781 <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">
1782 link
1783 </A></SPAN>
1784 <P class="">
1785<SPAN class="stylepoint_subsection">Names to Avoid</SPAN>
1786
1787 <ul>
1788 <li>single character names except for counters or iterators</li>
1789 <li>dashes (<code>-</code>) in any package/module name</li>
1790 <li>
1791<code>__double_leading_and_trailing_underscore__</code> names
1792 (reserved by Python)</li>
1793 </ul>
1794
1795 </P>
1796 <P class="">
1797<SPAN class="stylepoint_subsection">Naming Convention</SPAN>
1798
1799 <ul>
1800 <li>
1801 "Internal" means internal to a module or protected
1802 or private within a class.</li>
1803 <li>
1804 Prepending a single underscore (<code>_</code>) has some
1805 support for protecting module variables and functions (not included
1806 with <code>import * from</code>). Prepending a double underscore
1807 (<code>__</code>) to an instance variable or method
1808 effectively serves to make the variable or method private to its class
1809 (using name mangling).</li>
1810 <li>
1811 Place related classes and top-level functions together in a
1812 module. Unlike Java,
1813 there is no need to limit yourself to one class per module.</li>
1814 <li>
1815 Use CapWords for class names, but lower_with_under.py for module names.
1816 Although there are many existing modules named CapWords.py, this is now
1817 discouraged because it's confusing when the module happens to be
1818 named after a class. ("wait -- did I write
1819 <code>import StringIO</code> or <code>from StringIO import
1820 StringIO</code>?")</li>
1821 </ul>
1822
1823 </P>
1824 <P class="">
1825<SPAN class="stylepoint_subsection">Guidelines derived from Guido's Recommendations</SPAN>
1826
1827 <table rules="all" border="1" cellspacing="2" cellpadding="2">
1828
1829 <tr>
1830 <th>Type</th>
1831 <th>Public</th>
1832 <th>Internal</th>
1833 </tr>
1834
1835
1836
1837 <tr>
1838 <td>Packages</td>
1839 <td><code>lower_with_under</code></td>
1840 <td></td>
1841 </tr>
1842
1843 <tr>
1844 <td>Modules</td>
1845 <td><code>lower_with_under</code></td>
1846 <td><code>_lower_with_under</code></td>
1847 </tr>
1848
1849 <tr>
1850 <td>Classes</td>
1851 <td><code>CapWords</code></td>
1852 <td><code>_CapWords</code></td>
1853 </tr>
1854
1855 <tr>
1856 <td>Exceptions</td>
1857 <td><code>CapWords</code></td>
1858 <td></td>
1859 </tr>
1860
1861
1862
1863 <tr>
1864 <td>Functions</td>
1865 <td><code>lower_with_under()</code></td>
1866 <td><code>_lower_with_under()</code></td>
1867 </tr>
1868
1869 <tr>
1870 <td>Global/Class Constants</td>
1871 <td><code>CAPS_WITH_UNDER</code></td>
1872 <td><code>_CAPS_WITH_UNDER</code></td>
1873 </tr>
1874
1875 <tr>
1876 <td>Global/Class Variables</td>
1877 <td><code>lower_with_under</code></td>
1878 <td><code>_lower_with_under</code></td>
1879 </tr>
1880
1881 <tr>
1882 <td>Instance Variables</td>
1883 <td><code>lower_with_under</code></td>
1884 <td><code>_lower_with_under (protected) or __lower_with_under (private)</code></td>
1885 </tr>
1886
1887
1888
1889 <tr>
1890 <td>Method Names</td>
1891 <td><code>lower_with_under()</code></td>
1892 <td><code>_lower_with_under() (protected) or __lower_with_under() (private)</code></td>
1893 </tr>
1894
1895 <tr>
1896 <td>Function/Method Parameters</td>
1897 <td><code>lower_with_under</code></td>
1898 <td></td>
1899 </tr>
1900
1901 <tr>
1902 <td>Local Variables</td>
1903 <td><code>lower_with_under</code></td>
1904 <td></td>
1905 </tr>
1906
1907
1908 </table>
1909
1910
1911 </P>
1912 </SPAN></SPAN>
1913 </SPAN>
1914 <SPAN class=""><H3><A name="Main" id="Main">Main</A></H3>
1915<SPAN class="showhide_button" onclick="javascript:ShowHideByName('Main__body','Main__button')" name="Main__button" id="Main__button">▶</SPAN>
1916 <SPAN class="">
1917 Even a file meant to be used as a script should be importable and a
1918 mere import should not have the side effect of executing the script's
1919 main functionality. The main functionality should be in a main()
1920 function.
1921 </SPAN>
1922 <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">
1923 link
1924 </A></SPAN>
1925 <p>
1926 In Python,
1927 <code>pychecker</code>, <code>pydoc</code>, and unit tests
1928 require modules to be importable. Your code should always check
1929 <code>if __name__ == '__main__'</code> before executing your
1930 main program so that the main program is not executed when the
1931 module is imported.
1932
1933 </p>
1934
1935
1936
1937
1938
1939
1940
1941 <SPAN class=""><PRE>
1942<span class="external"></span>def main():
1943 <span class="external"> </span>...
1944
1945<span class="external"></span>if __name__ == '__main__':
1946 <span class="external"> </span>main()
1947<span class="external"></span>
1948</PRE></SPAN>
1949
1950 <p>
1951 All code at the top level will be executed when the module is
1952 imported. Be careful not to call functions, create objects, or
1953 perform other operations that should not be executed when the
1954 file is being <code>pycheck</code>ed or <code>pydoc</code>ed.
1955 </p>
1956 </SPAN></SPAN>
1957 </SPAN>
1958 </SPAN>
1959
1960<H2>Parting Words</H2>
1961 <p>
1962 <em>BE CONSISTENT</em>.
1963 </p>
1964
1965 <p>
1966 If you're editing code, take a few minutes to look at the code
1967 around you and determine its style. If they use spaces around
1968 all their arithmetic operators, you should too. If their
1969 comments have little boxes of hash marks around them, make your
1970 comments have little boxes of hash marks around them too.
1971 </p>
1972
1973 <p>
1974 The point of having style guidelines is to have a common vocabulary
1975 of coding so people can concentrate on what you're saying rather
1976 than on how you're saying it. We present global style rules here so
1977 people know the vocabulary, but local style is also important. If
1978 code you add to a file looks drastically different from the existing
1979 code around it, it throws readers out of their rhythm when they go to
1980 read it. Avoid this.
1981 </p>
1982
1983
1984
1985<p align="right">
mmentovaif7facf92009-10-23 21:01:49 +0000[diff] [blame^]1986Revision 2.14
apicard@google.comf900c2c2009-07-23 20:09:56 +0000[diff] [blame]1987</p>
1988
1989
1990<address>
1991 Amit Patel<br>
1992 Antoine Picard<br>
1993 Eugene Jhong<br>
1994 Jeremy Hylton<br>
1995 Matt Smart<br>
1996 Mike Shields<br>
1997</address>
1998</BODY>
1999</HTML>