mirror of
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svn+ssh://pythondev@svn.python.org/python/trunk ........ r61239 | andrew.kuchling | 2008-03-05 01:44:41 +0100 (Wed, 05 Mar 2008) | 1 line Add more items; add fragmentary notes ........ r61240 | amaury.forgeotdarc | 2008-03-05 02:50:33 +0100 (Wed, 05 Mar 2008) | 13 lines Issue#2238: some syntax errors from *args or **kwargs expressions would give bogus error messages, because of untested exceptions:: >>> f(**g(1=2)) XXX undetected error Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: 'int' object is not iterable instead of the expected SyntaxError: keyword can't be an expression Will backport. ........ r61241 | neal.norwitz | 2008-03-05 06:10:48 +0100 (Wed, 05 Mar 2008) | 3 lines Remove the files/dirs after closing the DB so the tests work on Windows. Patch from Trent Nelson. Also simplified removing a file by using test_support. ........ r61242 | neal.norwitz | 2008-03-05 06:14:18 +0100 (Wed, 05 Mar 2008) | 3 lines Get this test to pass even when there is no sound card in the system. Patch from Trent Nelson. (I can't test this.) ........ r61243 | neal.norwitz | 2008-03-05 06:20:44 +0100 (Wed, 05 Mar 2008) | 3 lines Catch OSError when trying to remove a file in case removal fails. This should prevent a failure in tearDown masking any real test failure. ........ r61244 | neal.norwitz | 2008-03-05 06:38:06 +0100 (Wed, 05 Mar 2008) | 5 lines Make the timeout longer to give slow machines a chance to pass the test before timing out. This doesn't change the duration of the test under normal circumstances. This is targetted at fixing the spurious failures on the FreeBSD buildbot primarily. ........ r61245 | neal.norwitz | 2008-03-05 06:49:03 +0100 (Wed, 05 Mar 2008) | 1 line Tabs -> spaces ........ r61246 | neal.norwitz | 2008-03-05 06:50:20 +0100 (Wed, 05 Mar 2008) | 1 line Use -u urlfetch to run more tests ........ r61247 | neal.norwitz | 2008-03-05 06:51:20 +0100 (Wed, 05 Mar 2008) | 1 line test_smtplib sometimes reports leaks too, suppress it ........ r61248 | jeffrey.yasskin | 2008-03-05 07:19:56 +0100 (Wed, 05 Mar 2008) | 5 lines Fix test_socketserver on Windows after r61099 added several signal.alarm() calls (which don't exist on non-Unix platforms). Thanks to Trent Nelson for the report and patch. ........ r61249 | georg.brandl | 2008-03-05 08:10:35 +0100 (Wed, 05 Mar 2008) | 2 lines Fix some rst. ........ r61252 | thomas.heller | 2008-03-05 15:53:39 +0100 (Wed, 05 Mar 2008) | 2 lines News entry for yesterdays commit. ........ r61253 | thomas.heller | 2008-03-05 16:34:29 +0100 (Wed, 05 Mar 2008) | 3 lines Issue 1872: Changed the struct module typecode from 't' to '?', for compatibility with PEP3118. ........ r61254 | skip.montanaro | 2008-03-05 17:41:09 +0100 (Wed, 05 Mar 2008) | 4 lines Elaborate on the role of the altinstall target when installing multiple versions. ........ r61255 | georg.brandl | 2008-03-05 20:31:44 +0100 (Wed, 05 Mar 2008) | 2 lines #2239: PYTHONPATH delimiter is os.pathsep. ........ r61256 | raymond.hettinger | 2008-03-05 21:59:58 +0100 (Wed, 05 Mar 2008) | 1 line C implementation of itertools.permutations(). ........ r61257 | raymond.hettinger | 2008-03-05 22:04:32 +0100 (Wed, 05 Mar 2008) | 1 line Small code cleanup. ........ r61260 | martin.v.loewis | 2008-03-05 23:24:31 +0100 (Wed, 05 Mar 2008) | 2 lines cd PCbuild only after deleting all pyc files. ........ r61261 | raymond.hettinger | 2008-03-06 02:15:52 +0100 (Thu, 06 Mar 2008) | 1 line Add examples. ........ r61262 | andrew.kuchling | 2008-03-06 02:36:27 +0100 (Thu, 06 Mar 2008) | 1 line Add two items ........ r61263 | georg.brandl | 2008-03-06 07:47:18 +0100 (Thu, 06 Mar 2008) | 2 lines #1725737: ignore other VC directories other than CVS and SVN's too. ........ r61264 | martin.v.loewis | 2008-03-06 07:55:22 +0100 (Thu, 06 Mar 2008) | 4 lines Patch #2232: os.tmpfile might fail on Windows if the user has no permission to create files in the root directory. Will backport to 2.5. ........ r61269 | georg.brandl | 2008-03-06 08:19:15 +0100 (Thu, 06 Mar 2008) | 2 lines Expand on re.split behavior with captured expressions. ........ r61270 | georg.brandl | 2008-03-06 08:22:09 +0100 (Thu, 06 Mar 2008) | 2 lines Little clarification of assignments. ........ r61271 | georg.brandl | 2008-03-06 08:31:34 +0100 (Thu, 06 Mar 2008) | 2 lines Add isinstance/issubclass to tutorial. ........ r61272 | georg.brandl | 2008-03-06 08:34:52 +0100 (Thu, 06 Mar 2008) | 2 lines Add missing NEWS entry for r61263. ........ r61273 | georg.brandl | 2008-03-06 08:41:16 +0100 (Thu, 06 Mar 2008) | 2 lines #2225: return nonzero status code from py_compile if not all files could be compiled. ........ r61274 | georg.brandl | 2008-03-06 08:43:02 +0100 (Thu, 06 Mar 2008) | 2 lines #2220: handle matching failure more gracefully. ........ r61275 | georg.brandl | 2008-03-06 08:45:52 +0100 (Thu, 06 Mar 2008) | 2 lines Bug #2220: handle rlcompleter attribute match failure more gracefully. ........ r61278 | martin.v.loewis | 2008-03-06 14:49:47 +0100 (Thu, 06 Mar 2008) | 1 line Rely on x64 platform configuration when building _bsddb on AMD64. ........ r61279 | martin.v.loewis | 2008-03-06 14:50:28 +0100 (Thu, 06 Mar 2008) | 1 line Update db-4.4.20 build procedure. ........ r61285 | raymond.hettinger | 2008-03-06 21:52:01 +0100 (Thu, 06 Mar 2008) | 1 line More tests. ........ r61286 | raymond.hettinger | 2008-03-06 23:51:36 +0100 (Thu, 06 Mar 2008) | 1 line Issue 2246: itertools grouper object did not participate in GC (should be backported). ........ r61288 | raymond.hettinger | 2008-03-07 02:33:20 +0100 (Fri, 07 Mar 2008) | 1 line Tweak recipes and tests ........ r61289 | jeffrey.yasskin | 2008-03-07 07:22:15 +0100 (Fri, 07 Mar 2008) | 5 lines Progress on issue #1193577 by adding a polling .shutdown() method to SocketServers. The core of the patch was written by Pedro Werneck, but any bugs are mine. I've also rearranged the code for timeouts in order to avoid interfering with the shutdown poll. ........ r61290 | nick.coghlan | 2008-03-07 15:13:28 +0100 (Fri, 07 Mar 2008) | 1 line Speed up with statements by storing the __exit__ method on the stack instead of in a temp variable (bumps the magic number for pyc files) ........ r61298 | andrew.kuchling | 2008-03-07 22:09:23 +0100 (Fri, 07 Mar 2008) | 1 line Grammar fix ........ r61303 | georg.brandl | 2008-03-08 10:54:06 +0100 (Sat, 08 Mar 2008) | 2 lines #2253: fix continue vs. finally docs. ........ r61304 | marc-andre.lemburg | 2008-03-08 11:01:43 +0100 (Sat, 08 Mar 2008) | 3 lines Add new name for Mandrake: Mandriva. ........ r61305 | georg.brandl | 2008-03-08 11:05:24 +0100 (Sat, 08 Mar 2008) | 2 lines #1533486: fix types in refcount intro. ........ r61312 | facundo.batista | 2008-03-08 17:50:27 +0100 (Sat, 08 Mar 2008) | 5 lines Issue 1106316. post_mortem()'s parameter, traceback, is now optional: it defaults to the traceback of the exception that is currently being handled. ........ r61313 | jeffrey.yasskin | 2008-03-08 19:26:54 +0100 (Sat, 08 Mar 2008) | 2 lines Add tests for with and finally performance to pybench. ........ r61314 | jeffrey.yasskin | 2008-03-08 21:08:21 +0100 (Sat, 08 Mar 2008) | 2 lines Fix pybench for pythons < 2.6, tested back to 2.3. ........ r61317 | jeffrey.yasskin | 2008-03-08 22:35:15 +0100 (Sat, 08 Mar 2008) | 3 lines Well that was dumb. platform.python_implementation returns a function, not a string. ........ r61329 | georg.brandl | 2008-03-09 16:11:39 +0100 (Sun, 09 Mar 2008) | 2 lines #2249: document assertTrue and assertFalse. ........ r61332 | neal.norwitz | 2008-03-09 20:03:42 +0100 (Sun, 09 Mar 2008) | 4 lines Introduce a lock to fix a race condition which caused an exception in the test. Some buildbots were consistently failing (e.g., amd64). Also remove a couple of semi-colons. ........ r61344 | raymond.hettinger | 2008-03-11 01:19:07 +0100 (Tue, 11 Mar 2008) | 1 line Add recipe to docs. ........ r61350 | guido.van.rossum | 2008-03-11 22:18:06 +0100 (Tue, 11 Mar 2008) | 3 lines Fix the overflows in expandtabs(). "This time for sure!" (Exploit at request.) ........ r61351 | raymond.hettinger | 2008-03-11 22:37:46 +0100 (Tue, 11 Mar 2008) | 1 line Improve docs for itemgetter(). Show that it works with slices. ........ r61363 | georg.brandl | 2008-03-13 08:15:56 +0100 (Thu, 13 Mar 2008) | 2 lines #2265: fix example. ........ r61364 | georg.brandl | 2008-03-13 08:17:14 +0100 (Thu, 13 Mar 2008) | 2 lines #2270: fix typo. ........ r61365 | georg.brandl | 2008-03-13 08:21:41 +0100 (Thu, 13 Mar 2008) | 2 lines #1720705: add docs about import/threading interaction, wording by Nick. ........ r61366 | andrew.kuchling | 2008-03-13 12:07:35 +0100 (Thu, 13 Mar 2008) | 1 line Add class decorators ........ r61367 | raymond.hettinger | 2008-03-13 17:43:17 +0100 (Thu, 13 Mar 2008) | 1 line Add 2-to-3 support for the itertools moved to builtins or renamed. ........ r61368 | raymond.hettinger | 2008-03-13 17:43:59 +0100 (Thu, 13 Mar 2008) | 1 line Consistent tense. ........ r61369 | raymond.hettinger | 2008-03-13 20:03:51 +0100 (Thu, 13 Mar 2008) | 1 line Issue 2274: Add heapq.heappushpop(). ........ r61370 | raymond.hettinger | 2008-03-13 20:33:34 +0100 (Thu, 13 Mar 2008) | 1 line Simplify the nlargest() code using heappushpop(). ........ r61371 | brett.cannon | 2008-03-13 21:27:00 +0100 (Thu, 13 Mar 2008) | 4 lines Move test_thread over to unittest. Commits GHOP 237. Thanks Benjamin Peterson for the patch. ........ r61372 | brett.cannon | 2008-03-13 21:33:10 +0100 (Thu, 13 Mar 2008) | 4 lines Move test_tokenize to doctest. Done as GHOP 238 by Josip Dzolonga. ........ r61373 | brett.cannon | 2008-03-13 21:47:41 +0100 (Thu, 13 Mar 2008) | 4 lines Convert test_contains, test_crypt, and test_select to unittest. Patch from GHOP 294 by David Marek. ........ r61374 | brett.cannon | 2008-03-13 22:02:16 +0100 (Thu, 13 Mar 2008) | 4 lines Move test_gdbm to use unittest. Closes issue #1960. Thanks Giampaolo Rodola. ........ r61375 | brett.cannon | 2008-03-13 22:09:28 +0100 (Thu, 13 Mar 2008) | 4 lines Convert test_fcntl to unittest. Closes issue #2055. Thanks Giampaolo Rodola. ........ r61376 | raymond.hettinger | 2008-03-14 06:03:44 +0100 (Fri, 14 Mar 2008) | 1 line Leave heapreplace() unchanged. ........ r61378 | martin.v.loewis | 2008-03-14 14:56:09 +0100 (Fri, 14 Mar 2008) | 2 lines Patch #2284: add -x64 option to rt.bat. ........ r61379 | martin.v.loewis | 2008-03-14 14:57:59 +0100 (Fri, 14 Mar 2008) | 2 lines Use -x64 flag. ........ r61382 | brett.cannon | 2008-03-14 15:03:10 +0100 (Fri, 14 Mar 2008) | 2 lines Remove a bad test. ........ r61383 | mark.dickinson | 2008-03-14 15:23:37 +0100 (Fri, 14 Mar 2008) | 9 lines Issue 705836: Fix struct.pack(">f", 1e40) to behave consistently across platforms: it should now raise OverflowError on all platforms. (Previously it raised OverflowError only on non IEEE 754 platforms.) Also fix the (already existing) test for this behaviour so that it actually raises TestFailed instead of just referencing it. ........ r61387 | thomas.heller | 2008-03-14 22:06:21 +0100 (Fri, 14 Mar 2008) | 1 line Remove unneeded initializer. ........ r61388 | martin.v.loewis | 2008-03-14 22:19:28 +0100 (Fri, 14 Mar 2008) | 2 lines Run debug version, cd to PCbuild. ........ r61392 | georg.brandl | 2008-03-15 00:10:34 +0100 (Sat, 15 Mar 2008) | 2 lines Remove obsolete paragraph. #2288. ........ r61395 | georg.brandl | 2008-03-15 01:20:19 +0100 (Sat, 15 Mar 2008) | 2 lines Fix lots of broken links in the docs, found by Sphinx' external link checker. ........ r61396 | skip.montanaro | 2008-03-15 03:32:49 +0100 (Sat, 15 Mar 2008) | 1 line note that fork and forkpty raise OSError on failure ........ r61402 | skip.montanaro | 2008-03-15 17:04:45 +0100 (Sat, 15 Mar 2008) | 1 line add %f format to datetime - issue 1158 ........ r61403 | skip.montanaro | 2008-03-15 17:07:11 +0100 (Sat, 15 Mar 2008) | 2 lines . ........
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.. _compound:
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*******************
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Compound statements
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*******************
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.. index:: pair: compound; statement
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Compound statements contain (groups of) other statements; they affect or control
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the execution of those other statements in some way. In general, compound
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statements span multiple lines, although in simple incarnations a whole compound
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statement may be contained in one line.
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The :keyword:`if`, :keyword:`while` and :keyword:`for` statements implement
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traditional control flow constructs. :keyword:`try` specifies exception
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handlers and/or cleanup code for a group of statements, while the
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:keyword:`with` statement allows the execution of initialization and
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finalization code around a block of code. Function and class definitions are
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also syntactically compound statements.
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.. index::
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single: clause
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single: suite
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Compound statements consist of one or more 'clauses.' A clause consists of a
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header and a 'suite.' The clause headers of a particular compound statement are
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all at the same indentation level. Each clause header begins with a uniquely
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identifying keyword and ends with a colon. A suite is a group of statements
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controlled by a clause. A suite can be one or more semicolon-separated simple
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statements on the same line as the header, following the header's colon, or it
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can be one or more indented statements on subsequent lines. Only the latter
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form of suite can contain nested compound statements; the following is illegal,
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mostly because it wouldn't be clear to which :keyword:`if` clause a following
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:keyword:`else` clause would belong::
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if test1: if test2: print(x)
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Also note that the semicolon binds tighter than the colon in this context, so
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that in the following example, either all or none of the :func:`print` calls are
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executed::
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if x < y < z: print(x); print(y); print(z)
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Summarizing:
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.. productionlist::
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compound_stmt: `if_stmt`
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: | `while_stmt`
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: | `for_stmt`
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: | `try_stmt`
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: | `with_stmt`
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: | `funcdef`
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: | `classdef`
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: | `decorated`
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suite: `stmt_list` NEWLINE | NEWLINE INDENT `statement`+ DEDENT
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statement: `stmt_list` NEWLINE | `compound_stmt`
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stmt_list: `simple_stmt` (";" `simple_stmt`)* [";"]
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.. index::
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single: NEWLINE token
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single: DEDENT token
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pair: dangling; else
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Note that statements always end in a ``NEWLINE`` possibly followed by a
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``DEDENT``. Also note that optional continuation clauses always begin with a
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keyword that cannot start a statement, thus there are no ambiguities (the
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'dangling :keyword:`else`' problem is solved in Python by requiring nested
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:keyword:`if` statements to be indented).
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The formatting of the grammar rules in the following sections places each clause
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on a separate line for clarity.
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.. _if:
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.. _elif:
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.. _else:
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The :keyword:`if` statement
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===========================
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.. index::
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statement: if
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keyword: elif
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keyword: else
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keyword: elif
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keyword: else
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The :keyword:`if` statement is used for conditional execution:
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.. productionlist::
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if_stmt: "if" `expression` ":" `suite`
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: ( "elif" `expression` ":" `suite` )*
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: ["else" ":" `suite`]
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It selects exactly one of the suites by evaluating the expressions one by one
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until one is found to be true (see section :ref:`booleans` for the definition of
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true and false); then that suite is executed (and no other part of the
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:keyword:`if` statement is executed or evaluated). If all expressions are
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false, the suite of the :keyword:`else` clause, if present, is executed.
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.. _while:
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The :keyword:`while` statement
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==============================
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.. index::
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statement: while
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keyword: else
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pair: loop; statement
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keyword: else
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The :keyword:`while` statement is used for repeated execution as long as an
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expression is true:
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.. productionlist::
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while_stmt: "while" `expression` ":" `suite`
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: ["else" ":" `suite`]
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This repeatedly tests the expression and, if it is true, executes the first
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suite; if the expression is false (which may be the first time it is tested) the
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suite of the :keyword:`else` clause, if present, is executed and the loop
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terminates.
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.. index::
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statement: break
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statement: continue
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A :keyword:`break` statement executed in the first suite terminates the loop
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without executing the :keyword:`else` clause's suite. A :keyword:`continue`
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statement executed in the first suite skips the rest of the suite and goes back
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to testing the expression.
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.. _for:
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The :keyword:`for` statement
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============================
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.. index::
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statement: for
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keyword: in
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keyword: else
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pair: target; list
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pair: loop; statement
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keyword: in
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keyword: else
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pair: target; list
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object: sequence
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The :keyword:`for` statement is used to iterate over the elements of a sequence
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(such as a string, tuple or list) or other iterable object:
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.. productionlist::
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for_stmt: "for" `target_list` "in" `expression_list` ":" `suite`
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: ["else" ":" `suite`]
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The expression list is evaluated once; it should yield an iterable object. An
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iterator is created for the result of the ``expression_list``. The suite is
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then executed once for each item provided by the iterator, in the order of
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ascending indices. Each item in turn is assigned to the target list using the
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standard rules for assignments (see :ref:`assignment`), and then the suite is
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executed. When the items are exhausted (which is immediately when the sequence
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is empty or an iterator raises a :exc:`StopIteration` exception), the suite in
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the :keyword:`else` clause, if present, is executed, and the loop terminates.
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.. index::
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statement: break
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statement: continue
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A :keyword:`break` statement executed in the first suite terminates the loop
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without executing the :keyword:`else` clause's suite. A :keyword:`continue`
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statement executed in the first suite skips the rest of the suite and continues
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with the next item, or with the :keyword:`else` clause if there was no next
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item.
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The suite may assign to the variable(s) in the target list; this does not affect
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the next item assigned to it.
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.. index::
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builtin: range
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Names in the target list are not deleted when the loop is finished, but if the
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sequence is empty, it will not have been assigned to at all by the loop. Hint:
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the built-in function :func:`range` returns an iterator of integers suitable to
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emulate the effect of Pascal's ``for i := a to b do``; e.g., ``range(3)``
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returns the list ``[0, 1, 2]``.
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.. warning::
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.. index::
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single: loop; over mutable sequence
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single: mutable sequence; loop over
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There is a subtlety when the sequence is being modified by the loop (this can
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only occur for mutable sequences, i.e. lists). An internal counter is used
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to keep track of which item is used next, and this is incremented on each
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iteration. When this counter has reached the length of the sequence the loop
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terminates. This means that if the suite deletes the current (or a previous)
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item from the sequence, the next item will be skipped (since it gets the
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index of the current item which has already been treated). Likewise, if the
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suite inserts an item in the sequence before the current item, the current
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item will be treated again the next time through the loop. This can lead to
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nasty bugs that can be avoided by making a temporary copy using a slice of
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the whole sequence, e.g., ::
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for x in a[:]:
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if x < 0: a.remove(x)
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.. _try:
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.. _except:
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.. _finally:
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The :keyword:`try` statement
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============================
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.. index::
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statement: try
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keyword: except
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keyword: finally
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.. index:: keyword: except
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The :keyword:`try` statement specifies exception handlers and/or cleanup code
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for a group of statements:
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.. productionlist::
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try_stmt: try1_stmt | try2_stmt
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try1_stmt: "try" ":" `suite`
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: ("except" [`expression` ["as" `target`]] ":" `suite`)+
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: ["else" ":" `suite`]
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: ["finally" ":" `suite`]
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try2_stmt: "try" ":" `suite`
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: "finally" ":" `suite`
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The :keyword:`except` clause(s) specify one or more exception handlers. When no
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exception occurs in the :keyword:`try` clause, no exception handler is executed.
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When an exception occurs in the :keyword:`try` suite, a search for an exception
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handler is started. This search inspects the except clauses in turn until one
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is found that matches the exception. An expression-less except clause, if
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present, must be last; it matches any exception. For an except clause with an
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expression, that expression is evaluated, and the clause matches the exception
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if the resulting object is "compatible" with the exception. An object is
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compatible with an exception if it is the class or a base class of the exception
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object or a tuple containing an item compatible with the exception.
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If no except clause matches the exception, the search for an exception handler
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continues in the surrounding code and on the invocation stack. [#]_
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If the evaluation of an expression in the header of an except clause raises an
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exception, the original search for a handler is canceled and a search starts for
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the new exception in the surrounding code and on the call stack (it is treated
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as if the entire :keyword:`try` statement raised the exception).
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When a matching except clause is found, the exception is assigned to the target
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specified after the :keyword:`as` keyword in that except clause, if present, and
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the except clause's suite is executed. All except clauses must have an
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executable block. When the end of this block is reached, execution continues
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normally after the entire try statement. (This means that if two nested
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handlers exist for the same exception, and the exception occurs in the try
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clause of the inner handler, the outer handler will not handle the exception.)
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When an exception has been assigned using ``as target``, it is cleared at the
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end of the except clause. This is as if ::
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|
|
except E as N:
|
|
foo
|
|
|
|
was translated to ::
|
|
|
|
except E as N:
|
|
try:
|
|
foo
|
|
finally:
|
|
N = None
|
|
del N
|
|
|
|
That means that you have to assign the exception to a different name if you want
|
|
to be able to refer to it after the except clause. The reason for this is that
|
|
with the traceback attached to them, exceptions will form a reference cycle with
|
|
the stack frame, keeping all locals in that frame alive until the next garbage
|
|
collection occurs.
|
|
|
|
.. index::
|
|
module: sys
|
|
object: traceback
|
|
|
|
Before an except clause's suite is executed, details about the exception are
|
|
stored in the :mod:`sys` module and can be access via :func:`sys.exc_info`.
|
|
:func:`sys.exc_info` returns a 3-tuple consisting of: ``exc_type``, the
|
|
exception class; ``exc_value``, the exception instance; ``exc_traceback``, a
|
|
traceback object (see section :ref:`types`) identifying the point in the program
|
|
where the exception occurred. :func:`sys.exc_info` values are restored to their
|
|
previous values (before the call) when returning from a function that handled an
|
|
exception.
|
|
|
|
.. index::
|
|
keyword: else
|
|
statement: return
|
|
statement: break
|
|
statement: continue
|
|
|
|
The optional :keyword:`else` clause is executed if and when control flows off
|
|
the end of the :keyword:`try` clause. [#]_ Exceptions in the :keyword:`else`
|
|
clause are not handled by the preceding :keyword:`except` clauses.
|
|
|
|
.. index:: keyword: finally
|
|
|
|
If :keyword:`finally` is present, it specifies a 'cleanup' handler. The
|
|
:keyword:`try` clause is executed, including any :keyword:`except` and
|
|
:keyword:`else` clauses. If an exception occurs in any of the clauses and is
|
|
not handled, the exception is temporarily saved. The :keyword:`finally` clause
|
|
is executed. If there is a saved exception, it is re-raised at the end of the
|
|
:keyword:`finally` clause. If the :keyword:`finally` clause raises another
|
|
exception or executes a :keyword:`return` or :keyword:`break` statement, the
|
|
saved exception is lost. The exception information is not available to the
|
|
program during execution of the :keyword:`finally` clause.
|
|
|
|
.. index::
|
|
statement: return
|
|
statement: break
|
|
statement: continue
|
|
|
|
When a :keyword:`return`, :keyword:`break` or :keyword:`continue` statement is
|
|
executed in the :keyword:`try` suite of a :keyword:`try`...\ :keyword:`finally`
|
|
statement, the :keyword:`finally` clause is also executed 'on the way out.' A
|
|
:keyword:`continue` statement is illegal in the :keyword:`finally` clause. (The
|
|
reason is a problem with the current implementation --- this restriction may be
|
|
lifted in the future).
|
|
|
|
Additional information on exceptions can be found in section :ref:`exceptions`,
|
|
and information on using the :keyword:`raise` statement to generate exceptions
|
|
may be found in section :ref:`raise`.
|
|
|
|
.. seealso::
|
|
|
|
:pep:`3110` - Catching exceptions in Python 3000
|
|
Describes the differences in :keyword:`try` statements between Python 2.x
|
|
and 3.0.
|
|
|
|
|
|
.. _with:
|
|
.. _as:
|
|
|
|
The :keyword:`with` statement
|
|
=============================
|
|
|
|
.. index:: statement: with
|
|
|
|
The :keyword:`with` statement is used to wrap the execution of a block with
|
|
methods defined by a context manager (see section :ref:`context-managers`).
|
|
This allows common :keyword:`try`...\ :keyword:`except`...\ :keyword:`finally`
|
|
usage patterns to be encapsulated for convenient reuse.
|
|
|
|
.. productionlist::
|
|
with_stmt: "with" `expression` ["as" `target`] ":" `suite`
|
|
|
|
The execution of the :keyword:`with` statement proceeds as follows:
|
|
|
|
#. The context expression is evaluated to obtain a context manager.
|
|
|
|
#. The context manager's :meth:`__enter__` method is invoked.
|
|
|
|
#. If a target was included in the :keyword:`with` statement, the return value
|
|
from :meth:`__enter__` is assigned to it.
|
|
|
|
.. note::
|
|
|
|
The :keyword:`with` statement guarantees that if the :meth:`__enter__`
|
|
method returns without an error, then :meth:`__exit__` will always be
|
|
called. Thus, if an error occurs during the assignment to the target
|
|
list, it will be treated the same as an error occurring within the suite
|
|
would be. See step 5 below.
|
|
|
|
#. The suite is executed.
|
|
|
|
#. The context manager's :meth:`__exit__` method is invoked. If an exception
|
|
caused the suite to be exited, its type, value, and traceback are passed as
|
|
arguments to :meth:`__exit__`. Otherwise, three :const:`None` arguments are
|
|
supplied.
|
|
|
|
If the suite was exited due to an exception, and the return value from the
|
|
:meth:`__exit__` method was false, the exception is reraised. If the return
|
|
value was true, the exception is suppressed, and execution continues with the
|
|
statement following the :keyword:`with` statement.
|
|
|
|
If the suite was exited for any reason other than an exception, the return
|
|
value from :meth:`__exit__` is ignored, and execution proceeds at the normal
|
|
location for the kind of exit that was taken.
|
|
|
|
|
|
In Python 2.5, the :keyword:`with` statement is only allowed when the
|
|
``with_statement`` feature has been enabled. It is always enabled in
|
|
Python 2.6.
|
|
|
|
.. seealso::
|
|
|
|
:pep:`0343` - The "with" statement
|
|
The specification, background, and examples for the Python :keyword:`with`
|
|
statement.
|
|
|
|
|
|
.. _function:
|
|
.. _def:
|
|
|
|
Function definitions
|
|
====================
|
|
|
|
.. index::
|
|
statement: def
|
|
pair: function; definition
|
|
pair: function; name
|
|
pair: name; binding
|
|
object: user-defined function
|
|
object: function
|
|
pair: function; name
|
|
pair: name; binding
|
|
|
|
A function definition defines a user-defined function object (see section
|
|
:ref:`types`):
|
|
|
|
.. productionlist::
|
|
funcdef: [`decorators`] "def" `funcname` "(" [`parameter_list`] ")" ["->" `expression`]? ":" `suite`
|
|
decorators: `decorator`+
|
|
decorator: "@" `dotted_name` ["(" [`argument_list` [","]] ")"] NEWLINE
|
|
funcdef: "def" `funcname` "(" [`parameter_list`] ")" ":" `suite`
|
|
dotted_name: `identifier` ("." `identifier`)*
|
|
parameter_list: (`defparameter` ",")*
|
|
: ( "*" [`parameter`] ("," `defparameter`)*
|
|
: [, "**" `parameter`]
|
|
: | "**" `parameter`
|
|
: | `defparameter` [","] )
|
|
parameter: `identifier` [":" `expression`]
|
|
defparameter: `parameter` ["=" `expression`]
|
|
funcname: `identifier`
|
|
|
|
|
|
A function definition is an executable statement. Its execution binds the
|
|
function name in the current local namespace to a function object (a wrapper
|
|
around the executable code for the function). This function object contains a
|
|
reference to the current global namespace as the global namespace to be used
|
|
when the function is called.
|
|
|
|
The function definition does not execute the function body; this gets executed
|
|
only when the function is called.
|
|
|
|
A function definition may be wrapped by one or more :term:`decorator` expressions.
|
|
Decorator expressions are evaluated when the function is defined, in the scope
|
|
that contains the function definition. The result must be a callable, which is
|
|
invoked with the function object as the only argument. The returned value is
|
|
bound to the function name instead of the function object. Multiple decorators
|
|
are applied in nested fashion. For example, the following code ::
|
|
|
|
@f1(arg)
|
|
@f2
|
|
def func(): pass
|
|
|
|
is equivalent to ::
|
|
|
|
def func(): pass
|
|
func = f1(arg)(f2(func))
|
|
|
|
.. index:: triple: default; parameter; value
|
|
|
|
When one or more parameters have the form *parameter* ``=`` *expression*, the
|
|
function is said to have "default parameter values." For a parameter with a
|
|
default value, the corresponding argument may be omitted from a call, in which
|
|
case the parameter's default value is substituted. If a parameter has a default
|
|
value, all following parameters up until the "``*``" must also have a default
|
|
value --- this is a syntactic restriction that is not expressed by the grammar.
|
|
|
|
**Default parameter values are evaluated when the function definition is
|
|
executed.** This means that the expression is evaluated once, when the function
|
|
is defined, and that that same "pre-computed" value is used for each call. This
|
|
is especially important to understand when a default parameter is a mutable
|
|
object, such as a list or a dictionary: if the function modifies the object
|
|
(e.g. by appending an item to a list), the default value is in effect modified.
|
|
This is generally not what was intended. A way around this is to use ``None``
|
|
as the default, and explicitly test for it in the body of the function, e.g.::
|
|
|
|
def whats_on_the_telly(penguin=None):
|
|
if penguin is None:
|
|
penguin = []
|
|
penguin.append("property of the zoo")
|
|
return penguin
|
|
|
|
Function call semantics are described in more detail in section :ref:`calls`. A
|
|
function call always assigns values to all parameters mentioned in the parameter
|
|
list, either from position arguments, from keyword arguments, or from default
|
|
values. If the form "``*identifier``" is present, it is initialized to a tuple
|
|
receiving any excess positional parameters, defaulting to the empty tuple. If
|
|
the form "``**identifier``" is present, it is initialized to a new dictionary
|
|
receiving any excess keyword arguments, defaulting to a new empty dictionary.
|
|
Parameters after "``*``" or "``*identifier``" are keyword-only parameters and
|
|
may only be passed used keyword arguments.
|
|
|
|
.. index:: pair: function; annotations
|
|
|
|
Parameters may have annotations of the form "``: expression``" following the
|
|
parameter name. Any parameter may have an annotation even those of the form
|
|
``*identifier`` or ``**identifier``. Functions may have "return" annotation of
|
|
the form "``-> expression``" after the parameter list. These annotations can be
|
|
any valid Python expression and are evaluated when the function definition is
|
|
executed. Annotations may be evaluated in a different order than they appear in
|
|
the source code. The presence of annotations does not change the semantics of a
|
|
function. The annotation values are available as values of a dictionary keyed
|
|
by the parameters' names in the :attr:`__annotations__` attribute of the
|
|
function object.
|
|
|
|
.. index:: pair: lambda; form
|
|
|
|
It is also possible to create anonymous functions (functions not bound to a
|
|
name), for immediate use in expressions. This uses lambda forms, described in
|
|
section :ref:`lambda`. Note that the lambda form is merely a shorthand for a
|
|
simplified function definition; a function defined in a ":keyword:`def`"
|
|
statement can be passed around or assigned to another name just like a function
|
|
defined by a lambda form. The ":keyword:`def`" form is actually more powerful
|
|
since it allows the execution of multiple statements and annotations.
|
|
|
|
**Programmer's note:** Functions are first-class objects. A "``def``" form
|
|
executed inside a function definition defines a local function that can be
|
|
returned or passed around. Free variables used in the nested function can
|
|
access the local variables of the function containing the def. See section
|
|
:ref:`naming` for details.
|
|
|
|
|
|
.. _class:
|
|
|
|
Class definitions
|
|
=================
|
|
|
|
.. index::
|
|
object: class
|
|
statement: class
|
|
pair: class; definition
|
|
pair: class; name
|
|
pair: name; binding
|
|
pair: execution; frame
|
|
single: inheritance
|
|
|
|
A class definition defines a class object (see section :ref:`types`):
|
|
|
|
.. XXX need to document PEP 3115 changes here (new metaclasses)
|
|
|
|
.. productionlist::
|
|
classdef: [`decorators`] "class" `classname` [`inheritance`] ":" `suite`
|
|
inheritance: "(" [`expression_list`] ")"
|
|
classname: `identifier`
|
|
|
|
|
|
A class definition is an executable statement. It first evaluates the
|
|
inheritance list, if present. Each item in the inheritance list should evaluate
|
|
to a class object or class type which allows subclassing. The class's suite is
|
|
then executed in a new execution frame (see section :ref:`naming`), using a
|
|
newly created local namespace and the original global namespace. (Usually, the
|
|
suite contains only function definitions.) When the class's suite finishes
|
|
execution, its execution frame is discarded but its local namespace is saved. A
|
|
class object is then created using the inheritance list for the base classes and
|
|
the saved local namespace for the attribute dictionary. The class name is bound
|
|
to this class object in the original local namespace.
|
|
|
|
Classes can also be decorated; as with functions, ::
|
|
|
|
@f1(arg)
|
|
@f2
|
|
class Foo: pass
|
|
|
|
is equivalent to ::
|
|
|
|
class Foo: pass
|
|
Foo = f1(arg)(f2(Foo))
|
|
|
|
**Programmer's note:** Variables defined in the class definition are class
|
|
can be set in a method with ``self.name = value``. Both class and instance
|
|
variables are accessible through the notation "``self.name``", and an instance
|
|
variable hides a class variable with the same name when accessed in this way.
|
|
Class variables can be used as defaults for instance variables, but using
|
|
mutable values there can lead to unexpected results. For :term:`new-style
|
|
class`\es, descriptors can be used to create instance variables with different
|
|
implementation details.
|
|
|
|
.. XXX add link to descriptor docs above
|
|
|
|
.. seealso::
|
|
|
|
:pep:`3129` - Class Decorators
|
|
|
|
Class definitions, like function definitions, may be wrapped by one or
|
|
more :term:`decorator` expressions. The evaluation rules for the
|
|
decorator expressions are the same as for functions. The result must
|
|
be a class object, which is then bound to the class name.
|
|
|
|
|
|
.. rubric:: Footnotes
|
|
|
|
.. [#] The exception is propagated to the invocation stack only if there is no
|
|
:keyword:`finally` clause that negates the exception.
|
|
|
|
.. [#] Currently, control "flows off the end" except in the case of an exception or the
|
|
execution of a :keyword:`return`, :keyword:`continue`, or :keyword:`break`
|
|
statement.
|