cpython/Doc/howto/doanddont.rst

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Merged revisions 68133-68134,68141-68142,68145-68146,68148-68149,68159-68162,68166,68171-68174,68179,68195-68196,68210,68214-68215,68217-68222 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk ........ r68133 | antoine.pitrou | 2009-01-01 16:38:03 +0100 (Thu, 01 Jan 2009) | 1 line fill in actual issue number in tests ........ r68134 | hirokazu.yamamoto | 2009-01-01 16:45:39 +0100 (Thu, 01 Jan 2009) | 2 lines Issue #4797: IOError.filename was not set when _fileio.FileIO failed to open file with `str' filename on Windows. ........ r68141 | benjamin.peterson | 2009-01-01 17:43:12 +0100 (Thu, 01 Jan 2009) | 1 line fix highlighting ........ r68142 | benjamin.peterson | 2009-01-01 18:29:49 +0100 (Thu, 01 Jan 2009) | 2 lines welcome to 2009, Python! ........ r68145 | amaury.forgeotdarc | 2009-01-02 01:03:54 +0100 (Fri, 02 Jan 2009) | 5 lines #4801 _collections module fails to build on cygwin. _PyObject_GC_TRACK is the macro version of PyObject_GC_Track, and according to documentation it should not be used for extension modules. ........ r68146 | ronald.oussoren | 2009-01-02 11:44:46 +0100 (Fri, 02 Jan 2009) | 2 lines Fix for issue4472: "configure --enable-shared doesn't work on OSX" ........ r68148 | ronald.oussoren | 2009-01-02 11:48:31 +0100 (Fri, 02 Jan 2009) | 2 lines Forgot to add a NEWS item in my previous checkin ........ r68149 | ronald.oussoren | 2009-01-02 11:50:48 +0100 (Fri, 02 Jan 2009) | 2 lines Fix for issue4780 ........ r68159 | ronald.oussoren | 2009-01-02 15:48:17 +0100 (Fri, 02 Jan 2009) | 2 lines Fix for issue 1627952 ........ r68160 | ronald.oussoren | 2009-01-02 15:52:09 +0100 (Fri, 02 Jan 2009) | 2 lines Fix for issue r1737832 ........ r68161 | ronald.oussoren | 2009-01-02 16:00:05 +0100 (Fri, 02 Jan 2009) | 3 lines Fix for issue 1149804 ........ r68162 | ronald.oussoren | 2009-01-02 16:06:00 +0100 (Fri, 02 Jan 2009) | 3 lines Fix for issue 4472 is incompatible with Cygwin, this patch should fix that. ........ r68166 | benjamin.peterson | 2009-01-02 19:26:23 +0100 (Fri, 02 Jan 2009) | 1 line document PyMemberDef ........ r68171 | georg.brandl | 2009-01-02 21:25:14 +0100 (Fri, 02 Jan 2009) | 3 lines #4811: fix markup glitches (mostly remains of the conversion), found by Gabriel Genellina. ........ r68172 | martin.v.loewis | 2009-01-02 21:32:55 +0100 (Fri, 02 Jan 2009) | 2 lines Issue #4075: Use OutputDebugStringW in Py_FatalError. ........ r68173 | martin.v.loewis | 2009-01-02 21:40:14 +0100 (Fri, 02 Jan 2009) | 2 lines Issue #4051: Prevent conflict of UNICODE macros in cPickle. ........ r68174 | benjamin.peterson | 2009-01-02 21:47:27 +0100 (Fri, 02 Jan 2009) | 1 line fix compilation on non-Windows platforms ........ r68179 | raymond.hettinger | 2009-01-02 22:26:45 +0100 (Fri, 02 Jan 2009) | 1 line Issue #4615. Document how to use itertools for de-duping. ........ r68195 | georg.brandl | 2009-01-03 14:45:15 +0100 (Sat, 03 Jan 2009) | 2 lines Remove useless string literal. ........ r68196 | georg.brandl | 2009-01-03 15:29:53 +0100 (Sat, 03 Jan 2009) | 2 lines Fix indentation. ........ r68210 | georg.brandl | 2009-01-03 20:10:12 +0100 (Sat, 03 Jan 2009) | 2 lines Set eol-style correctly for mp_distributing.py. ........ r68214 | georg.brandl | 2009-01-03 20:44:48 +0100 (Sat, 03 Jan 2009) | 2 lines Make indentation consistent. ........ r68215 | georg.brandl | 2009-01-03 21:15:14 +0100 (Sat, 03 Jan 2009) | 2 lines Fix role name. ........ r68217 | georg.brandl | 2009-01-03 21:30:15 +0100 (Sat, 03 Jan 2009) | 2 lines Add rstlint, a little tool to find subtle markup problems and inconsistencies in the Doc sources. ........ r68218 | georg.brandl | 2009-01-03 21:38:59 +0100 (Sat, 03 Jan 2009) | 2 lines Recognize usage of the default role. ........ r68219 | georg.brandl | 2009-01-03 21:47:01 +0100 (Sat, 03 Jan 2009) | 2 lines Fix uses of the default role. ........ r68220 | georg.brandl | 2009-01-03 21:55:06 +0100 (Sat, 03 Jan 2009) | 2 lines Remove trailing whitespace. ........ r68221 | georg.brandl | 2009-01-03 22:04:55 +0100 (Sat, 03 Jan 2009) | 2 lines Remove tabs from the documentation. ........ r68222 | georg.brandl | 2009-01-03 22:11:58 +0100 (Sat, 03 Jan 2009) | 2 lines Disable the line length checker by default. ........
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Idioms and Anti-Idioms in Python
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************************************
:Author: Moshe Zadka
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This document is placed in the public domain.
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.. topic:: Abstract
This document can be considered a companion to the tutorial. It shows how to use
Python, and even more importantly, how *not* to use Python.
Language Constructs You Should Not Use
======================================
While Python has relatively few gotchas compared to other languages, it still
has some constructs which are only useful in corner cases, or are plain
dangerous.
from module import \*
---------------------
Inside Function Definitions
^^^^^^^^^^^^^^^^^^^^^^^^^^^
``from module import *`` is *invalid* inside function definitions. While many
versions of Python do not check for the invalidity, it does not make it more
Merged revisions 72319-72320,72467,72661,72675-72679,72703,72708,72710,72712,72801-72802,72820,72822,72824,72826-72828,72830 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk ........ r72319 | georg.brandl | 2009-05-05 10:28:49 +0200 (Di, 05 Mai 2009) | 1 line #1309567: fix linecache behavior of stripping subdirectories from paths when looking for relative filename matches. Also add a linecache test suite. ........ r72320 | georg.brandl | 2009-05-05 10:30:28 +0200 (Di, 05 Mai 2009) | 1 line Add a news entry for r72319. ........ r72467 | georg.brandl | 2009-05-08 14:17:34 +0200 (Fr, 08 Mai 2009) | 1 line Fix name. ........ r72661 | georg.brandl | 2009-05-15 10:03:03 +0200 (Fr, 15 Mai 2009) | 1 line Fix example output for doctest-like demos. ........ r72675 | georg.brandl | 2009-05-16 13:13:21 +0200 (Sa, 16 Mai 2009) | 1 line #6034: clarify __reversed__ doc. ........ r72676 | georg.brandl | 2009-05-16 13:14:46 +0200 (Sa, 16 Mai 2009) | 1 line #6025: fix signature of parse(). ........ r72677 | georg.brandl | 2009-05-16 13:18:55 +0200 (Sa, 16 Mai 2009) | 1 line #6009: undocument default argument of Option as deprecated. ........ r72678 | georg.brandl | 2009-05-16 13:21:29 +0200 (Sa, 16 Mai 2009) | 1 line #2856: document 2.x os.listdir() behavior for undecodable filenames. ........ r72679 | georg.brandl | 2009-05-16 13:24:41 +0200 (Sa, 16 Mai 2009) | 1 line Fix about and bugs pages to match real workflow. ........ r72703 | georg.brandl | 2009-05-17 10:10:27 +0200 (So, 17 Mai 2009) | 1 line part of #4144: fix exception message in console session. ........ r72708 | georg.brandl | 2009-05-17 10:24:29 +0200 (So, 17 Mai 2009) | 1 line #6017: better document behavior of dictiterators when the dict is changed. ........ r72710 | georg.brandl | 2009-05-17 10:36:04 +0200 (So, 17 Mai 2009) | 1 line #5942: Copy over flag table from dbm.rst which is clearer. ........ r72712 | georg.brandl | 2009-05-17 10:55:00 +0200 (So, 17 Mai 2009) | 1 line #5935: mention that BROWSER is looked for in PATH. ........ r72801 | georg.brandl | 2009-05-20 20:31:14 +0200 (Mi, 20 Mai 2009) | 1 line #6055: refer to "sqlite3" consistently. ........ r72802 | georg.brandl | 2009-05-20 20:35:27 +0200 (Mi, 20 Mai 2009) | 1 line #6051: refer to email examples for better way to construct email messages. ........ r72820 | georg.brandl | 2009-05-22 09:23:32 +0200 (Fr, 22 Mai 2009) | 1 line Use raise X(y). ........ r72822 | georg.brandl | 2009-05-22 11:33:25 +0200 (Fr, 22 Mai 2009) | 1 line #6084: fix example. ........ r72824 | georg.brandl | 2009-05-22 11:43:17 +0200 (Fr, 22 Mai 2009) | 1 line Fix references to file-related functions and methods (os.* vs file.*). ........ r72826 | georg.brandl | 2009-05-22 11:49:42 +0200 (Fr, 22 Mai 2009) | 1 line Fix confusing wording. ........ r72827 | georg.brandl | 2009-05-22 11:50:30 +0200 (Fr, 22 Mai 2009) | 1 line s/use/call/ ........ r72828 | georg.brandl | 2009-05-22 11:58:48 +0200 (Fr, 22 Mai 2009) | 1 line Correction in softspace behavior description. ........ r72830 | georg.brandl | 2009-05-22 12:40:00 +0200 (Fr, 22 Mai 2009) | 1 line #6086: fix spelling and use a better exception to catch. ........
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valid, no more than having a smart lawyer makes a man innocent. Do not use it
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like that ever. Even in versions where it was accepted, it made the function
execution slower, because the compiler could not be certain which names are
local and which are global. In Python 2.1 this construct causes warnings, and
sometimes even errors.
At Module Level
^^^^^^^^^^^^^^^
While it is valid to use ``from module import *`` at module level it is usually
a bad idea. For one, this loses an important property Python otherwise has ---
you can know where each toplevel name is defined by a simple "search" function
in your favourite editor. You also open yourself to trouble in the future, if
some module grows additional functions or classes.
One of the most awful question asked on the newsgroup is why this code::
f = open("www")
f.read()
does not work. Of course, it works just fine (assuming you have a file called
"www".) But it does not work if somewhere in the module, the statement ``from os
import *`` is present. The :mod:`os` module has a function called :func:`open`
which returns an integer. While it is very useful, shadowing builtins is one of
its least useful properties.
Remember, you can never know for sure what names a module exports, so either
take what you need --- ``from module import name1, name2``, or keep them in the
module and access on a per-need basis --- ``import module;print module.name``.
When It Is Just Fine
^^^^^^^^^^^^^^^^^^^^
There are situations in which ``from module import *`` is just fine:
* The interactive prompt. For example, ``from math import *`` makes Python an
amazing scientific calculator.
* When extending a module in C with a module in Python.
* When the module advertises itself as ``from import *`` safe.
Unadorned :keyword:`exec`, :func:`execfile` and friends
-------------------------------------------------------
The word "unadorned" refers to the use without an explicit dictionary, in which
case those constructs evaluate code in the *current* environment. This is
dangerous for the same reasons ``from import *`` is dangerous --- it might step
over variables you are counting on and mess up things for the rest of your code.
Simply do not do that.
Bad examples::
>>> for name in sys.argv[1:]:
>>> exec "%s=1" % name
>>> def func(s, **kw):
>>> for var, val in kw.items():
>>> exec "s.%s=val" % var # invalid!
>>> execfile("handler.py")
>>> handle()
Good examples::
>>> d = {}
>>> for name in sys.argv[1:]:
>>> d[name] = 1
>>> def func(s, **kw):
>>> for var, val in kw.items():
>>> setattr(s, var, val)
>>> d={}
>>> execfile("handle.py", d, d)
>>> handle = d['handle']
>>> handle()
from module import name1, name2
-------------------------------
Merged revisions 72319-72320,72467,72661,72675-72679,72703,72708,72710,72712,72801-72802,72820,72822,72824,72826-72828,72830 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk ........ r72319 | georg.brandl | 2009-05-05 10:28:49 +0200 (Di, 05 Mai 2009) | 1 line #1309567: fix linecache behavior of stripping subdirectories from paths when looking for relative filename matches. Also add a linecache test suite. ........ r72320 | georg.brandl | 2009-05-05 10:30:28 +0200 (Di, 05 Mai 2009) | 1 line Add a news entry for r72319. ........ r72467 | georg.brandl | 2009-05-08 14:17:34 +0200 (Fr, 08 Mai 2009) | 1 line Fix name. ........ r72661 | georg.brandl | 2009-05-15 10:03:03 +0200 (Fr, 15 Mai 2009) | 1 line Fix example output for doctest-like demos. ........ r72675 | georg.brandl | 2009-05-16 13:13:21 +0200 (Sa, 16 Mai 2009) | 1 line #6034: clarify __reversed__ doc. ........ r72676 | georg.brandl | 2009-05-16 13:14:46 +0200 (Sa, 16 Mai 2009) | 1 line #6025: fix signature of parse(). ........ r72677 | georg.brandl | 2009-05-16 13:18:55 +0200 (Sa, 16 Mai 2009) | 1 line #6009: undocument default argument of Option as deprecated. ........ r72678 | georg.brandl | 2009-05-16 13:21:29 +0200 (Sa, 16 Mai 2009) | 1 line #2856: document 2.x os.listdir() behavior for undecodable filenames. ........ r72679 | georg.brandl | 2009-05-16 13:24:41 +0200 (Sa, 16 Mai 2009) | 1 line Fix about and bugs pages to match real workflow. ........ r72703 | georg.brandl | 2009-05-17 10:10:27 +0200 (So, 17 Mai 2009) | 1 line part of #4144: fix exception message in console session. ........ r72708 | georg.brandl | 2009-05-17 10:24:29 +0200 (So, 17 Mai 2009) | 1 line #6017: better document behavior of dictiterators when the dict is changed. ........ r72710 | georg.brandl | 2009-05-17 10:36:04 +0200 (So, 17 Mai 2009) | 1 line #5942: Copy over flag table from dbm.rst which is clearer. ........ r72712 | georg.brandl | 2009-05-17 10:55:00 +0200 (So, 17 Mai 2009) | 1 line #5935: mention that BROWSER is looked for in PATH. ........ r72801 | georg.brandl | 2009-05-20 20:31:14 +0200 (Mi, 20 Mai 2009) | 1 line #6055: refer to "sqlite3" consistently. ........ r72802 | georg.brandl | 2009-05-20 20:35:27 +0200 (Mi, 20 Mai 2009) | 1 line #6051: refer to email examples for better way to construct email messages. ........ r72820 | georg.brandl | 2009-05-22 09:23:32 +0200 (Fr, 22 Mai 2009) | 1 line Use raise X(y). ........ r72822 | georg.brandl | 2009-05-22 11:33:25 +0200 (Fr, 22 Mai 2009) | 1 line #6084: fix example. ........ r72824 | georg.brandl | 2009-05-22 11:43:17 +0200 (Fr, 22 Mai 2009) | 1 line Fix references to file-related functions and methods (os.* vs file.*). ........ r72826 | georg.brandl | 2009-05-22 11:49:42 +0200 (Fr, 22 Mai 2009) | 1 line Fix confusing wording. ........ r72827 | georg.brandl | 2009-05-22 11:50:30 +0200 (Fr, 22 Mai 2009) | 1 line s/use/call/ ........ r72828 | georg.brandl | 2009-05-22 11:58:48 +0200 (Fr, 22 Mai 2009) | 1 line Correction in softspace behavior description. ........ r72830 | georg.brandl | 2009-05-22 12:40:00 +0200 (Fr, 22 Mai 2009) | 1 line #6086: fix spelling and use a better exception to catch. ........
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This is a "don't" which is much weaker than the previous "don't"s but is still
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something you should not do if you don't have good reasons to do that. The
reason it is usually bad idea is because you suddenly have an object which lives
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in two separate namespaces. When the binding in one namespace changes, the
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binding in the other will not, so there will be a discrepancy between them. This
happens when, for example, one module is reloaded, or changes the definition of
a function at runtime.
Bad example::
# foo.py
a = 1
# bar.py
from foo import a
if something():
Merged revisions 68133-68134,68141-68142,68145-68146,68148-68149,68159-68162,68166,68171-68174,68179,68195-68196,68210,68214-68215,68217-68222 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk ........ r68133 | antoine.pitrou | 2009-01-01 16:38:03 +0100 (Thu, 01 Jan 2009) | 1 line fill in actual issue number in tests ........ r68134 | hirokazu.yamamoto | 2009-01-01 16:45:39 +0100 (Thu, 01 Jan 2009) | 2 lines Issue #4797: IOError.filename was not set when _fileio.FileIO failed to open file with `str' filename on Windows. ........ r68141 | benjamin.peterson | 2009-01-01 17:43:12 +0100 (Thu, 01 Jan 2009) | 1 line fix highlighting ........ r68142 | benjamin.peterson | 2009-01-01 18:29:49 +0100 (Thu, 01 Jan 2009) | 2 lines welcome to 2009, Python! ........ r68145 | amaury.forgeotdarc | 2009-01-02 01:03:54 +0100 (Fri, 02 Jan 2009) | 5 lines #4801 _collections module fails to build on cygwin. _PyObject_GC_TRACK is the macro version of PyObject_GC_Track, and according to documentation it should not be used for extension modules. ........ r68146 | ronald.oussoren | 2009-01-02 11:44:46 +0100 (Fri, 02 Jan 2009) | 2 lines Fix for issue4472: "configure --enable-shared doesn't work on OSX" ........ r68148 | ronald.oussoren | 2009-01-02 11:48:31 +0100 (Fri, 02 Jan 2009) | 2 lines Forgot to add a NEWS item in my previous checkin ........ r68149 | ronald.oussoren | 2009-01-02 11:50:48 +0100 (Fri, 02 Jan 2009) | 2 lines Fix for issue4780 ........ r68159 | ronald.oussoren | 2009-01-02 15:48:17 +0100 (Fri, 02 Jan 2009) | 2 lines Fix for issue 1627952 ........ r68160 | ronald.oussoren | 2009-01-02 15:52:09 +0100 (Fri, 02 Jan 2009) | 2 lines Fix for issue r1737832 ........ r68161 | ronald.oussoren | 2009-01-02 16:00:05 +0100 (Fri, 02 Jan 2009) | 3 lines Fix for issue 1149804 ........ r68162 | ronald.oussoren | 2009-01-02 16:06:00 +0100 (Fri, 02 Jan 2009) | 3 lines Fix for issue 4472 is incompatible with Cygwin, this patch should fix that. ........ r68166 | benjamin.peterson | 2009-01-02 19:26:23 +0100 (Fri, 02 Jan 2009) | 1 line document PyMemberDef ........ r68171 | georg.brandl | 2009-01-02 21:25:14 +0100 (Fri, 02 Jan 2009) | 3 lines #4811: fix markup glitches (mostly remains of the conversion), found by Gabriel Genellina. ........ r68172 | martin.v.loewis | 2009-01-02 21:32:55 +0100 (Fri, 02 Jan 2009) | 2 lines Issue #4075: Use OutputDebugStringW in Py_FatalError. ........ r68173 | martin.v.loewis | 2009-01-02 21:40:14 +0100 (Fri, 02 Jan 2009) | 2 lines Issue #4051: Prevent conflict of UNICODE macros in cPickle. ........ r68174 | benjamin.peterson | 2009-01-02 21:47:27 +0100 (Fri, 02 Jan 2009) | 1 line fix compilation on non-Windows platforms ........ r68179 | raymond.hettinger | 2009-01-02 22:26:45 +0100 (Fri, 02 Jan 2009) | 1 line Issue #4615. Document how to use itertools for de-duping. ........ r68195 | georg.brandl | 2009-01-03 14:45:15 +0100 (Sat, 03 Jan 2009) | 2 lines Remove useless string literal. ........ r68196 | georg.brandl | 2009-01-03 15:29:53 +0100 (Sat, 03 Jan 2009) | 2 lines Fix indentation. ........ r68210 | georg.brandl | 2009-01-03 20:10:12 +0100 (Sat, 03 Jan 2009) | 2 lines Set eol-style correctly for mp_distributing.py. ........ r68214 | georg.brandl | 2009-01-03 20:44:48 +0100 (Sat, 03 Jan 2009) | 2 lines Make indentation consistent. ........ r68215 | georg.brandl | 2009-01-03 21:15:14 +0100 (Sat, 03 Jan 2009) | 2 lines Fix role name. ........ r68217 | georg.brandl | 2009-01-03 21:30:15 +0100 (Sat, 03 Jan 2009) | 2 lines Add rstlint, a little tool to find subtle markup problems and inconsistencies in the Doc sources. ........ r68218 | georg.brandl | 2009-01-03 21:38:59 +0100 (Sat, 03 Jan 2009) | 2 lines Recognize usage of the default role. ........ r68219 | georg.brandl | 2009-01-03 21:47:01 +0100 (Sat, 03 Jan 2009) | 2 lines Fix uses of the default role. ........ r68220 | georg.brandl | 2009-01-03 21:55:06 +0100 (Sat, 03 Jan 2009) | 2 lines Remove trailing whitespace. ........ r68221 | georg.brandl | 2009-01-03 22:04:55 +0100 (Sat, 03 Jan 2009) | 2 lines Remove tabs from the documentation. ........ r68222 | georg.brandl | 2009-01-03 22:11:58 +0100 (Sat, 03 Jan 2009) | 2 lines Disable the line length checker by default. ........
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a = 2 # danger: foo.a != a
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Good example::
# foo.py
a = 1
# bar.py
import foo
if something():
foo.a = 2
except:
-------
Python has the ``except:`` clause, which catches all exceptions. Since *every*
error in Python raises an exception, this makes many programming errors look
like runtime problems, and hinders the debugging process.
The following code shows a great example::
try:
foo = opne("file") # misspelled "open"
except:
sys.exit("could not open file!")
The second line triggers a :exc:`NameError` which is caught by the except
clause. The program will exit, and you will have no idea that this has nothing
to do with the readability of ``"file"``.
The example above is better written ::
try:
foo = opne("file") # will be changed to "open" as soon as we run it
except IOError:
sys.exit("could not open file")
There are some situations in which the ``except:`` clause is useful: for
example, in a framework when running callbacks, it is good not to let any
callback disturb the framework.
Exceptions
==========
Exceptions are a useful feature of Python. You should learn to raise them
whenever something unexpected occurs, and catch them only where you can do
something about them.
The following is a very popular anti-idiom ::
def get_status(file):
if not os.path.exists(file):
print "file not found"
sys.exit(1)
return open(file).readline()
Consider the case the file gets deleted between the time the call to
:func:`os.path.exists` is made and the time :func:`open` is called. That means
the last line will throw an :exc:`IOError`. The same would happen if *file*
exists but has no read permission. Since testing this on a normal machine on
existing and non-existing files make it seem bugless, that means in testing the
results will seem fine, and the code will get shipped. Then an unhandled
:exc:`IOError` escapes to the user, who has to watch the ugly traceback.
Here is a better way to do it. ::
def get_status(file):
try:
return open(file).readline()
except (IOError, OSError):
print "file not found"
sys.exit(1)
In this version, \*either\* the file gets opened and the line is read (so it
works even on flaky NFS or SMB connections), or the message is printed and the
application aborted.
Still, :func:`get_status` makes too many assumptions --- that it will only be
used in a short running script, and not, say, in a long running server. Sure,
the caller could do something like ::
try:
status = get_status(log)
except SystemExit:
status = None
So, try to make as few ``except`` clauses in your code --- those will usually be
a catch-all in the :func:`main`, or inside calls which should always succeed.
So, the best version is probably ::
def get_status(file):
return open(file).readline()
The caller can deal with the exception if it wants (for example, if it tries
several files in a loop), or just let the exception filter upwards to *its*
caller.
The last version is not very good either --- due to implementation details, the
file would not be closed when an exception is raised until the handler finishes,
and perhaps not at all in non-C implementations (e.g., Jython). ::
def get_status(file):
fp = open(file)
try:
return fp.readline()
finally:
fp.close()
Using the Batteries
===================
Every so often, people seem to be writing stuff in the Python library again,
usually poorly. While the occasional module has a poor interface, it is usually
much better to use the rich standard library and data types that come with
Merged revisions 72319-72320,72467,72661,72675-72679,72703,72708,72710,72712,72801-72802,72820,72822,72824,72826-72828,72830 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk ........ r72319 | georg.brandl | 2009-05-05 10:28:49 +0200 (Di, 05 Mai 2009) | 1 line #1309567: fix linecache behavior of stripping subdirectories from paths when looking for relative filename matches. Also add a linecache test suite. ........ r72320 | georg.brandl | 2009-05-05 10:30:28 +0200 (Di, 05 Mai 2009) | 1 line Add a news entry for r72319. ........ r72467 | georg.brandl | 2009-05-08 14:17:34 +0200 (Fr, 08 Mai 2009) | 1 line Fix name. ........ r72661 | georg.brandl | 2009-05-15 10:03:03 +0200 (Fr, 15 Mai 2009) | 1 line Fix example output for doctest-like demos. ........ r72675 | georg.brandl | 2009-05-16 13:13:21 +0200 (Sa, 16 Mai 2009) | 1 line #6034: clarify __reversed__ doc. ........ r72676 | georg.brandl | 2009-05-16 13:14:46 +0200 (Sa, 16 Mai 2009) | 1 line #6025: fix signature of parse(). ........ r72677 | georg.brandl | 2009-05-16 13:18:55 +0200 (Sa, 16 Mai 2009) | 1 line #6009: undocument default argument of Option as deprecated. ........ r72678 | georg.brandl | 2009-05-16 13:21:29 +0200 (Sa, 16 Mai 2009) | 1 line #2856: document 2.x os.listdir() behavior for undecodable filenames. ........ r72679 | georg.brandl | 2009-05-16 13:24:41 +0200 (Sa, 16 Mai 2009) | 1 line Fix about and bugs pages to match real workflow. ........ r72703 | georg.brandl | 2009-05-17 10:10:27 +0200 (So, 17 Mai 2009) | 1 line part of #4144: fix exception message in console session. ........ r72708 | georg.brandl | 2009-05-17 10:24:29 +0200 (So, 17 Mai 2009) | 1 line #6017: better document behavior of dictiterators when the dict is changed. ........ r72710 | georg.brandl | 2009-05-17 10:36:04 +0200 (So, 17 Mai 2009) | 1 line #5942: Copy over flag table from dbm.rst which is clearer. ........ r72712 | georg.brandl | 2009-05-17 10:55:00 +0200 (So, 17 Mai 2009) | 1 line #5935: mention that BROWSER is looked for in PATH. ........ r72801 | georg.brandl | 2009-05-20 20:31:14 +0200 (Mi, 20 Mai 2009) | 1 line #6055: refer to "sqlite3" consistently. ........ r72802 | georg.brandl | 2009-05-20 20:35:27 +0200 (Mi, 20 Mai 2009) | 1 line #6051: refer to email examples for better way to construct email messages. ........ r72820 | georg.brandl | 2009-05-22 09:23:32 +0200 (Fr, 22 Mai 2009) | 1 line Use raise X(y). ........ r72822 | georg.brandl | 2009-05-22 11:33:25 +0200 (Fr, 22 Mai 2009) | 1 line #6084: fix example. ........ r72824 | georg.brandl | 2009-05-22 11:43:17 +0200 (Fr, 22 Mai 2009) | 1 line Fix references to file-related functions and methods (os.* vs file.*). ........ r72826 | georg.brandl | 2009-05-22 11:49:42 +0200 (Fr, 22 Mai 2009) | 1 line Fix confusing wording. ........ r72827 | georg.brandl | 2009-05-22 11:50:30 +0200 (Fr, 22 Mai 2009) | 1 line s/use/call/ ........ r72828 | georg.brandl | 2009-05-22 11:58:48 +0200 (Fr, 22 Mai 2009) | 1 line Correction in softspace behavior description. ........ r72830 | georg.brandl | 2009-05-22 12:40:00 +0200 (Fr, 22 Mai 2009) | 1 line #6086: fix spelling and use a better exception to catch. ........
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Python than inventing your own.
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A useful module very few people know about is :mod:`os.path`. It always has the
correct path arithmetic for your operating system, and will usually be much
Merged revisions 72319-72320,72467,72661,72675-72679,72703,72708,72710,72712,72801-72802,72820,72822,72824,72826-72828,72830 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk ........ r72319 | georg.brandl | 2009-05-05 10:28:49 +0200 (Di, 05 Mai 2009) | 1 line #1309567: fix linecache behavior of stripping subdirectories from paths when looking for relative filename matches. Also add a linecache test suite. ........ r72320 | georg.brandl | 2009-05-05 10:30:28 +0200 (Di, 05 Mai 2009) | 1 line Add a news entry for r72319. ........ r72467 | georg.brandl | 2009-05-08 14:17:34 +0200 (Fr, 08 Mai 2009) | 1 line Fix name. ........ r72661 | georg.brandl | 2009-05-15 10:03:03 +0200 (Fr, 15 Mai 2009) | 1 line Fix example output for doctest-like demos. ........ r72675 | georg.brandl | 2009-05-16 13:13:21 +0200 (Sa, 16 Mai 2009) | 1 line #6034: clarify __reversed__ doc. ........ r72676 | georg.brandl | 2009-05-16 13:14:46 +0200 (Sa, 16 Mai 2009) | 1 line #6025: fix signature of parse(). ........ r72677 | georg.brandl | 2009-05-16 13:18:55 +0200 (Sa, 16 Mai 2009) | 1 line #6009: undocument default argument of Option as deprecated. ........ r72678 | georg.brandl | 2009-05-16 13:21:29 +0200 (Sa, 16 Mai 2009) | 1 line #2856: document 2.x os.listdir() behavior for undecodable filenames. ........ r72679 | georg.brandl | 2009-05-16 13:24:41 +0200 (Sa, 16 Mai 2009) | 1 line Fix about and bugs pages to match real workflow. ........ r72703 | georg.brandl | 2009-05-17 10:10:27 +0200 (So, 17 Mai 2009) | 1 line part of #4144: fix exception message in console session. ........ r72708 | georg.brandl | 2009-05-17 10:24:29 +0200 (So, 17 Mai 2009) | 1 line #6017: better document behavior of dictiterators when the dict is changed. ........ r72710 | georg.brandl | 2009-05-17 10:36:04 +0200 (So, 17 Mai 2009) | 1 line #5942: Copy over flag table from dbm.rst which is clearer. ........ r72712 | georg.brandl | 2009-05-17 10:55:00 +0200 (So, 17 Mai 2009) | 1 line #5935: mention that BROWSER is looked for in PATH. ........ r72801 | georg.brandl | 2009-05-20 20:31:14 +0200 (Mi, 20 Mai 2009) | 1 line #6055: refer to "sqlite3" consistently. ........ r72802 | georg.brandl | 2009-05-20 20:35:27 +0200 (Mi, 20 Mai 2009) | 1 line #6051: refer to email examples for better way to construct email messages. ........ r72820 | georg.brandl | 2009-05-22 09:23:32 +0200 (Fr, 22 Mai 2009) | 1 line Use raise X(y). ........ r72822 | georg.brandl | 2009-05-22 11:33:25 +0200 (Fr, 22 Mai 2009) | 1 line #6084: fix example. ........ r72824 | georg.brandl | 2009-05-22 11:43:17 +0200 (Fr, 22 Mai 2009) | 1 line Fix references to file-related functions and methods (os.* vs file.*). ........ r72826 | georg.brandl | 2009-05-22 11:49:42 +0200 (Fr, 22 Mai 2009) | 1 line Fix confusing wording. ........ r72827 | georg.brandl | 2009-05-22 11:50:30 +0200 (Fr, 22 Mai 2009) | 1 line s/use/call/ ........ r72828 | georg.brandl | 2009-05-22 11:58:48 +0200 (Fr, 22 Mai 2009) | 1 line Correction in softspace behavior description. ........ r72830 | georg.brandl | 2009-05-22 12:40:00 +0200 (Fr, 22 Mai 2009) | 1 line #6086: fix spelling and use a better exception to catch. ........
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better than whatever you come up with yourself.
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Compare::
# ugh!
return dir+"/"+file
# better
return os.path.join(dir, file)
More useful functions in :mod:`os.path`: :func:`basename`, :func:`dirname` and
:func:`splitext`.
There are also many useful built-in functions people seem not to be aware of for
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some reason: :func:`min` and :func:`max` can find the minimum/maximum of any
sequence with comparable semantics, for example, yet many people write their own
:func:`max`/:func:`min`. Another highly useful function is :func:`reduce`. A
classical use of :func:`reduce` is something like ::
import sys, operator
nums = map(float, sys.argv[1:])
print reduce(operator.add, nums)/len(nums)
This cute little script prints the average of all numbers given on the command
line. The :func:`reduce` adds up all the numbers, and the rest is just some
pre- and postprocessing.
On the same note, note that :func:`float`, :func:`int` and :func:`long` all
accept arguments of type string, and so are suited to parsing --- assuming you
are ready to deal with the :exc:`ValueError` they raise.
Using Backslash to Continue Statements
======================================
Since Python treats a newline as a statement terminator, and since statements
Merged revisions 72319-72320,72467,72661,72675-72679,72703,72708,72710,72712,72801-72802,72820,72822,72824,72826-72828,72830 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk ........ r72319 | georg.brandl | 2009-05-05 10:28:49 +0200 (Di, 05 Mai 2009) | 1 line #1309567: fix linecache behavior of stripping subdirectories from paths when looking for relative filename matches. Also add a linecache test suite. ........ r72320 | georg.brandl | 2009-05-05 10:30:28 +0200 (Di, 05 Mai 2009) | 1 line Add a news entry for r72319. ........ r72467 | georg.brandl | 2009-05-08 14:17:34 +0200 (Fr, 08 Mai 2009) | 1 line Fix name. ........ r72661 | georg.brandl | 2009-05-15 10:03:03 +0200 (Fr, 15 Mai 2009) | 1 line Fix example output for doctest-like demos. ........ r72675 | georg.brandl | 2009-05-16 13:13:21 +0200 (Sa, 16 Mai 2009) | 1 line #6034: clarify __reversed__ doc. ........ r72676 | georg.brandl | 2009-05-16 13:14:46 +0200 (Sa, 16 Mai 2009) | 1 line #6025: fix signature of parse(). ........ r72677 | georg.brandl | 2009-05-16 13:18:55 +0200 (Sa, 16 Mai 2009) | 1 line #6009: undocument default argument of Option as deprecated. ........ r72678 | georg.brandl | 2009-05-16 13:21:29 +0200 (Sa, 16 Mai 2009) | 1 line #2856: document 2.x os.listdir() behavior for undecodable filenames. ........ r72679 | georg.brandl | 2009-05-16 13:24:41 +0200 (Sa, 16 Mai 2009) | 1 line Fix about and bugs pages to match real workflow. ........ r72703 | georg.brandl | 2009-05-17 10:10:27 +0200 (So, 17 Mai 2009) | 1 line part of #4144: fix exception message in console session. ........ r72708 | georg.brandl | 2009-05-17 10:24:29 +0200 (So, 17 Mai 2009) | 1 line #6017: better document behavior of dictiterators when the dict is changed. ........ r72710 | georg.brandl | 2009-05-17 10:36:04 +0200 (So, 17 Mai 2009) | 1 line #5942: Copy over flag table from dbm.rst which is clearer. ........ r72712 | georg.brandl | 2009-05-17 10:55:00 +0200 (So, 17 Mai 2009) | 1 line #5935: mention that BROWSER is looked for in PATH. ........ r72801 | georg.brandl | 2009-05-20 20:31:14 +0200 (Mi, 20 Mai 2009) | 1 line #6055: refer to "sqlite3" consistently. ........ r72802 | georg.brandl | 2009-05-20 20:35:27 +0200 (Mi, 20 Mai 2009) | 1 line #6051: refer to email examples for better way to construct email messages. ........ r72820 | georg.brandl | 2009-05-22 09:23:32 +0200 (Fr, 22 Mai 2009) | 1 line Use raise X(y). ........ r72822 | georg.brandl | 2009-05-22 11:33:25 +0200 (Fr, 22 Mai 2009) | 1 line #6084: fix example. ........ r72824 | georg.brandl | 2009-05-22 11:43:17 +0200 (Fr, 22 Mai 2009) | 1 line Fix references to file-related functions and methods (os.* vs file.*). ........ r72826 | georg.brandl | 2009-05-22 11:49:42 +0200 (Fr, 22 Mai 2009) | 1 line Fix confusing wording. ........ r72827 | georg.brandl | 2009-05-22 11:50:30 +0200 (Fr, 22 Mai 2009) | 1 line s/use/call/ ........ r72828 | georg.brandl | 2009-05-22 11:58:48 +0200 (Fr, 22 Mai 2009) | 1 line Correction in softspace behavior description. ........ r72830 | georg.brandl | 2009-05-22 12:40:00 +0200 (Fr, 22 Mai 2009) | 1 line #6086: fix spelling and use a better exception to catch. ........
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are often more than is comfortable to put in one line, many people do::
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if foo.bar()['first'][0] == baz.quux(1, 2)[5:9] and \
calculate_number(10, 20) != forbulate(500, 360):
pass
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You should realize that this is dangerous: a stray space after the ``\`` would
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make this line wrong, and stray spaces are notoriously hard to see in editors.
In this case, at least it would be a syntax error, but if the code was::
value = foo.bar()['first'][0]*baz.quux(1, 2)[5:9] \
+ calculate_number(10, 20)*forbulate(500, 360)
then it would just be subtly wrong.
It is usually much better to use the implicit continuation inside parenthesis:
This version is bulletproof::
Merged revisions 68133-68134,68141-68142,68145-68146,68148-68149,68159-68162,68166,68171-68174,68179,68195-68196,68210,68214-68215,68217-68222 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk ........ r68133 | antoine.pitrou | 2009-01-01 16:38:03 +0100 (Thu, 01 Jan 2009) | 1 line fill in actual issue number in tests ........ r68134 | hirokazu.yamamoto | 2009-01-01 16:45:39 +0100 (Thu, 01 Jan 2009) | 2 lines Issue #4797: IOError.filename was not set when _fileio.FileIO failed to open file with `str' filename on Windows. ........ r68141 | benjamin.peterson | 2009-01-01 17:43:12 +0100 (Thu, 01 Jan 2009) | 1 line fix highlighting ........ r68142 | benjamin.peterson | 2009-01-01 18:29:49 +0100 (Thu, 01 Jan 2009) | 2 lines welcome to 2009, Python! ........ r68145 | amaury.forgeotdarc | 2009-01-02 01:03:54 +0100 (Fri, 02 Jan 2009) | 5 lines #4801 _collections module fails to build on cygwin. _PyObject_GC_TRACK is the macro version of PyObject_GC_Track, and according to documentation it should not be used for extension modules. ........ r68146 | ronald.oussoren | 2009-01-02 11:44:46 +0100 (Fri, 02 Jan 2009) | 2 lines Fix for issue4472: "configure --enable-shared doesn't work on OSX" ........ r68148 | ronald.oussoren | 2009-01-02 11:48:31 +0100 (Fri, 02 Jan 2009) | 2 lines Forgot to add a NEWS item in my previous checkin ........ r68149 | ronald.oussoren | 2009-01-02 11:50:48 +0100 (Fri, 02 Jan 2009) | 2 lines Fix for issue4780 ........ r68159 | ronald.oussoren | 2009-01-02 15:48:17 +0100 (Fri, 02 Jan 2009) | 2 lines Fix for issue 1627952 ........ r68160 | ronald.oussoren | 2009-01-02 15:52:09 +0100 (Fri, 02 Jan 2009) | 2 lines Fix for issue r1737832 ........ r68161 | ronald.oussoren | 2009-01-02 16:00:05 +0100 (Fri, 02 Jan 2009) | 3 lines Fix for issue 1149804 ........ r68162 | ronald.oussoren | 2009-01-02 16:06:00 +0100 (Fri, 02 Jan 2009) | 3 lines Fix for issue 4472 is incompatible with Cygwin, this patch should fix that. ........ r68166 | benjamin.peterson | 2009-01-02 19:26:23 +0100 (Fri, 02 Jan 2009) | 1 line document PyMemberDef ........ r68171 | georg.brandl | 2009-01-02 21:25:14 +0100 (Fri, 02 Jan 2009) | 3 lines #4811: fix markup glitches (mostly remains of the conversion), found by Gabriel Genellina. ........ r68172 | martin.v.loewis | 2009-01-02 21:32:55 +0100 (Fri, 02 Jan 2009) | 2 lines Issue #4075: Use OutputDebugStringW in Py_FatalError. ........ r68173 | martin.v.loewis | 2009-01-02 21:40:14 +0100 (Fri, 02 Jan 2009) | 2 lines Issue #4051: Prevent conflict of UNICODE macros in cPickle. ........ r68174 | benjamin.peterson | 2009-01-02 21:47:27 +0100 (Fri, 02 Jan 2009) | 1 line fix compilation on non-Windows platforms ........ r68179 | raymond.hettinger | 2009-01-02 22:26:45 +0100 (Fri, 02 Jan 2009) | 1 line Issue #4615. Document how to use itertools for de-duping. ........ r68195 | georg.brandl | 2009-01-03 14:45:15 +0100 (Sat, 03 Jan 2009) | 2 lines Remove useless string literal. ........ r68196 | georg.brandl | 2009-01-03 15:29:53 +0100 (Sat, 03 Jan 2009) | 2 lines Fix indentation. ........ r68210 | georg.brandl | 2009-01-03 20:10:12 +0100 (Sat, 03 Jan 2009) | 2 lines Set eol-style correctly for mp_distributing.py. ........ r68214 | georg.brandl | 2009-01-03 20:44:48 +0100 (Sat, 03 Jan 2009) | 2 lines Make indentation consistent. ........ r68215 | georg.brandl | 2009-01-03 21:15:14 +0100 (Sat, 03 Jan 2009) | 2 lines Fix role name. ........ r68217 | georg.brandl | 2009-01-03 21:30:15 +0100 (Sat, 03 Jan 2009) | 2 lines Add rstlint, a little tool to find subtle markup problems and inconsistencies in the Doc sources. ........ r68218 | georg.brandl | 2009-01-03 21:38:59 +0100 (Sat, 03 Jan 2009) | 2 lines Recognize usage of the default role. ........ r68219 | georg.brandl | 2009-01-03 21:47:01 +0100 (Sat, 03 Jan 2009) | 2 lines Fix uses of the default role. ........ r68220 | georg.brandl | 2009-01-03 21:55:06 +0100 (Sat, 03 Jan 2009) | 2 lines Remove trailing whitespace. ........ r68221 | georg.brandl | 2009-01-03 22:04:55 +0100 (Sat, 03 Jan 2009) | 2 lines Remove tabs from the documentation. ........ r68222 | georg.brandl | 2009-01-03 22:11:58 +0100 (Sat, 03 Jan 2009) | 2 lines Disable the line length checker by default. ........
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value = (foo.bar()['first'][0]*baz.quux(1, 2)[5:9]
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+ calculate_number(10, 20)*forbulate(500, 360))