cpython/Doc/tutorial/stdlib.rst

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.. _tut-brieftour:
**********************************
Brief Tour of the Standard Library
**********************************
.. _tut-os-interface:
Operating System Interface
==========================
The :mod:`os` module provides dozens of functions for interacting with the
operating system::
>>> import os
>>> os.system('time 0:02')
0
>>> os.getcwd() # Return the current working directory
'C:\\Python26'
>>> os.chdir('/server/accesslogs')
Be sure to use the ``import os`` style instead of ``from os import *``. This
will keep :func:`os.open` from shadowing the built-in :func:`open` function which
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operates much differently.
.. index:: builtin: help
The built-in :func:`dir` and :func:`help` functions are useful as interactive
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aids for working with large modules like :mod:`os`::
>>> import os
>>> dir(os)
<returns a list of all module functions>
>>> help(os)
<returns an extensive manual page created from the module's docstrings>
For daily file and directory management tasks, the :mod:`shutil` module provides
a higher level interface that is easier to use::
>>> import shutil
>>> shutil.copyfile('data.db', 'archive.db')
>>> shutil.move('/build/executables', 'installdir')
.. _tut-file-wildcards:
File Wildcards
==============
The :mod:`glob` module provides a function for making file lists from directory
wildcard searches::
>>> import glob
>>> glob.glob('*.py')
['primes.py', 'random.py', 'quote.py']
.. _tut-command-line-arguments:
Command Line Arguments
======================
Common utility scripts often need to process command line arguments. These
arguments are stored in the :mod:`sys` module's *argv* attribute as a list. For
instance the following output results from running ``python demo.py one two
three`` at the command line::
>>> import sys
>>> print sys.argv
['demo.py', 'one', 'two', 'three']
The :mod:`getopt` module processes *sys.argv* using the conventions of the Unix
:func:`getopt` function. More powerful and flexible command line processing is
provided by the :mod:`optparse` module.
.. _tut-stderr:
Error Output Redirection and Program Termination
================================================
The :mod:`sys` module also has attributes for *stdin*, *stdout*, and *stderr*.
The latter is useful for emitting warnings and error messages to make them
visible even when *stdout* has been redirected::
>>> sys.stderr.write('Warning, log file not found starting a new one\n')
Warning, log file not found starting a new one
The most direct way to terminate a script is to use ``sys.exit()``.
.. _tut-string-pattern-matching:
String Pattern Matching
=======================
The :mod:`re` module provides regular expression tools for advanced string
processing. For complex matching and manipulation, regular expressions offer
succinct, optimized solutions::
>>> import re
>>> re.findall(r'\bf[a-z]*', 'which foot or hand fell fastest')
['foot', 'fell', 'fastest']
>>> re.sub(r'(\b[a-z]+) \1', r'\1', 'cat in the the hat')
'cat in the hat'
When only simple capabilities are needed, string methods are preferred because
they are easier to read and debug::
>>> 'tea for too'.replace('too', 'two')
'tea for two'
.. _tut-mathematics:
Mathematics
===========
The :mod:`math` module gives access to the underlying C library functions for
floating point math::
>>> import math
>>> math.cos(math.pi / 4.0)
0.70710678118654757
>>> math.log(1024, 2)
10.0
The :mod:`random` module provides tools for making random selections::
>>> import random
>>> random.choice(['apple', 'pear', 'banana'])
'apple'
>>> random.sample(xrange(100), 10) # sampling without replacement
[30, 83, 16, 4, 8, 81, 41, 50, 18, 33]
>>> random.random() # random float
0.17970987693706186
>>> random.randrange(6) # random integer chosen from range(6)
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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.. _tut-internet-access:
Internet Access
===============
There are a number of modules for accessing the internet and processing internet
protocols. Two of the simplest are :mod:`urllib2` for retrieving data from urls
and :mod:`smtplib` for sending mail::
>>> import urllib2
>>> for line in urllib2.urlopen('http://tycho.usno.navy.mil/cgi-bin/timer.pl'):
... if 'EST' in line or 'EDT' in line: # look for Eastern Time
... print line
<BR>Nov. 25, 09:43:32 PM EST
>>> import smtplib
>>> server = smtplib.SMTP('localhost')
>>> server.sendmail('soothsayer@example.org', 'jcaesar@example.org',
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... """To: jcaesar@example.org
... From: soothsayer@example.org
...
... Beware the Ides of March.
... """)
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>>> server.quit()
(Note that the second example needs a mailserver running on localhost.)
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.. _tut-dates-and-times:
Dates and Times
===============
The :mod:`datetime` module supplies classes for manipulating dates and times in
both simple and complex ways. While date and time arithmetic is supported, the
focus of the implementation is on efficient member extraction for output
formatting and manipulation. The module also supports objects that are timezone
aware. ::
# dates are easily constructed and formatted
>>> from datetime import date
>>> now = date.today()
>>> now
datetime.date(2003, 12, 2)
>>> now.strftime("%m-%d-%y. %d %b %Y is a %A on the %d day of %B.")
'12-02-03. 02 Dec 2003 is a Tuesday on the 02 day of December.'
# dates support calendar arithmetic
>>> birthday = date(1964, 7, 31)
>>> age = now - birthday
>>> age.days
14368
.. _tut-data-compression:
Data Compression
================
Common data archiving and compression formats are directly supported by modules
including: :mod:`zlib`, :mod:`gzip`, :mod:`bz2`, :mod:`zipfile` and
:mod:`tarfile`. ::
>>> import zlib
>>> s = 'witch which has which witches wrist watch'
>>> len(s)
41
>>> t = zlib.compress(s)
>>> len(t)
37
>>> zlib.decompress(t)
'witch which has which witches wrist watch'
>>> zlib.crc32(s)
226805979
.. _tut-performance-measurement:
Performance Measurement
=======================
Some Python users develop a deep interest in knowing the relative performance of
different approaches to the same problem. Python provides a measurement tool
that answers those questions immediately.
For example, it may be tempting to use the tuple packing and unpacking feature
instead of the traditional approach to swapping arguments. The :mod:`timeit`
module quickly demonstrates a modest performance advantage::
>>> from timeit import Timer
>>> Timer('t=a; a=b; b=t', 'a=1; b=2').timeit()
0.57535828626024577
>>> Timer('a,b = b,a', 'a=1; b=2').timeit()
0.54962537085770791
In contrast to :mod:`timeit`'s fine level of granularity, the :mod:`profile` and
:mod:`pstats` modules provide tools for identifying time critical sections in
larger blocks of code.
.. _tut-quality-control:
Quality Control
===============
One approach for developing high quality software is to write tests for each
function as it is developed and to run those tests frequently during the
development process.
The :mod:`doctest` module provides a tool for scanning a module and validating
tests embedded in a program's docstrings. Test construction is as simple as
cutting-and-pasting a typical call along with its results into the docstring.
This improves the documentation by providing the user with an example and it
allows the doctest module to make sure the code remains true to the
documentation::
def average(values):
"""Computes the arithmetic mean of a list of numbers.
>>> print average([20, 30, 70])
40.0
"""
return sum(values, 0.0) / len(values)
import doctest
doctest.testmod() # automatically validate the embedded tests
The :mod:`unittest` module is not as effortless as the :mod:`doctest` module,
but it allows a more comprehensive set of tests to be maintained in a separate
file::
import unittest
class TestStatisticalFunctions(unittest.TestCase):
def test_average(self):
self.assertEqual(average([20, 30, 70]), 40.0)
self.assertEqual(round(average([1, 5, 7]), 1), 4.3)
self.assertRaises(ZeroDivisionError, average, [])
self.assertRaises(TypeError, average, 20, 30, 70)
unittest.main() # Calling from the command line invokes all tests
.. _tut-batteries-included:
Batteries Included
==================
Python has a "batteries included" philosophy. This is best seen through the
sophisticated and robust capabilities of its larger packages. For example:
* The :mod:`xmlrpclib` and :mod:`SimpleXMLRPCServer` modules make implementing
remote procedure calls into an almost trivial task. Despite the modules
names, no direct knowledge or handling of XML is needed.
* The :mod:`email` package is a library for managing email messages, including
MIME and other RFC 2822-based message documents. Unlike :mod:`smtplib` and
:mod:`poplib` which actually send and receive messages, the email package has
a complete toolset for building or decoding complex message structures
(including attachments) and for implementing internet encoding and header
protocols.
* The :mod:`xml.dom` and :mod:`xml.sax` packages provide robust support for
parsing this popular data interchange format. Likewise, the :mod:`csv` module
supports direct reads and writes in a common database format. Together, these
modules and packages greatly simplify data interchange between python
applications and other tools.
* Internationalization is supported by a number of modules including
:mod:`gettext`, :mod:`locale`, and the :mod:`codecs` package.