The
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:\\Python25'
>>> os.chdir('/server/accesslogs')
Be sure to use the "import os" style instead of "from
os import *". This will keep os.open() from
shadowing the builtin open() function which operates
much differently.
The builtin dir()
and help() functions are useful as interactive aids
for working with large modules like 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
shutil module provides a higher level interface that
is easier to use:
Common utility scripts often need to process command line arguments. These
arguments are stored in the
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:
The
getopt module processes sys.argv using
the conventions of the Unix
getopt() function. More powerful and flexible command line processing is
provided by the
optparse module.
Error Output Redirection and Program Termination
The
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()".
String Pattern Matching
The
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'
Mathematics
The
math module gives access to the underlying C library
functions for floating point math:
The
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)
4
Internet Access
There are a number of modules for accessing the internet and processing
internet protocols. Two of the simplest are
urllib2 for retrieving data from urls and
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',
"""To: jcaesar@example.org
From: soothsayer@example.org
Beware the Ides of March.
""")
>>> server.quit()
Dates and Times
The 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
Data Compression
Common data archiving and compression formats are directly supported by
modules including:
zlib,
gzip,
bz2,
zipfile, and
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
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
timeit module quickly demonstrates a modest
performance advantage:
In contrast to timeit's fine level of granularity,
the
profile and pstats modules
provide tools for identifying time critical sections in larger blocks of code.
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
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
unittest module is not as effortless as the
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
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
xmlrpclib and
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
email package is a library for managing email
messages, including MIME and other RFC 2822-based message documents. Unlike
smtplib and 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
xml.dom and
xml.sax packages provide robust support for
parsing this popular data interchange format. Likewise, the
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
gettext,
locale, and the
codecs package.
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