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:\\Python24'
>>> 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 invoke processing 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.
10.4 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')
Warning, log file not found starting a new one
The most direct way to terminate a script is to use "sys.exit()".
10.5 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'
10.6 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
10.7 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: # look for Eastern Standard Time
... print line
<BR>Nov. 25, 09:43:32 PM EST
>>> import smtplib
>>> server = smtplib.SMTP('localhost')
>>> server.sendmail('soothsayer@tmp.org', 'jceasar@tmp.org',
"""To: jceasar@tmp.org
From: soothsayer@tmp.org
Beware the Ides of March.
""")
>>> server.quit()
10.8 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 time zone 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 or %d%b %Y is a %A on the %d day of %B")
'12-02-03 or 02Dec 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
10.9 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(t)
-1438085031
10.10 Performance Measurement
Some Python users develop a deep interest in knowing the relative
performance between 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 that the traditional approach is faster:
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.
10.11 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
10.12 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 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.
