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<H1><A NAME="SECTION0012000000000000000000"></A><A NAME="briefTour"></A>
<BR>
10. Brief Tour of the Standard Library 
</H1>

<P>

<H1><A NAME="SECTION0012100000000000000000"></A><A NAME="os-interface"></A>
<BR>
10.1 Operating System Interface
</H1>

<P>
The <a class="ulink" href="../lib/module-os.html"
  ><tt class="module">os</tt></a>
module provides dozens of functions for interacting with the
operating system:

<P>
<div class="verbatim"><pre>
&gt;&gt;&gt; import os
&gt;&gt;&gt; os.system('time 0:02')
0
&gt;&gt;&gt; os.getcwd()      # Return the current working directory
'C:\\Python24'
&gt;&gt;&gt; os.chdir('/server/accesslogs')
</pre></div>

<P>
Be sure to use the "<tt class="samp">import os</tt>" style instead of
"<tt class="samp">from os import *</tt>".  This will keep <tt class="function">os.open()</tt> from
shadowing the builtin <tt class="function">open()</tt> function which operates much
differently.

<P>
<a id='l2h-34' xml:id='l2h-34'></a>The builtin <tt class="function">dir()</tt> and <tt class="function">help()</tt> functions are useful
as interactive aids for working with large modules like <tt class="module">os</tt>:

<P>
<div class="verbatim"><pre>
&gt;&gt;&gt; import os
&gt;&gt;&gt; dir(os)
&lt;returns a list of all module functions&gt;
&gt;&gt;&gt; help(os)
&lt;returns an extensive manual page created from the module's docstrings&gt;
</pre></div>

<P>
For daily file and directory management tasks, the 
<a class="ulink" href="../lib/module-shutil.html"
  ><tt class="module">shutil</tt></a>
module provides a higher level interface that is easier to use:

<P>
<div class="verbatim"><pre>
&gt;&gt;&gt; import shutil
&gt;&gt;&gt; shutil.copyfile('data.db', 'archive.db')
&gt;&gt;&gt; shutil.move('/build/executables', 'installdir')
</pre></div>

<P>

<H1><A NAME="SECTION0012200000000000000000"></A><A NAME="file-wildcards"></A>
<BR>
10.2 File Wildcards
</H1>

<P>
The <a class="ulink" href="../lib/module-glob.html"
  ><tt class="module">glob</tt></a>
module provides a function for making file lists from directory
wildcard searches:

<P>
<div class="verbatim"><pre>
&gt;&gt;&gt; import glob
&gt;&gt;&gt; glob.glob('*.py')
['primes.py', 'random.py', 'quote.py']
</pre></div>

<P>

<H1><A NAME="SECTION0012300000000000000000"></A><A NAME="command-line-arguments"></A>
<BR>
10.3 Command Line Arguments
</H1>

<P>
Common utility scripts often need to process command line arguments.
These arguments are stored in the
<a class="ulink" href="../lib/module-sys.html"
  ><tt class="module">sys</tt></a> module's <var>argv</var>
attribute as a list.  For instance the following output results from
running "<tt class="samp">python demo.py one two three</tt>" at the command line:

<P>
<div class="verbatim"><pre>
&gt;&gt;&gt; import sys
&gt;&gt;&gt; print sys.argv
['demo.py', 'one', 'two', 'three']
</pre></div>

<P>
The <a class="ulink" href="../lib/module-getopt.html"
  ><tt class="module">getopt</tt></a>
module processes <var>sys.argv</var> using the conventions of the <span class="Unix">Unix</span>
<tt class="function">getopt()</tt> function.  More powerful and flexible command line
processing is provided by the
<a class="ulink" href="../lib/module-optparse.html"
  ><tt class="module">optparse</tt></a> module.

<P>

<H1><A NAME="SECTION0012400000000000000000"></A><A NAME="stderr"></A>
<BR>
10.4 Error Output Redirection and Program Termination
</H1>

<P>
The <a class="ulink" href="../lib/module-sys.html"
  ><tt class="module">sys</tt></a>
module also has attributes for <var>stdin</var>, <var>stdout</var>, and
<var>stderr</var>.  The latter is useful for emitting warnings and error
messages to make them visible even when <var>stdout</var> has been redirected:

<P>
<div class="verbatim"><pre>
&gt;&gt;&gt; sys.stderr.write('Warning, log file not found starting a new one\n')
Warning, log file not found starting a new one
</pre></div>

<P>
The most direct way to terminate a script is to use "<tt class="samp">sys.exit()</tt>".

<P>

<H1><A NAME="SECTION0012500000000000000000"></A><A NAME="string-pattern-matching"></A>
<BR>
10.5 String Pattern Matching
</H1>

<P>
The <a class="ulink" href="../lib/module-re.html"
  ><tt class="module">re</tt></a>
module provides regular expression tools for advanced string processing.
For complex matching and manipulation, regular expressions offer succinct,
optimized solutions:

<P>
<div class="verbatim"><pre>
&gt;&gt;&gt; import re
&gt;&gt;&gt; re.findall(r'\bf[a-z]*', 'which foot or hand fell fastest')
['foot', 'fell', 'fastest']
&gt;&gt;&gt; re.sub(r'(\b[a-z]+) \1', r'\1', 'cat in the the hat')
'cat in the hat'
</pre></div>

<P>
When only simple capabilities are needed, string methods are preferred
because they are easier to read and debug:

<P>
<div class="verbatim"><pre>
&gt;&gt;&gt; 'tea for too'.replace('too', 'two')
'tea for two'
</pre></div>

<P>

<H1><A NAME="SECTION0012600000000000000000"></A><A NAME="mathematics"></A>
<BR>
10.6 Mathematics
</H1>

<P>
The <a class="ulink" href="../lib/module-math.html"
  ><tt class="module">math</tt></a> module gives
access to the underlying C library functions for floating point math:

<P>
<div class="verbatim"><pre>
&gt;&gt;&gt; import math
&gt;&gt;&gt; math.cos(math.pi / 4.0)
0.70710678118654757
&gt;&gt;&gt; math.log(1024, 2)
10.0
</pre></div>

<P>
The <a class="ulink" href="../lib/module-random.html"
  ><tt class="module">random</tt></a>
module provides tools for making random selections:

<P>
<div class="verbatim"><pre>
&gt;&gt;&gt; import random
&gt;&gt;&gt; random.choice(['apple', 'pear', 'banana'])
'apple'
&gt;&gt;&gt; random.sample(xrange(100), 10)   # sampling without replacement
[30, 83, 16, 4, 8, 81, 41, 50, 18, 33]
&gt;&gt;&gt; random.random()    # random float
0.17970987693706186
&gt;&gt;&gt; random.randrange(6)    # random integer chosen from range(6)
4
</pre></div>

<P>

<H1><A NAME="SECTION0012700000000000000000"></A><A NAME="internet-access"></A>
<BR>
10.7 Internet Access
</H1>

<P>
There are a number of modules for accessing the internet and processing
internet protocols. Two of the simplest are
<a class="ulink" href="../lib/module-urllib2.html"
  ><tt class="module">urllib2</tt></a>
for retrieving data from urls and
<a class="ulink" href="../lib/module-smtplib.html"
  ><tt class="module">smtplib</tt></a> 
for sending mail:

<P>
<div class="verbatim"><pre>
&gt;&gt;&gt; import urllib2
&gt;&gt;&gt; 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
    
&lt;BR&gt;Nov. 25, 09:43:32 PM EST

&gt;&gt;&gt; import smtplib
&gt;&gt;&gt; server = smtplib.SMTP('localhost')
&gt;&gt;&gt; server.sendmail('soothsayer@example.org', 'jcaesar@example.org',
"""To: jcaesar@example.org
From: soothsayer@example.org

Beware the Ides of March.
""")
&gt;&gt;&gt; server.quit()
</pre></div>

<P>

<H1><A NAME="SECTION0012800000000000000000"></A><A NAME="dates-and-times"></A>
<BR>
10.8 Dates and Times
</H1>

<P>
The <a class="ulink" href="../lib/module-datetime.html"
  ><tt class="module">datetime</tt></a> 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.

<P>
<div class="verbatim"><pre>
# dates are easily constructed and formatted
&gt;&gt;&gt; from datetime import date
&gt;&gt;&gt; now = date.today()
&gt;&gt;&gt; now
datetime.date(2003, 12, 2)
&gt;&gt;&gt; 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
&gt;&gt;&gt; birthday = date(1964, 7, 31)
&gt;&gt;&gt; age = now - birthday
&gt;&gt;&gt; age.days
14368
</pre></div>

<P>

<H1><A NAME="SECTION0012900000000000000000"></A><A NAME="data-compression"></A>
<BR>
10.9 Data Compression
</H1>

<P>
Common data archiving and compression formats are directly supported
by modules including:
<a class="ulink" href="../lib/module-zlib.html"
  ><tt class="module">zlib</tt></a>,
<a class="ulink" href="../lib/module-gzip.html"
  ><tt class="module">gzip</tt></a>,
<a class="ulink" href="../lib/module-bz2.html"
  ><tt class="module">bz2</tt></a>,
<a class="ulink" href="../lib/module-zipfile.html"
  ><tt class="module">zipfile</tt></a>, and
<a class="ulink" href="../lib/module-tarfile.html"
  ><tt class="module">tarfile</tt></a>.

<P>
<div class="verbatim"><pre>
&gt;&gt;&gt; import zlib
&gt;&gt;&gt; s = 'witch which has which witches wrist watch'
&gt;&gt;&gt; len(s)
41
&gt;&gt;&gt; t = zlib.compress(s)
&gt;&gt;&gt; len(t)
37
&gt;&gt;&gt; zlib.decompress(t)
'witch which has which witches wrist watch'
&gt;&gt;&gt; zlib.crc32(s)
226805979
</pre></div>

<P>

<H1><A NAME="SECTION00121000000000000000000"></A><A NAME="performance-measurement"></A>
<BR>
10.10 Performance Measurement
</H1>

<P>
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.

<P>
For example, it may be tempting to use the tuple packing and unpacking
feature instead of the traditional approach to swapping arguments.
The <a class="ulink" href="../lib/module-timeit.html"
  ><tt class="module">timeit</tt></a> module
quickly demonstrates a modest performance advantage:

<P>
<div class="verbatim"><pre>
&gt;&gt;&gt; from timeit import Timer
&gt;&gt;&gt; Timer('t=a; a=b; b=t', 'a=1; b=2').timeit()
0.57535828626024577
&gt;&gt;&gt; Timer('a,b = b,a', 'a=1; b=2').timeit()
0.54962537085770791
</pre></div>

<P>
In contrast to <tt class="module">timeit</tt>'s fine level of granularity, the
<a class="ulink" href="../lib/module-profile.html"
  ><tt class="module">profile</tt></a> and <tt class="module">pstats</tt>
modules provide tools for identifying time critical sections in larger blocks
of code.

<P>

<H1><A NAME="SECTION00121100000000000000000"></A><A NAME="quality-control"></A>
<BR>
10.11 Quality Control
</H1>

<P>
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.

<P>
The <a class="ulink" href="../lib/module-doctest.html"
  ><tt class="module">doctest</tt></a> 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:

<P>
<div class="verbatim"><pre>
def average(values):
    """Computes the arithmetic mean of a list of numbers.

    &gt;&gt;&gt; print average([20, 30, 70])
    40.0
    """
    return sum(values, 0.0) / len(values)

import doctest
doctest.testmod()   # automatically validate the embedded tests
</pre></div>

<P>
The <a class="ulink" href="../lib/module-unittest.html"
  ><tt class="module">unittest</tt></a> module is not
as effortless as the <tt class="module">doctest</tt> module, but it allows a more
comprehensive set of tests to be maintained in a separate file:

<P>
<div class="verbatim"><pre>
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
</pre></div>

<P>

<H1><A NAME="SECTION00121200000000000000000"></A><A NAME="batteries-included"></A>
<BR>
10.12 Batteries Included
</H1>

<P>
Python has a ``batteries included'' philosophy.  This is best seen
through the sophisticated and robust capabilities of its larger
packages. For example:

<P>

<UL>
<LI>The <a class="ulink" href="../lib/module-xmlrpclib.html"
  ><tt class="module">xmlrpclib</tt></a> and
  <a class="ulink" href="../lib/module-SimpleXMLRPCServer.html"
  ><tt class="module">SimpleXMLRPCServer</tt></a>
  modules make implementing remote procedure calls into an almost trivial task.
  Despite the names, no direct knowledge or handling of XML is needed.
</LI>
<LI>The <a class="ulink" href="../lib/module-email.html"
  ><tt class="module">email</tt></a> package is a library
  for managing email messages, including MIME and other RFC 2822-based message
  documents. Unlike <tt class="module">smtplib</tt> and <tt class="module">poplib</tt> 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.
</LI>
<LI>The <a class="ulink" href="../lib/module-xml.dom.html"
  ><tt class="module">xml.dom</tt></a> and
  <a class="ulink" href="../lib/module-xml.sax.html"
  ><tt class="module">xml.sax</tt></a> packages provide robust
  support for parsing this popular data interchange format. Likewise, the
  <a class="ulink" href="../lib/module-csv.html"
  ><tt class="module">csv</tt></a> 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.
</LI>
<LI>Internationalization is supported by a number of modules including
  <a class="ulink" href="../lib/module-gettext.html"
  ><tt class="module">gettext</tt></a>,
  <a class="ulink" href="../lib/module-locale.html"
  ><tt class="module">locale</tt></a>, and the
  <a class="ulink" href="../lib/module-codecs.html"
  ><tt class="module">codecs</tt></a> package.
</LI>
</UL>

<P>

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	55	<!--Table of Child-Links-->
	56	<A NAME="CHILD_LINKS"><STRONG>Subsections</STRONG></a>
	57
	58	<UL CLASS="ChildLinks">
	59	<LI><A href="node12.html#SECTION0012100000000000000000">10.1 Operating System Interface</a>
	60	<LI><A href="node12.html#SECTION0012200000000000000000">10.2 File Wildcards</a>
	61	<LI><A href="node12.html#SECTION0012300000000000000000">10.3 Command Line Arguments</a>
	62	<LI><A href="node12.html#SECTION0012400000000000000000">10.4 Error Output Redirection and Program Termination</a>
	63	<LI><A href="node12.html#SECTION0012500000000000000000">10.5 String Pattern Matching</a>
	64	<LI><A href="node12.html#SECTION0012600000000000000000">10.6 Mathematics</a>
65	<LI><A href="node12.html#SECTION0012700000000000000000">10.7 Internet Access</a>
66	<LI><A href="node12.html#SECTION0012800000000000000000">10.8 Dates and Times</a>
67	<LI><A href="node12.html#SECTION0012900000000000000000">10.9 Data Compression</a>
68	<LI><A href="node12.html#SECTION00121000000000000000000">10.10 Performance Measurement</a>
69	<LI><A href="node12.html#SECTION00121100000000000000000">10.11 Quality Control</a>
70	<LI><A href="node12.html#SECTION00121200000000000000000">10.12 Batteries Included</a>
71	</ul>
72	<!--End of Table of Child-Links-->
73	</div>
74	<HR>
75
76	<H1><A NAME="SECTION0012000000000000000000"></A><A NAME="briefTour"></A>
77	<BR>
78	10. Brief Tour of the Standard Library
79	</H1>
80
81	<P>
82
83	<H1><A NAME="SECTION0012100000000000000000"></A><A NAME="os-interface"></A>
84	<BR>
85	10.1 Operating System Interface
86	</H1>
87
88	<P>
89	The <a class="ulink" href="../lib/module-os.html"
90	><tt class="module">os</tt></a>
91	module provides dozens of functions for interacting with the
92	operating system:
93
94	<P>
95	<div class="verbatim"><pre>
96	>>> import os
97	>>> os.system('time 0:02')
98	0
99	>>> os.getcwd() # Return the current working directory
100	'C:\\Python24'
101	>>> os.chdir('/server/accesslogs')
102	</pre></div>
103
104	<P>
105	Be sure to use the "<tt class="samp">import os</tt>" style instead of
106	"<tt class="samp">from os import *</tt>". This will keep <tt class="function">os.open()</tt> from
107	shadowing the builtin <tt class="function">open()</tt> function which operates much
108	differently.
109
110	<P>
111	<a id='l2h-34' xml:id='l2h-34'></a>The builtin <tt class="function">dir()</tt> and <tt class="function">help()</tt> functions are useful
112	as interactive aids for working with large modules like <tt class="module">os</tt>:
113
114	<P>
115	<div class="verbatim"><pre>
116	>>> import os
117	>>> dir(os)
118	<returns a list of all module functions>
119	>>> help(os)
120	<returns an extensive manual page created from the module's docstrings>
121	</pre></div>
122
123	<P>
124	For daily file and directory management tasks, the
125	<a class="ulink" href="../lib/module-shutil.html"
126	><tt class="module">shutil</tt></a>
127	module provides a higher level interface that is easier to use:
128
129	<P>
130	<div class="verbatim"><pre>
131	>>> import shutil
132	>>> shutil.copyfile('data.db', 'archive.db')
133	>>> shutil.move('/build/executables', 'installdir')
134	</pre></div>
135
136	<P>
137
138	<H1><A NAME="SECTION0012200000000000000000"></A><A NAME="file-wildcards"></A>
139	<BR>
140	10.2 File Wildcards
141	</H1>
142
143	<P>
144	The <a class="ulink" href="../lib/module-glob.html"
145	><tt class="module">glob</tt></a>
146	module provides a function for making file lists from directory
147	wildcard searches:
148
149	<P>
150	<div class="verbatim"><pre>
151	>>> import glob
152	>>> glob.glob('*.py')
153	['primes.py', 'random.py', 'quote.py']
154	</pre></div>
155
156	<P>
157
158	<H1><A NAME="SECTION0012300000000000000000"></A><A NAME="command-line-arguments"></A>
159	<BR>
160	10.3 Command Line Arguments
161	</H1>
162
163	<P>
164	Common utility scripts often need to process command line arguments.
165	These arguments are stored in the
166	<a class="ulink" href="../lib/module-sys.html"
167	><tt class="module">sys</tt></a> module's <var>argv</var>
168	attribute as a list. For instance the following output results from
169	running "<tt class="samp">python demo.py one two three</tt>" at the command line:
170
171	<P>
172	<div class="verbatim"><pre>
173	>>> import sys
174	>>> print sys.argv
175	['demo.py', 'one', 'two', 'three']
176	</pre></div>
177
178	<P>
179	The <a class="ulink" href="../lib/module-getopt.html"
180	><tt class="module">getopt</tt></a>
181	module processes <var>sys.argv</var> using the conventions of the <span class="Unix">Unix</span>
182	<tt class="function">getopt()</tt> function. More powerful and flexible command line
183	processing is provided by the
184	<a class="ulink" href="../lib/module-optparse.html"
185	><tt class="module">optparse</tt></a> module.
186
187	<P>
188
189	<H1><A NAME="SECTION0012400000000000000000"></A><A NAME="stderr"></A>
190	<BR>
191	10.4 Error Output Redirection and Program Termination
192	</H1>
193
194	<P>
195	The <a class="ulink" href="../lib/module-sys.html"
196	><tt class="module">sys</tt></a>
197	module also has attributes for <var>stdin</var>, <var>stdout</var>, and
198	<var>stderr</var>. The latter is useful for emitting warnings and error
199	messages to make them visible even when <var>stdout</var> has been redirected:
200
201	<P>
202	<div class="verbatim"><pre>
203	>>> sys.stderr.write('Warning, log file not found starting a new one\n')
204	Warning, log file not found starting a new one
205	</pre></div>
206
207	<P>
208	The most direct way to terminate a script is to use "<tt class="samp">sys.exit()</tt>".
209
210	<P>
211
212	<H1><A NAME="SECTION0012500000000000000000"></A><A NAME="string-pattern-matching"></A>
213	<BR>
214	10.5 String Pattern Matching
215	</H1>
216
217	<P>
218	The <a class="ulink" href="../lib/module-re.html"
219	><tt class="module">re</tt></a>
220	module provides regular expression tools for advanced string processing.
221	For complex matching and manipulation, regular expressions offer succinct,
222	optimized solutions:
223
224	<P>
225	<div class="verbatim"><pre>
226	>>> import re
227	>>> re.findall(r'\bf[a-z]*', 'which foot or hand fell fastest')
228	['foot', 'fell', 'fastest']
229	>>> re.sub(r'(\b[a-z]+) \1', r'\1', 'cat in the the hat')
230	'cat in the hat'
231	</pre></div>
232
233	<P>
234	When only simple capabilities are needed, string methods are preferred
235	because they are easier to read and debug:
236
237	<P>
238	<div class="verbatim"><pre>
239	>>> 'tea for too'.replace('too', 'two')
240	'tea for two'
241	</pre></div>
242
243	<P>
244
245	<H1><A NAME="SECTION0012600000000000000000"></A><A NAME="mathematics"></A>
246	<BR>
247	10.6 Mathematics
248	</H1>
249
250	<P>
251	The <a class="ulink" href="../lib/module-math.html"
252	><tt class="module">math</tt></a> module gives
253	access to the underlying C library functions for floating point math:
254
255	<P>
256	<div class="verbatim"><pre>
257	>>> import math
258	>>> math.cos(math.pi / 4.0)
259	0.70710678118654757
260	>>> math.log(1024, 2)
261	10.0
262	</pre></div>
263
264	<P>
265	The <a class="ulink" href="../lib/module-random.html"
266	><tt class="module">random</tt></a>
267	module provides tools for making random selections:
268
269	<P>
270	<div class="verbatim"><pre>
271	>>> import random
272	>>> random.choice(['apple', 'pear', 'banana'])
273	'apple'
274	>>> random.sample(xrange(100), 10) # sampling without replacement
275	[30, 83, 16, 4, 8, 81, 41, 50, 18, 33]
276	>>> random.random() # random float
277	0.17970987693706186
278	>>> random.randrange(6) # random integer chosen from range(6)
279	4
280	</pre></div>
281
282	<P>
283
284	<H1><A NAME="SECTION0012700000000000000000"></A><A NAME="internet-access"></A>
285	<BR>
286	10.7 Internet Access
287	</H1>
288
289	<P>
290	There are a number of modules for accessing the internet and processing
291	internet protocols. Two of the simplest are
292	<a class="ulink" href="../lib/module-urllib2.html"
293	><tt class="module">urllib2</tt></a>
294	for retrieving data from urls and
295	<a class="ulink" href="../lib/module-smtplib.html"
296	><tt class="module">smtplib</tt></a>
297	for sending mail:
298
299	<P>
300	<div class="verbatim"><pre>
301	>>> import urllib2
302	>>> for line in urllib2.urlopen('http://tycho.usno.navy.mil/cgi-bin/timer.pl'):
303	... if 'EST' in line: # look for Eastern Standard Time
304	... print line
305
306	<BR>Nov. 25, 09:43:32 PM EST
307
308	>>> import smtplib
309	>>> server = smtplib.SMTP('localhost')
310	>>> server.sendmail('soothsayer@example.org', 'jcaesar@example.org',
311	"""To: jcaesar@example.org
312	From: soothsayer@example.org
313
314	Beware the Ides of March.
315	""")
316	>>> server.quit()
317	</pre></div>
318
319	<P>
320
321	<H1><A NAME="SECTION0012800000000000000000"></A><A NAME="dates-and-times"></A>
322	<BR>
323	10.8 Dates and Times
324	</H1>
325
326	<P>
327	The <a class="ulink" href="../lib/module-datetime.html"
328	><tt class="module">datetime</tt></a> module
329	supplies classes for manipulating dates and times in both simple
330	and complex ways. While date and time arithmetic is supported, the
331	focus of the implementation is on efficient member extraction for
332	output formatting and manipulation. The module also supports objects
333	that are time zone aware.
334
335	<P>
336	<div class="verbatim"><pre>
337	# dates are easily constructed and formatted
338	>>> from datetime import date
339	>>> now = date.today()
340	>>> now
341	datetime.date(2003, 12, 2)
342	>>> now.strftime("%m-%d-%y. %d %b %Y is a %A on the %d day of %B.")
343	'12-02-03. 02 Dec 2003 is a Tuesday on the 02 day of December.'
344
345	# dates support calendar arithmetic
346	>>> birthday = date(1964, 7, 31)
347	>>> age = now - birthday
348	>>> age.days
349	14368
350	</pre></div>
351
352	<P>
353
354	<H1><A NAME="SECTION0012900000000000000000"></A><A NAME="data-compression"></A>
355	<BR>
356	10.9 Data Compression
357	</H1>
358
359	<P>
360	Common data archiving and compression formats are directly supported
361	by modules including:
362	<a class="ulink" href="../lib/module-zlib.html"
363	><tt class="module">zlib</tt></a>,
364	<a class="ulink" href="../lib/module-gzip.html"
365	><tt class="module">gzip</tt></a>,
366	<a class="ulink" href="../lib/module-bz2.html"
367	><tt class="module">bz2</tt></a>,
368	<a class="ulink" href="../lib/module-zipfile.html"
369	><tt class="module">zipfile</tt></a>, and
370	<a class="ulink" href="../lib/module-tarfile.html"
371	><tt class="module">tarfile</tt></a>.
372
373	<P>
374	<div class="verbatim"><pre>
375	>>> import zlib
376	>>> s = 'witch which has which witches wrist watch'
377	>>> len(s)
378	41
379	>>> t = zlib.compress(s)
380	>>> len(t)
381	37
382	>>> zlib.decompress(t)
383	'witch which has which witches wrist watch'
384	>>> zlib.crc32(s)
385	226805979
386	</pre></div>
387
388	<P>
389
390	<H1><A NAME="SECTION00121000000000000000000"></A><A NAME="performance-measurement"></A>
391	<BR>
392	10.10 Performance Measurement
393	</H1>
394
395	<P>
396	Some Python users develop a deep interest in knowing the relative
397	performance of different approaches to the same problem.
398	Python provides a measurement tool that answers those questions
399	immediately.
400
401	<P>
402	For example, it may be tempting to use the tuple packing and unpacking
403	feature instead of the traditional approach to swapping arguments.
404	The <a class="ulink" href="../lib/module-timeit.html"
405	><tt class="module">timeit</tt></a> module
406	quickly demonstrates a modest performance advantage:
407
408	<P>
409	<div class="verbatim"><pre>
410	>>> from timeit import Timer
411	>>> Timer('t=a; a=b; b=t', 'a=1; b=2').timeit()
412	0.57535828626024577
413	>>> Timer('a,b = b,a', 'a=1; b=2').timeit()
414	0.54962537085770791
415	</pre></div>
416
417	<P>
418	In contrast to <tt class="module">timeit</tt>'s fine level of granularity, the
419	<a class="ulink" href="../lib/module-profile.html"
420	><tt class="module">profile</tt></a> and <tt class="module">pstats</tt>
421	modules provide tools for identifying time critical sections in larger blocks
422	of code.
423
424	<P>
425
426	<H1><A NAME="SECTION00121100000000000000000"></A><A NAME="quality-control"></A>
427	<BR>
428	10.11 Quality Control
429	</H1>
430
431	<P>
432	One approach for developing high quality software is to write tests for
433	each function as it is developed and to run those tests frequently during
434	the development process.
435
436	<P>
437	The <a class="ulink" href="../lib/module-doctest.html"
438	><tt class="module">doctest</tt></a> module provides
439	a tool for scanning a module and validating tests embedded in a program's
440	docstrings. Test construction is as simple as cutting-and-pasting a
441	typical call along with its results into the docstring. This improves
442	the documentation by providing the user with an example and it allows the
443	doctest module to make sure the code remains true to the documentation:
444
445	<P>
446	<div class="verbatim"><pre>
447	def average(values):
448	"""Computes the arithmetic mean of a list of numbers.
449
450	>>> print average([20, 30, 70])
451	40.0
452	"""
453	return sum(values, 0.0) / len(values)
454
455	import doctest
456	doctest.testmod() # automatically validate the embedded tests
457	</pre></div>
458
459	<P>
460	The <a class="ulink" href="../lib/module-unittest.html"
461	><tt class="module">unittest</tt></a> module is not
462	as effortless as the <tt class="module">doctest</tt> module, but it allows a more
463	comprehensive set of tests to be maintained in a separate file:
464
465	<P>
466	<div class="verbatim"><pre>
467	import unittest
468
469	class TestStatisticalFunctions(unittest.TestCase):
470
471	def test_average(self):
472	self.assertEqual(average([20, 30, 70]), 40.0)
473	self.assertEqual(round(average([1, 5, 7]), 1), 4.3)
474	self.assertRaises(ZeroDivisionError, average, [])
475	self.assertRaises(TypeError, average, 20, 30, 70)
476
477	unittest.main() # Calling from the command line invokes all tests
478	</pre></div>
479
480	<P>
481
482	<H1><A NAME="SECTION00121200000000000000000"></A><A NAME="batteries-included"></A>
483	<BR>
484	10.12 Batteries Included
485	</H1>
486
487	<P>
488	Python has a ``batteries included'' philosophy. This is best seen
489	through the sophisticated and robust capabilities of its larger
490	packages. For example:
491
492	<P>
493
494	<UL>
495	<LI>The <a class="ulink" href="../lib/module-xmlrpclib.html"
496	><tt class="module">xmlrpclib</tt></a> and
497	<a class="ulink" href="../lib/module-SimpleXMLRPCServer.html"
498	><tt class="module">SimpleXMLRPCServer</tt></a>
499	modules make implementing remote procedure calls into an almost trivial task.
500	Despite the names, no direct knowledge or handling of XML is needed.
501	</LI>
502	<LI>The <a class="ulink" href="../lib/module-email.html"
503	><tt class="module">email</tt></a> package is a library
504	for managing email messages, including MIME and other RFC 2822-based message
505	documents. Unlike <tt class="module">smtplib</tt> and <tt class="module">poplib</tt> which actually send
506	and receive messages, the email package has a complete toolset for building
507	or decoding complex message structures (including attachments) and for
508	implementing internet encoding and header protocols.
509	</LI>
510	<LI>The <a class="ulink" href="../lib/module-xml.dom.html"
511	><tt class="module">xml.dom</tt></a> and
512	<a class="ulink" href="../lib/module-xml.sax.html"
513	><tt class="module">xml.sax</tt></a> packages provide robust
514	support for parsing this popular data interchange format. Likewise, the
515	<a class="ulink" href="../lib/module-csv.html"
516	><tt class="module">csv</tt></a> module supports direct reads and
517	writes in a common database format. Together, these modules and packages
518	greatly simplify data interchange between python applications and other
519	tools.
520	</LI>
521	<LI>Internationalization is supported by a number of modules including
522	<a class="ulink" href="../lib/module-gettext.html"
523	><tt class="module">gettext</tt></a>,
524	<a class="ulink" href="../lib/module-locale.html"
525	><tt class="module">locale</tt></a>, and the
526	<a class="ulink" href="../lib/module-codecs.html"
527	><tt class="module">codecs</tt></a> package.
528	</LI>
529	</UL>
530
531	<P>
532
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