author | mikek |
Mon, 17 May 2010 18:37:02 +0100 | |
changeset 16 | ccc8ba7d117c |
parent 1 | 2fb8b9db1c86 |
permissions | -rw-r--r-- |
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.. _tut-brieftour: |
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********************************** |
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Brief Tour of the Standard Library |
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********************************** |
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.. _tut-os-interface: |
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Operating System Interface |
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========================== |
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The :mod:`os` module provides dozens of functions for interacting with the |
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operating system:: |
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>>> import os |
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>>> os.system('time 0:02') |
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0 |
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>>> os.getcwd() # Return the current working directory |
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'C:\\Python26' |
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>>> os.chdir('/server/accesslogs') |
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Be sure to use the ``import os`` style instead of ``from os import *``. This |
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will keep :func:`os.open` from shadowing the builtin :func:`open` function which |
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operates much differently. |
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.. index:: builtin: help |
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The builtin :func:`dir` and :func:`help` functions are useful as interactive |
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aids for working with large modules like :mod:`os`:: |
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>>> import os |
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>>> dir(os) |
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<returns a list of all module functions> |
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>>> help(os) |
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<returns an extensive manual page created from the module's docstrings> |
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For daily file and directory management tasks, the :mod:`shutil` module provides |
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a higher level interface that is easier to use:: |
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>>> import shutil |
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>>> shutil.copyfile('data.db', 'archive.db') |
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>>> shutil.move('/build/executables', 'installdir') |
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.. _tut-file-wildcards: |
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File Wildcards |
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============== |
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The :mod:`glob` module provides a function for making file lists from directory |
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wildcard searches:: |
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>>> import glob |
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>>> glob.glob('*.py') |
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['primes.py', 'random.py', 'quote.py'] |
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.. _tut-command-line-arguments: |
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Command Line Arguments |
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====================== |
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Common utility scripts often need to process command line arguments. These |
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arguments are stored in the :mod:`sys` module's *argv* attribute as a list. For |
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instance the following output results from running ``python demo.py one two |
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three`` at the command line:: |
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>>> import sys |
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>>> print sys.argv |
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['demo.py', 'one', 'two', 'three'] |
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The :mod:`getopt` module processes *sys.argv* using the conventions of the Unix |
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:func:`getopt` function. More powerful and flexible command line processing is |
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provided by the :mod:`optparse` module. |
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.. _tut-stderr: |
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Error Output Redirection and Program Termination |
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================================================ |
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The :mod:`sys` module also has attributes for *stdin*, *stdout*, and *stderr*. |
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The latter is useful for emitting warnings and error messages to make them |
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visible even when *stdout* has been redirected:: |
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>>> sys.stderr.write('Warning, log file not found starting a new one\n') |
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Warning, log file not found starting a new one |
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The most direct way to terminate a script is to use ``sys.exit()``. |
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.. _tut-string-pattern-matching: |
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String Pattern Matching |
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======================= |
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The :mod:`re` module provides regular expression tools for advanced string |
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processing. For complex matching and manipulation, regular expressions offer |
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succinct, optimized solutions:: |
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>>> import re |
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>>> re.findall(r'\bf[a-z]*', 'which foot or hand fell fastest') |
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['foot', 'fell', 'fastest'] |
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>>> re.sub(r'(\b[a-z]+) \1', r'\1', 'cat in the the hat') |
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'cat in the hat' |
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When only simple capabilities are needed, string methods are preferred because |
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they are easier to read and debug:: |
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>>> 'tea for too'.replace('too', 'two') |
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'tea for two' |
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113 |
|
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114 |
|
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.. _tut-mathematics: |
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116 |
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Mathematics |
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=========== |
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119 |
|
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The :mod:`math` module gives access to the underlying C library functions for |
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floating point math:: |
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122 |
|
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>>> import math |
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>>> math.cos(math.pi / 4.0) |
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125 |
0.70710678118654757 |
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126 |
>>> math.log(1024, 2) |
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10.0 |
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128 |
|
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The :mod:`random` module provides tools for making random selections:: |
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130 |
|
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>>> import random |
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>>> random.choice(['apple', 'pear', 'banana']) |
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133 |
'apple' |
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>>> random.sample(xrange(100), 10) # sampling without replacement |
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[30, 83, 16, 4, 8, 81, 41, 50, 18, 33] |
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>>> random.random() # random float |
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137 |
0.17970987693706186 |
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>>> random.randrange(6) # random integer chosen from range(6) |
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139 |
4 |
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140 |
|
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141 |
|
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142 |
.. _tut-internet-access: |
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143 |
|
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144 |
Internet Access |
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145 |
=============== |
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146 |
|
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There are a number of modules for accessing the internet and processing internet |
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protocols. Two of the simplest are :mod:`urllib2` for retrieving data from urls |
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and :mod:`smtplib` for sending mail:: |
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150 |
|
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151 |
>>> import urllib2 |
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>>> for line in urllib2.urlopen('http://tycho.usno.navy.mil/cgi-bin/timer.pl'): |
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... if 'EST' in line or 'EDT' in line: # look for Eastern Time |
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|
154 |
... print line |
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155 |
|
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|
156 |
<BR>Nov. 25, 09:43:32 PM EST |
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157 |
|
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|
158 |
>>> import smtplib |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
159 |
>>> server = smtplib.SMTP('localhost') |
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|
160 |
>>> server.sendmail('soothsayer@example.org', 'jcaesar@example.org', |
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|
161 |
... """To: jcaesar@example.org |
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|
162 |
... From: soothsayer@example.org |
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|
163 |
... |
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164 |
... Beware the Ides of March. |
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165 |
... """) |
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|
166 |
>>> server.quit() |
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|
167 |
|
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168 |
(Note that the second example needs a mailserver running on localhost.) |
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|
169 |
|
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|
170 |
|
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|
171 |
.. _tut-dates-and-times: |
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172 |
|
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|
173 |
Dates and Times |
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|
174 |
=============== |
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|
175 |
|
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|
176 |
The :mod:`datetime` module supplies classes for manipulating dates and times in |
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|
177 |
both simple and complex ways. While date and time arithmetic is supported, the |
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|
178 |
focus of the implementation is on efficient member extraction for output |
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|
179 |
formatting and manipulation. The module also supports objects that are timezone |
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|
180 |
aware. :: |
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|
181 |
|
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|
182 |
# dates are easily constructed and formatted |
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|
183 |
>>> from datetime import date |
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|
184 |
>>> now = date.today() |
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|
185 |
>>> now |
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|
186 |
datetime.date(2003, 12, 2) |
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|
187 |
>>> now.strftime("%m-%d-%y. %d %b %Y is a %A on the %d day of %B.") |
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|
188 |
'12-02-03. 02 Dec 2003 is a Tuesday on the 02 day of December.' |
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|
189 |
|
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|
190 |
# dates support calendar arithmetic |
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|
191 |
>>> birthday = date(1964, 7, 31) |
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|
192 |
>>> age = now - birthday |
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|
193 |
>>> age.days |
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|
194 |
14368 |
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|
195 |
|
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|
196 |
|
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197 |
.. _tut-data-compression: |
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|
198 |
|
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|
199 |
Data Compression |
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|
200 |
================ |
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|
201 |
|
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|
202 |
Common data archiving and compression formats are directly supported by modules |
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|
203 |
including: :mod:`zlib`, :mod:`gzip`, :mod:`bz2`, :mod:`zipfile` and |
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|
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:mod:`tarfile`. :: |
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|
205 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
206 |
>>> import zlib |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
207 |
>>> s = 'witch which has which witches wrist watch' |
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|
208 |
>>> len(s) |
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|
209 |
41 |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
210 |
>>> t = zlib.compress(s) |
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|
211 |
>>> len(t) |
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|
212 |
37 |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
213 |
>>> zlib.decompress(t) |
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'witch which has which witches wrist watch' |
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|
215 |
>>> zlib.crc32(s) |
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|
216 |
226805979 |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
217 |
|
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|
218 |
|
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|
219 |
.. _tut-performance-measurement: |
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220 |
|
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Performance Measurement |
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======================= |
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Some Python users develop a deep interest in knowing the relative performance of |
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different approaches to the same problem. Python provides a measurement tool |
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that answers those questions immediately. |
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For example, it may be tempting to use the tuple packing and unpacking feature |
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instead of the traditional approach to swapping arguments. The :mod:`timeit` |
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module quickly demonstrates a modest performance advantage:: |
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>>> from timeit import Timer |
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>>> Timer('t=a; a=b; b=t', 'a=1; b=2').timeit() |
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0.57535828626024577 |
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>>> Timer('a,b = b,a', 'a=1; b=2').timeit() |
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0.54962537085770791 |
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In contrast to :mod:`timeit`'s fine level of granularity, the :mod:`profile` and |
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:mod:`pstats` modules provide tools for identifying time critical sections in |
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larger blocks of code. |
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.. _tut-quality-control: |
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Quality Control |
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=============== |
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One approach for developing high quality software is to write tests for each |
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function as it is developed and to run those tests frequently during the |
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development process. |
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The :mod:`doctest` module provides a tool for scanning a module and validating |
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tests embedded in a program's docstrings. Test construction is as simple as |
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cutting-and-pasting a typical call along with its results into the docstring. |
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This improves the documentation by providing the user with an example and it |
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allows the doctest module to make sure the code remains true to the |
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documentation:: |
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def average(values): |
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"""Computes the arithmetic mean of a list of numbers. |
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>>> print average([20, 30, 70]) |
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40.0 |
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""" |
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return sum(values, 0.0) / len(values) |
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import doctest |
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doctest.testmod() # automatically validate the embedded tests |
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The :mod:`unittest` module is not as effortless as the :mod:`doctest` module, |
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but it allows a more comprehensive set of tests to be maintained in a separate |
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file:: |
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import unittest |
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class TestStatisticalFunctions(unittest.TestCase): |
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def test_average(self): |
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self.assertEqual(average([20, 30, 70]), 40.0) |
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self.assertEqual(round(average([1, 5, 7]), 1), 4.3) |
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self.assertRaises(ZeroDivisionError, average, []) |
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self.assertRaises(TypeError, average, 20, 30, 70) |
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unittest.main() # Calling from the command line invokes all tests |
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.. _tut-batteries-included: |
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Batteries Included |
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================== |
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Python has a "batteries included" philosophy. This is best seen through the |
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sophisticated and robust capabilities of its larger packages. For example: |
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* The :mod:`xmlrpclib` and :mod:`SimpleXMLRPCServer` modules make implementing |
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remote procedure calls into an almost trivial task. Despite the modules |
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names, no direct knowledge or handling of XML is needed. |
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* The :mod:`email` package is a library for managing email messages, including |
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MIME and other RFC 2822-based message documents. Unlike :mod:`smtplib` and |
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:mod:`poplib` which actually send and receive messages, the email package has |
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a complete toolset for building or decoding complex message structures |
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(including attachments) and for implementing internet encoding and header |
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protocols. |
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* The :mod:`xml.dom` and :mod:`xml.sax` packages provide robust support for |
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parsing this popular data interchange format. Likewise, the :mod:`csv` module |
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supports direct reads and writes in a common database format. Together, these |
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modules and packages greatly simplify data interchange between python |
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applications and other tools. |
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* Internationalization is supported by a number of modules including |
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:mod:`gettext`, :mod:`locale`, and the :mod:`codecs` package. |
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