Interfaz al sistema operativo

El módulo :mod:`os` provee docenas de funciones para interactuar con el sistema operativo:

>>> import os
>>> os.system('time 0:02')
0
>>> os.getcwd()      # devuelve el directorio de trabajo actual
'C:\\Python26'
>>> os.chdir('/server/accesslogs')

Asegurate de usar el estilo import os en lugar de from os import *. Esto evitará que :func:`os.open` oculte a la función integrada :func:`open`, que trabaja bastante diferente.

.. index:: builtin: help

Las funciones integradas :func:`dir` y :func:`help` son útiles como ayudas interactivas para trabajr con módulos grandes como :mod:`os`:

>>> import os
>>> dir(os)
<devuelve una lista de todas las funciones del módulo>
>>> help(os)
<devuelve un manual creado a partir de las documentaciones del módulo>

Para tareas diarias de administración de archivos y directorios, el módulo :mod:`shutil` provee una interfaz de más alto nivel que es más fácil de usar:

>>> import shutil
>>> shutil.copyfile('datos.db', 'archivo.db')
>>> shutil.move('/build/executables', 'dir_instalac')

Comodines de archivos

El módulo :mod:`glob` provee una función para hacer listas de archivos a partir de búsquedas con comodines en directorios:

>>> import glob
>>> glob.glob('*.py')
['primes.py', 'random.py', 'quote.py']

Argumentos de linea de órdenes

Los programas frecuentemente necesitan procesar argumentos de linea de órdes. Estos argumentos se almacenan en el atributo argv del módulo :mod:`sys` como una lista. Por ejemplo, la siguiente salida resulta de ejecutar python demo.py uno dos tres en la línea de órdenes:

>>> import sys
>>> print sys.argv
['demo.py', 'uno', 'dos', 'tres']

El módulo :mod:`getopt` procesa sys.argv usando las convenciones de la función de Unix :func:`getopt`. El módulo :mod:`optparse` provee un procesamiento más flexible de la linea de órdenes.

Redirección de la salida de error y finalización del programa

El módulo :mod:`sys` también tiene atributos para stdin, stdout, y stderr. Este último es útil para emitir mensajes de alerta y error para que se vean incluso cuando se haya redireccionado stdout:

>>> sys.stderr.write('Alerta, archivo de log no encontrado\n')
Alerta, archivo de log no encontrado

La forma más directa de terminar un programa es usar sys.exit().

Coincidencia en patrones de cadenas

El módulo :mod:`re` provee herramientas de expresiones regulares para un procesamiento avanzado de cadenas. Para manipulación y coincidencias complejas, las expresiones regulares ofrecen soluciones concisas y optimizadas:

>>> 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 :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)
4

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',
... """To: jcaesar@example.org
... From: soothsayer@example.org
...
... Beware the Ides of March.
... """)
>>> server.quit()

(Note that the second example needs a mailserver running on localhost.)

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

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

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.

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

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.

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• 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.

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• 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.

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• Internationalization is supported by a number of modules including :mod:`gettext`, :mod:`locale`, and the :mod:`codecs` package.

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