cpython/Doc/tutorial/stdlib.rst

.. _tut-brieftour:

**********************************
Brief Tour of the Standard Library
**********************************


.. _tut-os-interface:

Operating System Interface
==========================

The :mod:`os` module provides dozens of functions for interacting with the
operating system::

   >>> import os
   >>> os.getcwd()      # Return the current working directory
   'C:\\Python37'
   >>> os.chdir('/server/accesslogs')   # Change current working directory
   >>> os.system('mkdir today')   # Run the command mkdir in the system shell
   0

Be sure to use the ``import os`` style instead of ``from os import *``.  This
will keep :func:`os.open` from shadowing the built-in :func:`open` function which
operates much differently.

.. index:: builtin: help

The built-in :func:`dir` and :func:`help` functions are useful as interactive
aids for working with large modules like :mod:`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 :mod:`shutil` module provides
a higher level interface that is easier to use::

   >>> import shutil
   >>> shutil.copyfile('data.db', 'archive.db')
   'archive.db'
   >>> shutil.move('/build/executables', 'installdir')
   'installdir'


.. _tut-file-wildcards:

File Wildcards
==============

The :mod:`glob` module provides a function for making file lists from directory
wildcard searches::

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


.. _tut-command-line-arguments:

Command Line Arguments
======================

Common utility scripts often need to process command line arguments. These
arguments are stored in the :mod:`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::

   >>> import sys
   >>> print(sys.argv)
   ['demo.py', 'one', 'two', 'three']

The :mod:`argparse` module provides a mechanism to process command line arguments.
It should always be preferred over directly processing ``sys.argv`` manually.

Take, for example, the below snippet of code::

   >>> import argparse
   >>> from getpass import getuser
   >>> parser = argparse.ArgumentParser(description='An argparse example.')
   >>> parser.add_argument('name', nargs='?', default=getuser(), help='The name of someone to greet.')
   >>> parser.add_argument('--verbose', '-v', action='count')
   >>> args = parser.parse_args()
   >>> greeting = ["Hi", "Hello", "Greetings! its very nice to meet you"][args.verbose % 3]
   >>> print(f'{greeting}, {args.name}')
   >>> if not args.verbose:
   >>>     print('Try running this again with multiple "-v" flags!')

.. _tut-stderr:

Error Output Redirection and Program Termination
================================================

The :mod:`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()``.


.. _tut-string-pattern-matching:

String Pattern Matching
=======================

The :mod:`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'


.. _tut-mathematics:

Mathematics
===========

The :mod:`math` module gives access to the underlying C library functions for
floating point math::

   >>> import math
   >>> math.cos(math.pi / 4)
   0.70710678118654757
   >>> 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(range(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

The :mod:`statistics` module calculates basic statistical properties
(the mean, median, variance, etc.) of numeric data::

    >>> import statistics
    >>> data = [2.75, 1.75, 1.25, 0.25, 0.5, 1.25, 3.5]
    >>> statistics.mean(data)
    1.6071428571428572
    >>> statistics.median(data)
    1.25
    >>> statistics.variance(data)
    1.3720238095238095

The SciPy project <https://scipy.org> has many other modules for numerical
computations.

.. _tut-internet-access:

Internet Access
===============

There are a number of modules for accessing the internet and processing internet
protocols. Two of the simplest are :mod:`urllib.request` for retrieving data
from URLs and :mod:`smtplib` for sending mail::

   >>> from urllib.request import urlopen
   >>> with urlopen('http://tycho.usno.navy.mil/cgi-bin/timer.pl') as response:
   ...     for line in response:
   ...         line = line.decode('utf-8')  # Decoding the binary data to text.
   ...         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.)


.. _tut-dates-and-times:

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


.. _tut-data-compression:

Data Compression
================

Common data archiving and compression formats are directly supported by modules
including: :mod:`zlib`, :mod:`gzip`, :mod:`bz2`, :mod:`lzma`, :mod:`zipfile` and
:mod:`tarfile`. ::

   >>> import zlib
   >>> s = b'witch which has which witches wrist watch'
   >>> len(s)
   41
   >>> t = zlib.compress(s)
   >>> len(t)
   37
   >>> zlib.decompress(t)
   b'witch which has which witches wrist watch'
   >>> zlib.crc32(s)
   226805979


.. _tut-performance-measurement:

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.


.. _tut-quality-control:

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) / 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)
           with self.assertRaises(ZeroDivisionError):
               average([])
           with self.assertRaises(TypeError):
               average(20, 30, 70)

   unittest.main()  # Calling from the command line invokes all tests


.. _tut-batteries-included:

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:`xmlrpc.client` and :mod:`xmlrpc.server` 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 :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.

* The :mod:`json` package provides robust support for parsing this
  popular data interchange format.  The :mod:`csv` module supports
  direct reading and writing of files in Comma-Separated Value format,
  commonly supported by databases and spreadsheets.  XML processing is
  supported by the :mod:`xml.etree.ElementTree`, :mod:`xml.dom` and
  :mod:`xml.sax` packages. Together, these modules and packages
  greatly simplify data interchange between Python applications and
  other tools.

* The :mod:`sqlite3` module is a wrapper for the SQLite database
  library, providing a persistent database that can be updated and
  accessed using slightly nonstandard SQL syntax.

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