Python OG

Brief Tour of the Standard Library

10.1. Operating System Interface

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

  • os.getcwd(), get current working directory.

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    >>> os.getcwd()      # Return the current working directory
    'C:\\Python39'

  • os.chdir(), change current working directory

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    >>> os.chdir('/')
    >>> os.getcwd()
    '/'

  • os.system('cmd'), run a cmd in system shell.

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    >>> os.system('ls')
    ....
    0

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

  • If you don’t know how to use a module, you can use help(modulename) to check the manual or The Python Standard Library

For daily file and directory management tasks, the shutil module provides a higher level interface that is easier to use

the shutil stand for shell utilities

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

10.2. File Wildcards

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

A wildcard is a symbol such as \ or ? used in some computing commands or searches in order to represent any character or range of characters.*

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>>> import glob
>>> glob.glob('*.py')
['primes.py', 'random.py', 'quote.py'
  • A glob pattern is a text expression that matches one or more file names using wild cards familiar to most users of a command line. For example, * is a glob that matches any name at all. And glob is a Filename globbing utility.
  • There are only three functions in this module, you can learn it quickly.

10.3. Command Line Arguments

  • As has mentioned before, cmd arguments are stored in sys.argv
  • The argparse module provides a more sophisticated mechanism to process command line arguments.
  • argparse will be useful if you want to write some cmdline tools

10.4. Error Output Redirection and Program Termination

  • Use sys.stderr to prevent error massage from being redirected.

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    import sys
    sys.stderr.write('Error!\n')
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    $ python3 ptest.py > trash
    Error!

  • you can use sys.stderr as a file

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    >>> import sys
    >>> file = open('temp' , 'w+')
    >>> type(sys.stderr) == type(file)
    True

  • The most direct way to terminate a script is to use sys.exit().

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    exit(status=None, /)
        Exit the interpreter by raising SystemExit(status).
        
        If the status is omitted or None, it defaults to zero (i.e., success).
        If the status is an integer, it will be used as the system exit status.
        If it is another kind of object, it will be printed and the system
        exit status will be one (i.e., failure).

    10.5. String Pattern Matching

  • use re for regular expression

  • for simple string operation, use string methods like s.replace('old','new')

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

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

If these modules can not meet your requests, have a look at SciPy

10.7. Internet Access

There are a number of modules for accessing the internet and processing internet protocol.

Two of the simplest are:

  • urllib.request —- retrive data from url (requests can be more handy)

  • smtplib—-send emails(need a mail server running)

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    $ sudo python -m smtpd -n -c DebuggingServer localhost:25
    # set up a fake server for debugging
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    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()
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    $ python3 ptest.py
    ---------- MESSAGE FOLLOWS ----------
    To: jcaesar@example.org
    From: soothsayer@example.org
    X-Peer: 127.0.0.1

    Beware the Ides of March.
    ------------ END MESSAGE ------------

    10.8. Dates and Times

The datetime module supplies classes for manipulating dates and times in both simple and complex ways.

+ - are supported. 

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# get date of today
>>> import datetime
>>> datetime.date.today()
datetime.date(2021, 2, 12)
>>> birthday = datetime.date(1964, 7, 31) # sub test
>>> datetime.date.today() - birthday
datetime.timedelta(days=20653)

10.9. Data Compression

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

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>>> 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

10.10. Performance Measurement

knowing the relative performance of different approaches to the same problem.

  • use timeit for simple tests

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    >>> 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 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.

  • doctest, scanning a module and validating tests embedded in a program’s docstrings.

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    >>> 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
    TestResults(failed=0, attempted=1)

    >>> def average(values):
        	"""Computes the arithmetic mean of a list of numbers.
    	
        	>>> print(average([20, 30, 7]))
        	40.0
        	"""
        	return sum(values) / len(values)
    >>> doctest.testmod()
    **********************************************************************
    File "__main__", line 4, in __main__.average
    Failed example:
        print(average([20, 30, 7]))
    Expected:
        40.0
    Got:
        19.0
    **********************************************************************
    1 items had failures:
       1 of   1 in __main__.average
    ***Test Failed*** 1 failures.
    TestResults(failed=1, attempted=1)

  • unittest, allows a more comprehensive set of tests to be maintained in a separate file.

10.12. Batteries Included

Python has a “batteries included” philosophy.

It means that something is self-sufficient, comes out-of-the-box ready to use, with everything that is needed.(开箱即用)

You can almost find every thing you need in The Python Standard Library.