chapter-7-os.md

10.1. Operating System Interface

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

>>>

>>> import os
>>> os.getcwd()      # Return the current working directory
'C:\\Python38'
>>> 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 os.open() from shadowing the built-in open() function which operates much differently.

The built-in dir() and help() functions are useful as interactive aids for working with large modules like 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 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'

10.2. File Wildcards

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

>>>

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

10.3. Command Line Arguments

Common utility scripts often need to process command line arguments. These arguments are stored in the 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 argparse module provides a more sophisticated mechanism to process command line arguments. The following script extracts one or more filenames and an optional number of lines to be displayed:

import argparse

parser = argparse.ArgumentParser(prog = 'top',
    description = 'Show top lines from each file')
parser.add_argument('filenames', nargs='+')
parser.add_argument('-l', '--lines', type=int, default=10)
args = parser.parse_args()
print(args)

When run at the command line with python top.py --lines=5 alpha.txt beta.txt, the script sets args.lines to 5 and args.filenames to ['alpha.txt', 'beta.txt'].