1.2.7. Standard Library


Reference document for this section: os module: operating system functionality

“A portable way of using operating system dependent functionality.”

Directory and file manipulation

Current directory:

In [17]: os.getcwd()
Out[17]: '/Users/cburns/src/scipy2009/scipy_2009_tutorial/source'

List a directory:

In [31]: os.listdir(os.curdir)

Make a directory:

In [32]: os.mkdir('junkdir')
In [33]: 'junkdir' in os.listdir(os.curdir)
Out[33]: True

Rename the directory:

In [36]: os.rename('junkdir', 'foodir')
In [37]: 'junkdir' in os.listdir(os.curdir)
Out[37]: False
In [38]: 'foodir' in os.listdir(os.curdir)
Out[38]: True
In [41]: os.rmdir('foodir')
In [42]: 'foodir' in os.listdir(os.curdir)
Out[42]: False

Delete a file:

In [44]: fp = open('junk.txt', 'w')
In [45]: fp.close()
In [46]: 'junk.txt' in os.listdir(os.curdir)
Out[46]: True
In [47]: os.remove('junk.txt')
In [48]: 'junk.txt' in os.listdir(os.curdir)
Out[48]: False

os.path: path manipulations

os.path provides common operations on pathnames.

In [70]: fp = open('junk.txt', 'w')
In [71]: fp.close()
In [72]: a = os.path.abspath('junk.txt')
In [73]: a
Out[73]: '/Users/cburns/src/scipy2009/scipy_2009_tutorial/source/junk.txt'
In [74]: os.path.split(a)
Out[74]: ('/Users/cburns/src/scipy2009/scipy_2009_tutorial/source',
In [78]: os.path.dirname(a)
Out[78]: '/Users/cburns/src/scipy2009/scipy_2009_tutorial/source'
In [79]: os.path.basename(a)
Out[79]: 'junk.txt'
In [80]: os.path.splitext(os.path.basename(a))
Out[80]: ('junk', '.txt')
In [84]: os.path.exists('junk.txt')
Out[84]: True
In [86]: os.path.isfile('junk.txt')
Out[86]: True
In [87]: os.path.isdir('junk.txt')
Out[87]: False
In [88]: os.path.expanduser('~/local')
Out[88]: '/Users/cburns/local'
In [92]: os.path.join(os.path.expanduser('~'), 'local', 'bin')
Out[92]: '/Users/cburns/local/bin'

Running an external command

In [8]: os.system('ls')
basic_types.rst demo.py functions.rst python_language.rst standard_library.rst
control_flow.rst exceptions.rst io.rst python-logo.png
demo2.py first_steps.rst oop.rst reusing_code.rst


Alternative to os.system

A noteworthy alternative to os.system is the sh module. Which provides much more convenient ways to obtain the output, error stream and exit code of the external command.

In [20]: import sh
In [20]: com = sh.ls()
In [21]: print(com)
basic_types.rst exceptions.rst oop.rst standard_library.rst
control_flow.rst first_steps.rst python_language.rst
demo2.py functions.rst python-logo.png
demo.py io.rst reusing_code.rst
In [22]: print(com.exit_code)
In [23]: type(com)
Out[23]: sh.RunningCommand

Walking a directory

os.path.walk generates a list of filenames in a directory tree.

In [10]: for dirpath, dirnames, filenames in os.walk(os.curdir):
....: for fp in filenames:
....: print(os.path.abspath(fp))

Environment variables:

In [9]: import os
In [11]: os.environ.keys()
In [12]: os.environ['PYTHONPATH']
Out[12]: '.:/Users/cburns/src/utils:/Users/cburns/src/nitools:
In [16]: os.getenv('PYTHONPATH')
Out[16]: '.:/Users/cburns/src/utils:/Users/cburns/src/nitools:
/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5' shutil: high-level file operations

The shutil provides useful file operations:

  • shutil.rmtree: Recursively delete a directory tree.
  • shutil.move: Recursively move a file or directory to another location.
  • shutil.copy: Copy files or directories. glob: Pattern matching on files

The glob module provides convenient file pattern matching.

Find all files ending in .txt:

In [18]: import glob
In [19]: glob.glob('*.txt')
Out[19]: ['holy_grail.txt', 'junk.txt', 'newfile.txt'] sys module: system-specific information

System-specific information related to the Python interpreter.

  • Which version of python are you running and where is it installed:

    In [117]: sys.platform
    Out[117]: 'darwin'
    In [118]: sys.version
    Out[118]: '2.5.2 (r252:60911, Feb 22 2008, 07:57:53) \n
    [GCC 4.0.1 (Apple Computer, Inc. build 5363)]'
    In [119]: sys.prefix
    Out[119]: '/Library/Frameworks/Python.framework/Versions/2.5'
  • List of command line arguments passed to a Python script:

    In [100]: sys.argv
    Out[100]: ['/Users/cburns/local/bin/ipython']

sys.path is a list of strings that specifies the search path for modules. Initialized from PYTHONPATH:

In [121]: sys.path
... pickle: easy persistence

Useful to store arbitrary objects to a file. Not safe or fast!

In [1]: import pickle
In [2]: l = [1, None, 'Stan']
In [3]: pickle.dump(l, file('test.pkl', 'w'))
In [4]: pickle.load(file('test.pkl'))
Out[4]: [1, None, 'Stan']


Write a program to search your PYTHONPATH for the module site.py.

The PYTHONPATH Search Solution