If you want to run an external process from within Python, and are not interested in the error that process may emit, there are a few ways, and I will demonstrate by open a Python shell in a directory that isn't version-controlled by Mercurial:

  1. So, we don't want to see this ugliness:

    >>> from subprocess import call
    >>> call('hg status'.split())
    abort: no repository found in '/home/tshepang/projects/pyramid' (.hg not found)!
    255
    
  2. So, here's the simplest solution:

    >>> import os
    >>> from subprocess import call
    >>> call('hg status'.split(), stderr=open(os.devnull))
    255
    

    Problem: it leaves the file descriptor open... not good.

  3. We fix:

    >>> import os
    >>> from subprocess import call
    >>> DEVNULL = open(os.devnull)
    >>> call('hg status'.split(), stderr=DEVNULL)
    255
    >>> DEVNULL.close()
    

    Much better, much uglier. (example usage)

  4. Something better:

    >>> import os
    >>> from subprocess import call
    >>> with open(os.devnull) as DEVNULL:
    ...     call('hg status'.split(), stderr=DEVNULL)
    255
    

    Looks much nicer, and is more convenient, unless we wanted to do something similar multiple times, in which case the previous example would be preferable.

  5. Best solution:

    >>> import os
    >>> from subprocess import call, DEVNULL
    >>> call('hg status'.split(), stderr=DEVNULL)
    255
    

This small and wonderful feature is new as of Python 3.3.