me likes subprocess.DEVNULL

If you want to run an external process from within Python, and are not interested in any errors that process may emit, there are a few ways you can go about it. I will demonstrate this by opening 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.

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.