code: def _svn_revision(filename):
    
    p = subprocess.Popen(['svn', 'status', '-v', filename],
               stdout=subprocess.PIPE,
               stderr=subprocess.PIPE)
    (stdout, stderr) = p.communicate()
    if p.returncode != 0 or stderr or not stdout:
      raise ValueError('Error determining svn_revision for %s: %s' %
               (os.path.split(filename)[1], textwrap.fill(stderr)))
    return stdout.split()[2]
docs: Helper for ``build_index()``: Calculate the subversion revision
    number for a given file (by using ``subprocess`` to run ``svn``).