gistfile1.py

def which(prog, error_msg=None):
    """
    Execute and return output of 'which' command for argument `prog'.

    @param error_msg: exception string for OSError
    @type error_msg: string or None
    @raises OSError: program 'prog' or 'which' system command not found
    """
    try:
        result = subprocess.Popen(('which', prog), stdout=subprocess.PIPE
                                  ).stdout.read().strip()
    except OSError as err:
        if err.errno != errno.ENOENT:
            raise
        raise OSError("`which' not found in environment PATH: {0}"
                      .format(os.environ.get('PATH')))
    if not result:
        raise OSError(error_msg or "`{0}' not found in environment PATH: {0}"
                      .format(prog, os.environ.get('PATH')))
    return result