def extract_positional_arg(mock_obj, call_number, position):
    """
    Given a unittest.mock object (which has been used to mock some callable), 
    a call number (less than the number of times the mock object has been 
    called) and a position, return the positional argument at that position 
    for the given call of the object.
    """
    if len(mock_obj.call_args_list) <= call_number:
        return None
    call_args = mock_obj.call_args_list[call_number]
    positional_args = call_args[0]
    if len(positional_args) <= position:
        return None
    return positional_args[position]