<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def assertNone(expr, message=None):
 """ Assert that expr is None. :param expr: expression. :param message: Message set to raised Exception :raises: TestStepFail if expr is not None. """

    if expr is not None:
        raise TestStepFail(
            format_message(message) if message is not None else "Assert: %s != None" % str(expr))