<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_revocation_key(self, user):
 """ When the value returned by this method changes, this revocates tokens. It always includes the password so that changing the password revokes existing tokens. In addition, for one-time tokens, it also contains the last login datetime so that logging in revokes existing tokens. """

        value = ''
        if self.invalidate_on_password_change:
            value += user.password
        if self.one_time:
            value += str(user.last_login)
        return value