<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _check_user(self, user):
 """ Checks to make sure that a user is valid. First, checks that the user is not None. If this check fails, a MissingUserError is raised. Next, checks if the user has a validation method. If the method does not exist, the check passes. If the method exists, it is called. If the result of the call is not truthy, an InvalidUserError is raised """

        MissingUserError.require_condition(
            user is not None,
            'Could not find the requested user',
        )
        user_validate_method = getattr(
            user, self.user_class_validation_method, None
        )
        if user_validate_method is None:
            return
        InvalidUserError.require_condition(
            user_validate_method(),
            "The user is not valid or has had access revoked",
        )