<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def values_list(self, *args, **kwargs):
 """Return the primary keys as a list. The only valid call is values_list('pk', flat=True) """

        flat = kwargs.pop('flat', False)
        assert flat is True
        assert len(args) == 1
        assert args[0] == self.model._meta.pk.name
        return self.pks