<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_pk(self):
 """ Return None if we don't have any filter on a pk, the pk if we have one, or raise a ValueError if we have more than one. For internal use only. """

        pk = None
        if self._lazy_collection['pks']:
            if len(self._lazy_collection['pks']) > 1:
                raise ValueError('Too much pks !')
            pk = list(self._lazy_collection['pks'])[0]
        return pk