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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def user_id(self):
 """ User id of the current API user """

        if self._user_id is None:
            if self.has_logged_in:
                self._user_id = self._req_get_user_aq()['data']['uid']
            else:
                raise AuthenticationError('Not logged in.')
        return self._user_id