code: def verify(cls, user_id, verification_hash):
          
          user = yield cls.get(user_id)
  
          # If user does not have verification hash then this means they have already been verified
          if 'verification_hash' not in user._resource:
              raise Return(user)
  
          if user.verification_hash != verification_hash:
              raise exceptions.ValidationError('Invalid verification hash')
  
          del user.verification_hash
          yield user._save()
  
          raise Return(user)
docs: Verify a user using the verification hash
  
          The verification hash is removed from the user once verified
  
          :param user_id: the user ID
          :param verification_hash: the verification hash
          :returns: a User instance