# view
    def retrieve(self, request, *args, **kwargs):
        data = {request.GET.get('field'): request.GET.get('value')}
        user = User.objects.filter(**data)
        serializer = self.get_serializer(user, many=True)
        return Response(serializer.data)


# Test variant №1
    def test_retrieve_user(self):
        for i in [{'field': 'pk', 'value': '1'},
                  {'field': 'username', 'value': 'user1'},
                  {'field': 'email', 'value': 'email@gmail.com'}]:
            response = self.client.get('/user/', {'field': i['field'], 'value': i['value']})
            self.assertEqual(response.status_code, 200)


# Test variant №2
    def test_retrieve_user_pk(self):
        response = self.client.get('/user/', {'field': 'pk', 'value': '1'})
        self.assertEqual(response.status_code, 200)

    def test_retrieve_user_username(self):
        response = self.client.get('/user/', {'field': 'username', 'value': 'user1'})
        self.assertEqual(response.status_code, 200)

    def test_retrieve_user_email(self):
        response = self.client.get('/user/', {'field': 'email', 'value': 'email@gmail.com'})
        self.assertEqual(response.status_code, 200)