f.py

def lookup_xml_response(*args, **kwargs):
    print args # returning (), which makes sense, because of how it's getting called on #15
    m = Mock()
    m.status_code = 200
    m.text = '< .... />' // Do the stuff to determine what response to send back based on args/kwargs
    return m

class MyTestCase(TestCase):
    def setUp(self):
        self.api = MyApi()
        self.maxDiff = None

    @patch('requests.post') 
    def test_search(self, requests_mock):
        requests_mock.side_effect = lookup_xml_response
        self.api.search('tasty things')