harness PayPalInterface._call with tests

requirements.txt

@@ -1,2 +1,3 @@
 nose
-requests
+mock
+requests

tests/test_express_checkout.py

@@ -3,9 +3,13 @@
 import unittest
 import warnings
 
+from mock import patch, Mock
+
 from . import interface_factory
 from . import api_details
+from paypal.exceptions import PayPalAPIResponseError
 from paypal.interface import PayPalInterface
+from paypal.response import PayPalResponse
 
 interface = interface_factory.get_interface_obj()
 
@@ -175,3 +179,32 @@ def test_returns_certificate_call_params(self):
                                 'timeout': interface.config.HTTP_TIMEOUT,
                                 'verify': interface.config.API_CA_CERTS}
         self.assertEqual(expected_call_params, call_params)
+
+
+class CallTest(unittest.TestCase):
+
+    def test_posts_params(self):
+        with patch('paypal.interface.requests.post') as post_mock:
+            post_mock.return_value = Mock(text='ACK=SUCCESS')
+            paypal_response = interface._call('some_method',
+                                              param_a='a1',
+                                              param_b='b2')
+        expected_data = {'PARAM_A': 'a1',
+                         'PARAM_B': 'b2',
+                         'USER': interface.config.API_USERNAME,
+                         'PWD': interface.config.API_PASSWORD,
+                         'SIGNATURE': interface.config.API_SIGNATURE,
+                         'METHOD': 'some_method',
+                         'VERSION': interface.config.API_VERSION}
+        post_mock.assert_called_once_with(interface.config.API_ENDPOINT,
+                                          timeout=interface.config.HTTP_TIMEOUT,
+                                          verify=interface.config.API_CA_CERTS,
+                                          data=expected_data)
+        self.assertIsInstance(paypal_response, PayPalResponse)
+        self.assertTrue(paypal_response.success)
+
+    def test_raises_configerror_on_error_response(self):
+        with patch('paypal.interface.requests.post') as post_mock:
+            post_mock.return_value = Mock(text='ACK=NO_SUCCESS')
+            with self.assertRaises(PayPalAPIResponseError):
+                interface._call('some_method', param='a')