Tests: fix MockRequestsBackend.get_api_call_arg edge cases

get_api_call_arg had incorrectly returned None if a kwarg was passed
to the mocked function with a False-y value (e.g., [] or {})

get_api_call_json had only considered data param, ignoring json param
requests added a while back

tests/mock_requests_backend.py

@@ -51,20 +51,16 @@ class RequestsBackendMockAPITestCase(SimpleTestCase, AnymailTestMixin):
         # This assumes the last (or only) call to requests.Session.request is the API call of interest.
         if self.mock_request.call_args is None:
             raise AssertionError("No ESP API was called")
-        (args, kwargs) = self.mock_request.call_args
-        try:
-            actual_method = kwargs.get('method', None) or args[1]
-            actual_url = kwargs.get('url', None) or args[2]
-        except IndexError:
-            raise AssertionError("API was called without a method or url (?!)")
-        if actual_method != method:
-            raise AssertionError("API was not called using %s. (%s was used instead.)"
-                                 % (method, actual_method))
-        if not actual_url.endswith(url):
-            raise AssertionError("API was not called at %s\n(It was called at %s)"
-                                 % (url, actual_url))
+        if method is not None:
+            actual_method = self.get_api_call_arg('method')
+            if actual_method != method:
+                self.fail("API was not called using %s. (%s was used instead.)" % (method, actual_method))
+        if url is not None:
+            actual_url = self.get_api_call_arg('url')
+            if not actual_url.endswith(url):
+                self.fail("API was not called at %s\n(It was called at %s)" % (url, actual_url))
 
-    def get_api_call_arg(self, kwarg, pos, required=True):
+    def get_api_call_arg(self, kwarg, required=True):
         """Returns an argument passed to the mock ESP API.
 
         Fails test if API wasn't called.
@@ -73,36 +69,51 @@ class RequestsBackendMockAPITestCase(SimpleTestCase, AnymailTestMixin):
             raise AssertionError("API was not called")
         (args, kwargs) = self.mock_request.call_args
         try:
-            return kwargs.get(kwarg, None) or args[pos]
-        except IndexError:
-            if required:
-                raise AssertionError("API was called without required %s" % kwarg)
-            else:
-                return None
+            return kwargs[kwarg]
+        except KeyError:
+            pass
+
+        try:
+            # positional arg? This is the order of requests.Session.request params:
+            pos = ('method', 'url', 'params', 'data', 'headers', 'cookies', 'files', 'auth',
+                   'timeout', 'allow_redirects', 'proxies', 'hooks', 'stream', 'verify', 'cert', 'json',
+                   ).index(kwarg)
+            return args[pos]
+        except (ValueError, IndexError):
+            pass
+
+        if required:
+            self.fail("API was called without required arg '%s'" % kwarg)
+        return None
 
     def get_api_call_params(self, required=True):
         """Returns the query params sent to the mock ESP API."""
-        return self.get_api_call_arg('params', 3, required)
+        return self.get_api_call_arg('params', required)
 
     def get_api_call_data(self, required=True):
         """Returns the raw data sent to the mock ESP API."""
-        return self.get_api_call_arg('data', 4, required)
+        return self.get_api_call_arg('data', required)
 
     def get_api_call_json(self, required=True):
         """Returns the data sent to the mock ESP API, json-parsed"""
-        return json.loads(self.get_api_call_data(required))
+        # could be either the data param (as json str) or the json param (needing formatting)
+        value = self.get_api_call_arg('data', required=False)
+        if value is not None:
+            return json.loads(value)
+        else:
+            return self.get_api_call_arg('json', required)
 
     def get_api_call_headers(self, required=True):
         """Returns the headers sent to the mock ESP API"""
-        return self.get_api_call_arg('headers', 5, required)
+        return self.get_api_call_arg('headers', required)
 
     def get_api_call_files(self, required=True):
         """Returns the files sent to the mock ESP API"""
-        return self.get_api_call_arg('files', 7, required)
+        return self.get_api_call_arg('files', required)
 
     def get_api_call_auth(self, required=True):
         """Returns the auth sent to the mock ESP API"""
-        return self.get_api_call_arg('auth', 8, required)
+        return self.get_api_call_arg('auth', required)
 
     def assert_esp_not_called(self, msg=None):
         if self.mock_request.called: