@@ -50,7 +50,7 @@ def response_content(
 
 
 def _transform_types(
-    data: Dict[str, Any], custom_types: dict, transform_files: Optional[bool] = True
+    data: Dict[str, Any], custom_types: dict, *, transform_files: Optional[bool] = True
 ) -> Tuple[dict, dict]:
     """Copy the data dict with attributes that have custom types and transform them
     before being sent to the server.
@@ -67,7 +67,7 @@ def _transform_types(
     files = {}
 
     for attr_name, type_cls in custom_types.items():
-        if attr_name not in data.keys():
+        if attr_name not in data:
             continue
 
         type_obj = type_cls(data[attr_name])
 
@@ -97,14 +97,16 @@ def test_warn(self):
         assert warn_source == warning.source
 
 
-def test_transform_types_copies_data():
+def test_transform_types_copies_data_with_empty_files():
     data = {"attr": "spam"}
-    new_data, _ = utils._transform_types(data, {})
+    new_data, files = utils._transform_types(data, {})
 
     assert new_data is not data
+    assert new_data == data
+    assert files == {}
 
 
-def test_transform_types_with_transform_files():
+def test_transform_types_with_transform_files_populates_files():
     custom_types = {"attr": types.FileAttribute}
     data = {"attr": "spam"}
     new_data, files = utils._transform_types(data, custom_types)
@@ -113,7 +115,7 @@ def test_transform_types_with_transform_files():
     assert files["attr"] == ("attr", "spam")
 
 
-def test_transform_types_without_transform_files():
+def test_transform_types_without_transform_files_populates_data_with_empty_files():
     custom_types = {"attr": types.FileAttribute}
     data = {"attr": "spam"}
     new_data, files = utils._transform_types(data, custom_types, transform_files=False)