[Python] Add test object serialization for multipart requests (#18156)

* [Python] add test object serialization for multipart requests #18140

* [python] update samples

* [python] update samples

samples/openapi3/client/petstore/python/tests/test_rest.py

@@ -0,0 +1,31 @@
+import json
+import unittest
+from unittest.mock import patch, Mock
+
+import pytest
+
+import petstore_api
+
+
+class TestMultipleResponseTypes(unittest.TestCase):
+    def setUp(self):
+        self.api_client = petstore_api.ApiClient()
+        self.fake_api = petstore_api.FakeApi(self.api_client)
+
+    def test_multipart_requests(self):
+        mock_resp = Mock()
+        mock_resp.status = 200
+        mock_resp.data = b"some text"
+        mock_resp.headers = {}
+
+        marker = petstore_api.TestObjectForMultipartRequestsRequestMarker(
+            name="name",
+        )
+
+        with patch(
+            "urllib3.PoolManager.urlopen", return_value=mock_resp
+        ):
+            returned = self.fake_api.test_object_for_multipart_requests(
+                marker=marker
+            )
+        assert returned is None