test(generator): add tests for _compat.py fallback

Author: hebaalazzeh

packages/gapic-generator/gapic/generator/generator.py

@@ -267,7 +267,7 @@ def _render_template(
             return answer
 
         # Only render _compat.py.j2 if the API schema has auto_populated_fields
-        if template_name.endswith("_compat.py.j2"):  # pragma: NO COVER
+        if template_name.endswith("_compat.py.j2") or template_name.endswith("test__compat.py.j2"):  # pragma: NO COVER
             has_auto_populated = any(  # pragma: NO COVER
                 m_settings and getattr(m_settings, "auto_populated_fields", None)  # pragma: NO COVER
                 for m_settings in api_schema.all_method_settings.values()  # pragma: NO COVER

packages/gapic-generator/gapic/templates/tests/unit/gapic/%name_%version/%sub/test__compat.py.j2

@@ -0,0 +1,57 @@
+# -*- coding: utf-8 -*-
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+import builtins
+import sys
+
+import mock
+import pytest
+
+from {{ api.naming.module_namespace }} import _compat
+
+
+def test_setup_request_id_fallback():
+    with mock.patch("sys.modules", dict(sys.modules)):
+        if "google.api_core.gapic_v1.request" in sys.modules:
+            del sys.modules["google.api_core.gapic_v1.request"]
+            
+        real_import = builtins.__import__
+
+        def mock_import(name, globals=None, locals=None, fromlist=(), level=0):
+            if name == "google.api_core.gapic_v1.request":
+                raise ImportError("Mocked ImportError for gapic_v1.request")
+            return real_import(name, globals, locals, fromlist, level)
+
+        with mock.patch("builtins.__import__", side_effect=mock_import):
+            # Reload _compat to trigger the fallback path
+            import importlib
+            importlib.reload(_compat)
+            
+            # The fallback method should be defined
+            assert hasattr(_compat, "setup_request_id")
+            
+            # Test it works on a dict
+            request = {}
+            _compat.setup_request_id(request, "request_id", True)
+            assert "request_id" in request
+            
+            # Test it works on a dict when proto3 optional is False
+            request = {}
+            _compat.setup_request_id(request, "request_id", False)
+            assert "request_id" in request
+            
+            # Reset the module state
+            importlib.reload(_compat)