chore(generator): add compatability layer templates (#17886)

This PR consolidates the common code into a temporary compatibility
layer as a fallback implementation as we move this code to
google-api-core. Once, the minimum supported version of google-api-core
has these functions we will clean up this layer.

Towards: #17883

Author: hebaalazzeh

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

@@ -37,6 +37,12 @@
 from google.protobuf.compiler.plugin_pb2 import CodeGeneratorResponse
 
 
+ALLOWED_PRIVATE_TEMPLATES = (
+    "__init__.py.j2",
+    "_compat.py.j2",
+)
+
+
 class Generator:
     """A protoc code generator for client libraries.
 
@@ -118,9 +124,9 @@ def get_response(self, api_schema: api.API, opts: Options) -> CodeGeneratorRespo
         # and instead of iterating over it/them, we iterate over samples
         # and plug those into the template.
         for template_name in client_templates:
-            # Quick check: Skip "private" templates.
+            # Quick check: Skip "private" templates except explicitly allowed ones.
             filename = template_name.split("/")[-1]
-            if filename.startswith("_") and filename != "__init__.py.j2":
+            if filename.startswith("_") and filename not in ALLOWED_PRIVATE_TEMPLATES:
                 continue
 
             # Append to the output files dictionary.
@@ -413,7 +419,7 @@ def _get_file(
 
         # Quick check: Do not render empty files.
         if utils.empty(cgr_file.content) and not fn.endswith(
-            ("py.typed", "__init__.py")
+            ("py.typed", "__init__.py", "_compat.py")
         ):
             return {}
 

packages/gapic-generator/gapic/templates/%namespace/%name_%version/%sub/_compat.py.j2

@@ -0,0 +1,15 @@
+{% extends "_base.py.j2" %}
+
+{% block content %}
+
+"""A compatibility module for older versions of google-api-core."""
+{# TODO(https://github.com/googleapis/google-cloud-python/issues/17813):
+Clean up this file/functions when the minimum supported version of 
+google-api-core has the functions in `_compat.py.j2`. #}
+{# TODO(https://github.com/googleapis/google-cloud-python/issues/17884): 
+Add conditional logic to check if static code exists in google-api-core and use it from there, 
+falling back to the local implementation if not present. #}
+{# TODO(https://github.com/googleapis/google-cloud-python/issues/17883): 
+Backfill compatibility functions being removed from the client layer. #}
+
+{% endblock %}

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

@@ -0,0 +1,12 @@
+{% extends "_base.py.j2" %}
+
+{% block content %}
+
+"""Tests for the compatibility module for older versions of google-api-core."""
+{# TODO(https://github.com/googleapis/google-cloud-python/issues/17813):
+Clean up this file/tests when the minimum supported version of 
+google-api-core has the functions in `_compat.py.j2`. #}
+{# TODO(https://github.com/googleapis/google-cloud-python/issues/17883): 
+Backfill compatibility functions tests being removed from the client layer. #}
+
+{% endblock %}

packages/gapic-generator/tests/integration/goldens/asset/google/cloud/asset_v1/_compat.py

@@ -0,0 +1,16 @@
+# -*- 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.
+#
+"""A compatibility module for older versions of google-api-core."""

packages/gapic-generator/tests/integration/goldens/asset/tests/unit/gapic/asset_v1/test_compat.py

@@ -0,0 +1,16 @@
+# -*- 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.
+#
+"""Tests for the compatibility module for older versions of google-api-core."""

packages/gapic-generator/tests/integration/goldens/credentials/google/iam/credentials_v1/_compat.py

@@ -0,0 +1,16 @@
+# -*- 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.
+#
+"""A compatibility module for older versions of google-api-core."""

packages/gapic-generator/tests/integration/goldens/credentials/tests/unit/gapic/credentials_v1/test_compat.py

@@ -0,0 +1,16 @@
+# -*- 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.
+#
+"""Tests for the compatibility module for older versions of google-api-core."""

packages/gapic-generator/tests/integration/goldens/eventarc/google/cloud/eventarc_v1/_compat.py

@@ -0,0 +1,16 @@
+# -*- 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.
+#
+"""A compatibility module for older versions of google-api-core."""

packages/gapic-generator/tests/integration/goldens/eventarc/tests/unit/gapic/eventarc_v1/test_compat.py

@@ -0,0 +1,16 @@
+# -*- 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.
+#
+"""Tests for the compatibility module for older versions of google-api-core."""

packages/gapic-generator/tests/integration/goldens/logging/google/cloud/logging_v2/_compat.py

@@ -0,0 +1,16 @@
+# -*- 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.
+#
+"""A compatibility module for older versions of google-api-core."""