feat(api-core): centralize rest transcoding helpers

[hebaalazzeh]

introduces a generic REST Transcoding helper under google.api_core.gapic_v1.rest_transcoding.

Scope of Changes:

• rest_helpers.py: A centralized, parameters-driven generic utility transcode() that replaces the redundant code generated inside the client REST transport constructors (query parameter JSON parsing, required field default checking, body parsing, and numeric enum serialization).
• __init__.py: Registered lazy-loading for the module.
• conftest.py: Added session-scoped mock fixtures for mTLS variables to prevent local workstation client certificate leakage during python unit tests.
• test_rest_transcoding.py: A comprehensive test suite validating transcoding behavior for query parameters, nested fields, required field default injection, and numeric enums.

This is the runtime package companion to the generator PR.

[gemini-code-assist]

Code Review

This pull request introduces a new rest_transcoding module under google/api_core/gapic_v1 to handle transcoding of requests into HTTP methods, URIs, bodies, and query parameters. It also adds comprehensive unit tests for this new functionality, configures a session-scoped pytest fixture to isolate tests from workstation mTLS environments, and updates Python version configurations. The feedback suggests optimizing the conversion of protobuf messages to Python dictionaries in rest_transcoding.py by using json_format.MessageToDict instead of serializing to JSON and deserializing it back.

[hebaalazzeh]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces a new helper module rest_helpers in google.api_core.gapic_v1 containing a transcode function, which converts protobuf or proto-plus request messages into HTTP methods, URIs, JSON bodies, and query parameters. It also includes comprehensive unit tests for this function and a new session-scoped test fixture to isolate unit tests from workstation mTLS environments. There are no review comments, and I have no additional feedback to provide.

[daniel-sanche]

1. Can you point to the code that this is replacing in a generated client? I'm having toruble finding it
2. It could be confusing that this method has the same name as path_template.transcode, which this is built on top of

Edit: I guess this is replacing a number of methods in rest_base? Can you show a diff of what the changes will look like in the generated code?

[hebaalazzeh]

1. Can you point to the code that this is replacing in a generated client? I'm having toruble finding it
2. It could be confusing that this method has the same name as path_template.transcode, which this is built on top of

Edit: I guess this is replacing a number of methods in rest_base? Can you show a diff of what the changes will look like in the generated code?

Regarding the code this is replacing: This is replacing the three private helper methods generated for each REST RPC: _get_transcoded_request, _get_request_body_json, and _get_query_params_json (defined in rest_base.py.j2:L128-167 and originally called within the template at _shared_macros.j2:L209-216). By consolidating them, we avoid generating duplicate request serialization, JSON body conversion, and default required field query parameter injection logic for every REST endpoint.

Regarding the method name: Great point. I've renamed transcode in rest_helpers.py to transcode_request to make it clear that it transcodes a full request and to avoid any confusion with path_template.transcode.

for a diff example:

# rest_base.py
    class _BaseEcho:
        @staticmethod
        def _get_http_options():
            return [{'method': 'post', 'uri': '/v1beta1/echo:echo', 'body': '*'}]

-       @staticmethod
-       def _get_transcoded_request(http_options, request):
-           pb_request = request
-           return path_template.transcode(http_options, pb_request)
-
-       @staticmethod
-       def _get_request_body_json(transcoded_request):
-           return json_format.MessageToJson(transcoded_request['body'], use_integers_for_enums=False)
-
-       @staticmethod
-       def _get_query_params_json(transcoded_request):
-           return json.loads(json_format.MessageToJson(transcoded_request['query_params'], use_integers_for_enums=False))

# rest.py
-           transcoded_request = _BaseEchoRestTransport._BaseEcho._get_transcoded_request(http_options, request)
-           body = _BaseEchoRestTransport._BaseEcho._get_request_body_json(transcoded_request)
-           query_params = _BaseEchoRestTransport._BaseEcho._get_query_params_json(transcoded_request)
+           transcoded_request, body, query_params = rest_helpers.transcode_request(
+               http_options,
+               request,
+               required_fields_default_values=getattr(
+                   _BaseEchoRestTransport._BaseEcho,
+                   "__REQUIRED_FIELDS_DEFAULT_VALUES",
+                   None,
+               ),
+               rest_numeric_enums=False,
+           )

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