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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[hebaalazzeh]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces the transcode_request utility (and its alias transcode) in rest_helpers.py to convert request messages into HTTP methods, URIs, bodies, and query parameters, accompanied by comprehensive unit tests and an mTLS environment isolation fixture. The reviewer identified a potential issue where merging default values for required fields could lead to duplicate parameters if those fields are already bound to the URI path or request body. To resolve this, the reviewer suggested importing re and adding logic to identify bound fields from the HTTP options so they can be excluded from the query parameters, along with corresponding test cases to validate this behavior.

[daniel-sanche]

I'd like to see how this is used in the generated code before approving, but it looks like that PR doesn't have the generated files yet: #17766