[BUG][Python] Header parameter with content: application/json is not JSON serialized

[AndreyVMarkelov]

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator?
• Have you tested with the latest
  master
  to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature
  request
  (example)

Description

The Python client generator does not JSON-serialize header parameters
defined using content: application/json.

For a header parameter backed by an object schema, the generated client
assigns the model directly to the header:

_header_params['X-Json-Arg'] = x_json_arg

ApiClient.sanitize_for_serialization() later converts the model to a
Python dict, but that value is not JSON-encoded before it is used as
an HTTP header value.

As a result, a parameter declared with content: application/json does
not produce the JSON string required by the OpenAPI declaration.

Regular schema-based header parameters should keep their existing
behavior.

openapi-generator version

7.26.0-SNAPSHOT

Reproduced against the latest master.

OpenAPI declaration file content or url

Minimal repro:

openapi: 3.1.0
info:
  title: JSON header content test
  version: 1.0.0
paths:
  /test:
    post:
      operationId: testJsonHeader
      parameters:
        - name: X-Json-Arg
          in: header
          required: true
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/HeaderArg'
        - name: X-Plain-Arg
          in: header
          required: false
          schema:
            type: string
      responses:
        '204':
          description: No content
components:
  schemas:
    HeaderArg:
      type: object
      properties:
        path:
          type: string
        recursive:
          type: boolean

Generation Details

Generator: python

java -jar modules/openapi-generator-cli/target/openapi-generator-cli.jar generate \
  -i /tmp/json-header-content.yaml \
  -g python \
  -o /tmp/python-json-header

Steps to reproduce

1. Build the current master.
2. Save the OpenAPI document above as /tmp/json-header-content.yaml.
3. Generate a Python client using the command above.
4. Inspect openapi_client/api/default_api.py.

Actual generated code:

if x_json_arg is not None:
    _header_params['X-Json-Arg'] = x_json_arg

Expected behavior:

if x_json_arg is not None:
    _header_params['X-Json-Arg'] = json.dumps(
        self.api_client.sanitize_for_serialization(x_json_arg)
    )

The regular header should remain unchanged:

_header_params['X-Plain-Arg'] = x_plain_arg

Related issues/PRs

Related Java generator issue/PR: #25056

This is the equivalent problem in the Python client generator.

Suggest a fix

CodegenParameter.contentType is populated for parameters defined using
content, so the Python generator can detect JSON header parameters
during parameter post-processing.

A possible fix is to: - mark header parameters whose contentType is
JSON; - JSON-encode only those values in python/api.mustache; -
preserve the existing behavior for ordinary header parameters.