[python] fix: omit content-type header when optional body is absent

[msyyc]

For #11253

Problem

For an operation with an optional body whose content-type is modeled as required/constant (a single specific media type, e.g. azure-batch's application/json; odata=minimalmetadata), the generated client crashes when called without a body:

ValueError: No value for given attribute

The operation layer correctly computed content_type = None when no body was passed, but the generated request builder declared content_type: str = kwargs.pop("content_type") (required) and serialized the header unconditionally, so _SERIALIZER.header("content_type", None, "str") raised.

The asymmetry was the defect: the operation layer knew the body was optional, but the request-builder layer did not.

Fix

Thread body-optionality into the request-builder layer so a None content-type is omitted instead of serialized.

• parameter_serializer.py
  • serialize_query_header now accepts is_body_optional and guards the content-type header with if content_type is not None: when the body is optional.
  • pop_kwargs_from_signature accepts an explicit is_body_optional and declares the content-type kwarg as Optional[str] = kwargs.pop("content_type", ...None) even when the param is otherwise required.
• builder_serializer.py
  • The request builder's pop_kwargs_from_signature and serialize_headers pass is_body_optional (derived from body_parameter.optional, reliable even in dpg mode where the request-builder body isn't in the signature).

Operation-layer behavior is unchanged (its content_type = content_type if body else None reassignment is not emitted in the request builder).

Generated code (after)

def build_..._request(**kwargs: Any) -> HttpRequest:
    _headers = case_insensitive_dict(kwargs.pop("headers", {}) or {})
    content_type: Optional[str] = kwargs.pop("content_type", _headers.pop("content-type", None))
    ...
    # Construct headers
    if content_type is not None:
        _headers["content-type"] = _SERIALIZER.header("content_type", content_type, "str")
    ...

Verification

• Reproduced the bug with a minimal spec, then confirmed the regenerated request builder now declares content_type: Optional[str] and guards the header.
• Full regeneration of both flavors (unbranded + azure): zero generated diffs — no regressions.
• Added tests/unit/test_content_type_optional_body.py (4 tests); full unit suite passes.

Code Diff

No code diff caused by this PR

[pkg-pr-new]

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npm i https://pkg.pr.new/@typespec/http-client-python@11272

commit: 3a6fa97

[github-actions]

All changed packages have been documented.

• ✅ @typespec/http-client-python

Show changes

@typespec/http-client-python - fix ✏️

Fix generated request builders serializing a None content-type header for an,> operation with an optional body whose content-type is required/constant. The,> content-type kwarg is now declared Optional[str] and the header is omitted,> when it is None, instead of raising ValueError: No value for given attribute.

[Copilot]

Pull request overview

Fixes the TypeSpec Python HTTP client generator so that generated request builders omit the content-type header when the request body is optional and absent, preventing _SERIALIZER.header(..., None, ...) from raising ValueError: No value for given attribute (issue #11253).

Changes:

• Thread is_body_optional into request-builder kwarg popping and header serialization so content_type becomes Optional[str] with a None default and the header is conditionally emitted.
• Update request-builder serialization to derive body optionality from the modeled body parameter (including dpg scenarios where the body isn’t in the request-builder signature).
• Add unit regression tests and a Chronus changelog entry.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
packages/http-client-python/generator/pygen/codegen/serializers/parameter_serializer.py	Adds is_body_optional handling to treat content-type as nullable/guarded when body is optional.
packages/http-client-python/generator/pygen/codegen/serializers/builder_serializer.py	Passes body optionality through to request-builder kwarg/header serialization.
packages/http-client-python/tests/unit/test_content_type_optional_body.py	Adds regression unit tests covering guarded header emission and optionalized content_type kwarg.
.chronus/changes/http-client-python_fix-optional-body-content-type-2026-7-16-0-0-0.md	Records the generator fix in the changelog.

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