[azure-core] Fix wrong content-type on multipart request with no file (#39163) by manmohanmirkar123 · Pull Request #47831 · Azure/azure-sdk-for-python

manmohanmirkar123 · 2026-07-03T10:40:59Z

Summary

When a multipart payload's file part is optional and omitted, azure-core inferred that the request was not multipart — because it decided based solely on whether files was non-empty — and set Content-Type: application/x-www-form-urlencoded. This mislabels a request that should be sent as multipart/form-data (e.g. a TypeSpec-defined body whose only file part is optional).

Change

Add an is_multipart_payload flag to HttpRequest, threaded into _set_body. When set, the urlencoded Content-Type is not applied, so the transport can set the correct multipart/form-data boundary even when no file is present.

# before: no way to express intent; forced to urlencoded
# after:
HttpRequest("POST", url, data={"field": "value"}, is_multipart_payload=True)
# -> Content-Type is not forced to application/x-www-form-urlencoded

Default behavior is unchanged: a data-only request without the flag is still urlencoded.

Testing

Added regression tests in test_rest_http_request.py:
- a multipart request with form data and no file no longer sets the urlencoded Content-Type
- a data-only request without the flag still sets application/x-www-form-urlencoded
Existing url-encoded / multipart / json request tests still pass.

Changelog

Added a Bugs Fixed entry under an unreleased azure-core section.

When a multipart payload's file part is optional and omitted, azure-core inferred the request was not multipart (because `files` was empty) and set Content-Type to application/x-www-form-urlencoded. Add an `is_multipart_payload` flag to HttpRequest so callers can mark such a request as multipart; when set, the urlencoded content-type is not applied and the transport sets the correct multipart boundary. Fixes Azure#39163

github-actions · 2026-07-03T10:42:51Z

Thank you for your contribution @manmohanmirkar123! We will review the pull request and get back to you soon.

manmohanmirkar123 · 2026-07-03T10:46:11Z

@microsoft-github-policy-service agree

Copilot

Pull request overview

This PR targets azure-core’s REST HttpRequest body handling to avoid incorrectly forcing Content-Type: application/x-www-form-urlencoded for multipart form payloads when the (optional) file part is omitted, which breaks TypeSpec-generated multipart requests (issue #39163).

Changes:

Threads a new internal is_multipart_payload intent flag into HttpRequest._set_body to avoid applying the urlencoded Content-Type.
Adds regression tests covering the new multipart-without-file scenario and ensuring the default urlencoded behavior remains unchanged.
Adds an unreleased changelog entry documenting the fix.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
sdk/core/azure-core/azure/core/rest/_rest_py3.py	Adds `is_multipart_payload` plumbing into request body setup to avoid urlencoded `Content-Type` for intended multipart payloads.
sdk/core/azure-core/tests/test_rest_http_request.py	Adds regression tests for multipart form data without files and for default urlencoding behavior.
sdk/core/azure-core/CHANGELOG.md	Documents the behavioral change under an unreleased version entry.

        if files:
            default_headers, self._files = set_multipart_body(files)
        if data:
-            default_headers, self._data = set_urlencoded_body(data, has_files=bool(files))
+            # Treat the payload as multipart when explicitly requested, so a multipart
+            # request whose only file part is optional (and omitted) is not encoded as
+            # application/x-www-form-urlencoded.
+            default_headers, self._data = set_urlencoded_body(
+                data, has_files=bool(files) or is_multipart_payload
+            )


+    assert "Content-Type" not in request.headers
+    assert request.content == {"test": "123"}


+
+### Bugs Fixed
+
+- Fixed `HttpRequest` setting a `Content-Type` of `application/x-www-form-urlencoded` on multipart requests that carry form data but no file (e.g. when the file part is optional and omitted). Such requests can now be flagged with the internal `is_multipart_payload` keyword so the transport sets the correct multipart `Content-Type`. #39163


Addresses review feedback: previously `is_multipart_payload` only suppressed the urlencoded Content-Type, but transports decide multipart vs urlencoded based on `request.files`, so a data-only payload was still sent urlencoded. When `is_multipart_payload` is set and no files are provided, encode the form fields as file-less multipart parts (name, (None, value)) in `self._files` and clear `self._data`, so both the requests and aiohttp transports build a real multipart/form-data body. Update the regression test to assert the multipart parts and the changelog to match the wire behavior.

manmohanmirkar123 · 2026-07-03T10:59:01Z

Thanks for the review — addressed the multipart feedback in e7632bb1: data-only multipart payloads are now encoded as file-less multipart parts (self._files) with self._data cleared, so the requests and aiohttp transports build a real multipart/form-data body. Test and changelog updated to match.

The CHANGELOG has a `1.42.0 (Unreleased)` section for the multipart fix, but `_version.py` was still `1.41.0` (already released), so the changelog verification failed. Bump the package version to match.

Copilot AI review requested due to automatic review settings July 3, 2026 10:40

manmohanmirkar123 requested review from iscai-msft, kashifkhan and xirzec as code owners July 3, 2026 10:41

github-actions Bot added Community Contribution Community members are working on the issue customer-reported Issues that are reported by GitHub users external to the Azure organization. labels Jul 3, 2026

Copilot started reviewing on behalf of manmohanmirkar123 July 3, 2026 10:42 View session

Copilot AI reviewed Jul 3, 2026

View reviewed changes

[azure-core] Bump version to 1.42.0 for unreleased changelog entry

4d36648

The CHANGELOG has a `1.42.0 (Unreleased)` section for the multipart fix, but `_version.py` was still `1.41.0` (already released), so the changelog verification failed. Bump the package version to match.

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[azure-core] Fix wrong content-type on multipart request with no file (#39163)#47831

[azure-core] Fix wrong content-type on multipart request with no file (#39163)#47831
manmohanmirkar123 wants to merge 3 commits into
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manmohanmirkar123:fix/39163-multipart-content-type

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		assert "Content-Type" not in request.headers
		assert request.content == {"test": "123"}


		### Bugs Fixed

		- Fixed `HttpRequest` setting a `Content-Type` of `application/x-www-form-urlencoded` on multipart requests that carry form data but no file (e.g. when the file part is optional and omitted). Such requests can now be flagged with the internal `is_multipart_payload` keyword so the transport sets the correct multipart `Content-Type`. #39163

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manmohanmirkar123 commented Jul 3, 2026

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manmohanmirkar123 commented Jul 3, 2026 •

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