fix: ensure BodyPartReader.read() returns bytes, not bytearray by CrepuscularIRIS · Pull Request #12413 · aio-libs/aiohttp

CrepuscularIRIS · 2026-04-22T14:10:02Z

What

BodyPartReader.read() (with and without decode=True) returns a bytearray instead of bytes, violating the documented return type.

Why

The internal accumulation buffer in read() is a bytearray. Both return paths handed it back directly without conversion:

data = bytearray()
# ... fill data ...
return data          # bytearray, not bytes
# and with decode=True:
decoded_data = bytearray()
# ...
return decoded_data  # bytearray, not bytes

The documented return type is bytes.

Fixes #12404

How

Convert the accumulated buffer to bytes before returning:

return bytes(data)
return bytes(decoded_data)

This is a zero-copy-free, minimal change that makes the return type match the documentation and type hints without altering any other behaviour.

No alternatives were necessary — the fix is unambiguous.

Scope

Included: Fix the two return statements in BodyPartReader.read(); add two regression tests asserting isinstance(result, bytes) for both code paths
NOT included: Changes to filename property, decode_iter(), or any other method. Those can be addressed separately if needed.

Verification

pytest tests/test_multipart.py::TestPartReader::test_read_returns_bytes_not_bytearray — passes
pytest tests/test_multipart.py::TestPartReader::test_read_decode_returns_bytes_not_bytearray — passes
pytest tests/test_multipart.py::TestPartReader — 56/56 pass (2 new tests included)
One pre-existing failure (test_body_part_reader_payload_write) confirmed unrelated — reproduces on master without any changes

Checklist

I think the code is well written
Unit tests for the changes exist
Documentation reflects the changes
If you provide code modification, please add yourself to CONTRIBUTORS.txt
Add a new news fragment into the CHANGES folder (CHANGES/12404.bugfix.rst)

AI Disclosure

AI tools were used to assist with research and drafting. All code changes were reviewed, verified manually, and tested before submission.

BodyPartReader.read() accumulated data in a bytearray buffer and returned it directly, violating the documented return type of bytes. Both the plain and decode=True code paths were affected. Convert the internal bytearray to bytes before returning so the API contract matches the documentation and type hints. Fixes aio-libs#12404

for more information, see https://pre-commit.ci

codecov · 2026-04-22T14:17:03Z

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.92%. Comparing base (b0601d6) to head (e650dc4).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #12413   +/-   ##
=======================================
  Coverage   98.92%   98.92%           
=======================================
  Files         134      134           
  Lines       46741    46758   +17     
  Branches     2429     2430    +1     
=======================================
+ Hits        46239    46256   +17     
  Misses        373      373           
  Partials      129      129

Flag	Coverage Δ
CI-GHA	`98.98% <100.00%> (+<0.01%)`	⬆️
OS-Linux	`98.72% <100.00%> (-0.01%)`	⬇️
OS-Windows	`96.98% <100.00%> (+<0.01%)`	⬆️
OS-macOS	`97.89% <100.00%> (-0.01%)`	⬇️
Py-3.10.11	`97.39% <100.00%> (-0.01%)`	⬇️
Py-3.10.20	`97.86% <100.00%> (+<0.01%)`	⬆️
Py-3.11.15	`98.12% <100.00%> (+<0.01%)`	⬆️
Py-3.11.9	`97.65% <100.00%> (-0.01%)`	⬇️
Py-3.12.10	`97.73% <100.00%> (+<0.01%)`	⬆️
Py-3.12.13	`98.20% <100.00%> (-0.01%)`	⬇️
Py-3.13.13	`98.45% <100.00%> (-0.01%)`	⬇️
Py-3.14.4	`98.50% <100.00%> (-0.01%)`	⬇️
Py-3.14.4t	`97.52% <100.00%> (+<0.01%)`	⬆️
Py-pypy3.11.15-7.3.21	`97.35% <100.00%> (+<0.01%)`	⬆️
VM-macos	`97.89% <100.00%> (-0.01%)`	⬇️
VM-ubuntu	`98.72% <100.00%> (-0.01%)`	⬇️
VM-windows	`96.98% <100.00%> (+<0.01%)`	⬆️
cython-coverage	`38.07% <22.22%> (-0.02%)`	⬇️

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codspeed-hq · 2026-04-22T14:23:42Z

Merging this PR will not alter performance

✅ 67 untouched benchmarks
⏩ 4 skipped benchmarks¹

_{Comparing CrepuscularIRIS:fix/body-part-reader-returns-bytes (e650dc4) with master (365f717)}

4 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

webknjaz · 2026-04-22T21:42:01Z

+            assert isinstance(
+                result, bytes


No need to have the assertion in the CM.

Done in e650dc4 — the with pytest.raises(...) block no longer contains an assert.

webknjaz · 2026-04-22T21:43:09Z

+            result = await obj.read()
+            assert isinstance(
+                result, bytes
+            ), f"Expected bytes, got {type(result).__name__}"


No need to have a custom assertion text, the test runner already reveals the local vars better.

Done in e650dc4 — the custom match string is removed; pytest's default output is enough.

webknjaz · 2026-04-22T21:44:26Z

+                result, bytes
+            ), f"Expected bytes, got {type(result).__name__}"
+
+    async def test_read_decode_returns_bytes_not_bytearray(self) -> None:


Use parametrization instead of duplicating the same check with only small difference but exactly the same semantics.

Done in e650dc4 — the two copies are now a single @pytest.mark.parametrize("kwargs", [{}, {"decode": True}]) test.

webknjaz · 2026-04-22T21:45:02Z

@@ -0,0 +1,3 @@
+Fixed ``BodyPartReader.read()`` and ``BodyPartReader.read(decode=True)`` returning
+``bytearray`` instead of ``bytes``, violating the documented API contract.


Use RST roles to link the types.

Done in e650dc4 — both types are now linked with :class:\bytearray`and:class:`bytes``.

webknjaz · 2026-04-22T21:47:26Z

The documented return type is bytes.

Opening this doc shows the opposite. Why did you claim having checked what your robot generated?

CrepuscularIRIS · 2026-04-22T22:51:33Z

You're absolutely right, and I apologize for the misleading description.

When writing the PR I was looking at the Python source-code annotation (async def read(self, *, decode: bool = False) -> bytes:) rather than the rendered Sphinx documentation at the linked URL — which has always said :rtype: bytearray. That was a careless mistake, and the AI-generated draft made the same error.

The documentation was therefore consistent with the implementation (both said bytearray); the only inconsistency was the type annotation in the source. The PR body should have said "the type annotation says bytes but the implementation and documentation both return bytearray".

I've now updated docs/multipart_reference.rst to change :rtype: bytearray → :rtype: bytes for read() (commit c3f30a1), completing the fix so that the annotation, documentation, and implementation all agree on bytes.

Collapse the two duplicate test methods into one parametrized test, move the isinstance assertion outside the Stream context manager, drop the custom failure message, and use :class: RST roles in the changelog entry — all per code-review feedback.

CrepuscularIRIS · 2026-04-23T06:13:41Z

Pushed e650dc4 — combined the two tests into one parametrised case with the assert moved outside the CM and the custom message dropped, and switched to :class: RST roles in the changelog. Happy to adjust if anything still looks off.

CrepuscularIRIS requested review from asvetlov and webknjaz as code owners April 22, 2026 14:10

psf-chronographer Bot added the bot:chronographer:provided There is a change note present in this PR label Apr 22, 2026

[pre-commit.ci] auto fixes from pre-commit.com hooks

e31bc14

for more information, see https://pre-commit.ci

webknjaz requested changes Apr 22, 2026

View reviewed changes

docs: update BodyPartReader.read() rtype from bytearray to bytes

c3f30a1

		@@ -0,0 +1,3 @@
		Fixed ``BodyPartReader.read()`` and ``BodyPartReader.read(decode=True)`` returning
		``bytearray`` instead of ``bytes``, violating the documented API contract.

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CrepuscularIRIS commented Apr 22, 2026

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Merging this PR will not alter performance

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webknjaz commented Apr 22, 2026

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CrepuscularIRIS commented Apr 22, 2026

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CrepuscularIRIS commented Apr 23, 2026

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codecov Bot commented Apr 22, 2026 •

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