fix(serializers): set MsgPackSerializer DEFAULT_ENCODING to None

[gaoflow]

Problem

MsgPackSerializer inherits DEFAULT_ENCODING = "utf-8" from BaseSerializer. Backends that decode stored bytes before passing them to the serializer (Memcached, Valkey/Redis) therefore call value.decode("utf-8") on raw msgpack-packed bytes. Since msgpack produces arbitrary binary, this raises UnicodeDecodeError for virtually any non-trivial value.

Minimal reproduction:

import asyncio
from aiocache import MemcachedCache
from aiocache.serializers import MsgPackSerializer

async def test():
    cache = MemcachedCache(serializer=MsgPackSerializer())
    await cache.set("key", {"k": "v"})
    await cache.get("key")  # UnicodeDecodeError: 'utf-8' codec can't decode byte 0x82 …

asyncio.run(test())

Fixes #834.

Fix

PickleSerializer already handles this correctly by setting DEFAULT_ENCODING = None, which tells backends to skip the .decode() step and pass raw bytes straight to loads(). Apply the same fix to MsgPackSerializer.

The raw parameter passed to msgpack.loads() is updated so that both None and "utf-8" produce raw=False (Python strings), preserving the existing round-trip behaviour while the encoding hint to backends is corrected.

---

This pull request was prepared with the assistance of AI, under my direction and review.

[Dreamsorcerer]

Can you cherry-pick the tests from #836?

[gaoflow]

Done in affebc7.

I added the MsgPackSerializer acceptance tests from #836 for:

• set / get
• add / get
• multi_set / multi_get

Local verification:

uv run --with-requirements requirements-dev.txt --with-editable . python -m pytest tests/ut/test_serializers.py -q
# 61 passed

uv run --with-requirements requirements-dev.txt --with-editable . python -m pytest tests/acceptance/test_serializers.py::TestMsgPackSerializer -q -m 'not memcached'
# 36 passed, 18 deselected

uv run --with-requirements requirements-dev.txt --with-editable . python -m pytest tests/ut/test_serializers.py tests/acceptance/test_serializers.py::TestMsgPackSerializer -q -m 'not memcached'
# 97 passed, 18 deselected

uv run --with black --with-editable . python -m black --check aiocache/serializers/serializers.py tests/ut/test_serializers.py tests/acceptance/test_serializers.py
# passed

uv run --with-requirements requirements-dev.txt --with-editable . python -m flake8 aiocache/serializers/serializers.py tests/ut/test_serializers.py tests/acceptance/test_serializers.py
# passed

git diff --check
# passed

I also tried the full new acceptance selection locally, but the memcached_cache parametrization fails because no memcached service is running on 127.0.0.1:11211. The project contributing docs note that acceptance/functional tests need Docker/backend services, so I verified the runnable memory/valkey subset locally and left the memcached coverage for CI.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.85%. Comparing base (43f6a24) to head (affebc7).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1079   +/-   ##
=======================================
  Coverage   98.84%   98.85%           
=======================================
  Files          32       32           
  Lines        3560     3579   +19     
  Branches      125      125           
=======================================
+ Hits         3519     3538   +19     
  Misses         41       41           

Files with missing lines	Coverage Δ
aiocache/serializers/serializers.py	95.52% <100.00%> (+0.06%)	⬆️
tests/acceptance/test_serializers.py	100.00% <100.00%> (ø)
tests/ut/test_serializers.py	100.00% <100.00%> (ø)

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