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Types inheriting from GenericAlias types (such as Mapping) cannot be decoded. #957
Description
Description
If you try to make a generic class that inherits from one of the _SpecialGenericAlias types (such as Mapping), the type inference fails to work.
A minimal example below (Python 3.13, msgspec 0.20.0):
from collections.abc import Mapping
from dataclasses import dataclass
import msgspec
@dataclass
class Bar[T](Mapping[str, T]):
data: dict[str, T]
def __getitem__(self, x):
return self.data[x]
def __len__(self):
return len(self.data)
def __iter__(self):
return iter(self.data)
x: Bar[int] = Bar({"x": 3})
msgspec.msgpack.decode(msgspec.msgpack.encode(x), type=Bar[int])with traceback
Traceback (most recent call last):
File "<censored>", line 31, in <module>
msgspec.msgpack.decode(msgspec.msgpack.encode(x), type=Bar[int])
~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/opt/venv/lib/python3.13/site-packages/msgspec/_utils.py", line 252, in get_dataclass_info
hints = get_class_annotations(obj)
File "/opt/venv/lib/python3.13/site-packages/msgspec/_utils.py", line 149, in get_class_annotations
mro, typevar_mappings = _get_class_mro_and_typevar_mappings(obj)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^
File "/opt/venv/lib/python3.13/site-packages/msgspec/_utils.py", line 126, in _get_class_mro_and_typevar_mappings
inner(obj, {})
~~~~~^^^^^^^^^
File "/opt/venv/lib/python3.13/site-packages/msgspec/_utils.py", line 124, in inner
inner(b, new_scope)
~~~~~^^^^^^^^^^^^^^
File "/opt/venv/lib/python3.13/site-packages/msgspec/_utils.py", line 116, in inner
params = cls.__parameters__
^^^^^^^^^^^^^^^^^^
AttributeError: type object 'Mapping' has no attribute '__parameters__'
I believe this is caused by the logic in inner(...) in the utils. For this, we look up __origin__ - however, in these specific cases, the types library has typing.Mapping as a typing._SpecialGenericAlias, which has different behaviour - it's __origin__ is not typing.Mapping, it's collections.abc.Mapping. This has no generic type parameters so inference fails.
This looks extremely tough to wade through so not sure what a good workaround is here - not a typing expert!
I'll note briefly that this behaviour goes away if it's not generic in inheritance, but it still fails if I use msgspec.Struct and a combined metaclass from StructMeta and ABCMeta so is not an artifact of dataclasses or anything, I think.
Activity
provinzkraut commentedon Nov 29, 2025
I've been looking at this and I agree. This is a tough one! But I think I've got it figured out, I'll submit a PR soon(ish).
There are lots of things at play here. At first I thought it was an easy fix, because we just need to fix the inspection. But fixing that uncovered a whole bunch of other bugs that needed fixing in order to make your example work.
collections.abc.Mapping), it produces atypes.GenericAlias(vs. the "old style"typing._GenericAlias), which msgspec did not handle correctly during inspectiontyping._GenericAliashas a__dict__, so you can just assign attributes to it.types.GenericAliashowever has__slots__, so we can't cache our type information on thereprovinzkraut commentedon Nov 29, 2025
@mwaddoups I've put up a PR at #962. Can you confirm that (with msgspec installed from that branch) this fixes your original issue?
mwaddoups commentedon Nov 30, 2025
Happy to confirm that branch solves both the toy issue described in our situation, as well as our original slightly more complex use case.
I glanced at the PR in the forlorn hope that I could understand the details, it remained a hope, but thanks for looking into this one!
Siyet commentedon Apr 8, 2026
Ported to the community fork msgspec-arise: issue #24, PR #31
provinzkraut commentedon Jun 19, 2026
@mwaddoups Sorry this took so long, but we managed to get it fixed now!