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Fix for when obj is UnionType in _utils._apply_params #769

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#772
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Fix for when obj is UnionType in _utils._apply_params#769
#772

Description

@treykeown
treykeown
opened on Nov 12, 2024

Description

Hi. I'm using Python 3.13 and installing the package directly from this git repo.

I had an issue with _utils._apply_params - I don't have a minimal example, but here's the traceback:

...
  File ".../mycode.py", line 264, in decode
    return msgspec.msgpack.decode(item, **kwargs)
    ^^^^^^^^^^^^^^^
  File ".../.venv/lib/python3.13/site-packages/msgspec/_utils.py", line 111, in get_class_annotations
    mro, typevar_mappings = _get_class_mro_and_typevar_mappings(obj)
      ^^^^^^^^^^^^^^^^^
  File ".../.venv/lib/python3.13/site-packages/msgspec/_utils.py", line 88, in _get_class_mro_and_typevar_mappings
    inner(obj, {})
      ~~~^^^^^^^^^
  File ".../.venv/lib/python3.13/site-packages/msgspec/_utils.py", line 86, in inner
    inner(b, new_scope)
    ^^^^^^^
  File ".../.venv/lib/python3.13/site-packages/msgspec/_utils.py", line 79, in inner
    args = tuple(_apply_params(a, scope) for a in c.__args__)
    ^^^^^^^^^^^
  File ".../.venv/lib/python3.13/site-packages/msgspec/_utils.py", line 79, in <genexpr>
    args = tuple(_apply_params(a, scope) for a in c.__args__)
    ^^^^^^^^^^^
  File ".../.venv/lib/python3.13/site-packages/msgspec/_utils.py", line 55, in _apply_params
    args = tuple(mapping.get(p, p) for p in params)
    ^^^^^^^^^^^^^^^
TypeError: 'getset_descriptor' object is not iterable

I found that this was due to _utils._apply_params being called with obj == types.UnionType. UnionType returns a descriptor for __parameters__.

>>> type(int | str).__parameters__
<attribute '__parameters__' of 'types.UnionType' objects>

I've pasted my workaround below:

def _apply_params(obj, mapping):
    import types
    if (params := getattr(obj, "__parameters__", None)) and (obj != types.UnionType):
        args = tuple(mapping.get(p, p) for p in params)
        return obj[args]
    elif isinstance(obj, typing.TypeVar):
        return mapping.get(obj, obj)
    return obj

Activity

  1. jcrist commented on Nov 12, 2024

    @jcrist
    jcrist
    on Nov 12, 2024
    Member

    Thanks for opening this - can you please spend the time producing a minimal example? Without an example demonstrating the failure we have no way to check if the fix is correct.

  2. treykeown commented on Nov 15, 2024

    @treykeown
    treykeown
    on Nov 15, 2024
    Author
    from types import UnionType

import msgspec

if __name__ == "__main__":
    class Base[T](msgspec.Struct, frozen=True, kw_only=True, tag=True): ...

    class Buggy(Base[UnionType], frozen=True, kw_only=True): ...
    # This works fine:
    # from typing import Union
    # class Buggy(Base[Union], frozen=True, kw_only=True): ...

    class Container(msgspec.Struct, frozen=True):
        item: Buggy

    enc = msgspec.msgpack.encode(Container(item=Buggy()))
    msgspec.msgpack.decode(enc, type=Container)

  3. jcrist commented on Nov 15, 2024

    @jcrist
    jcrist
    on Nov 15, 2024
    Member

    Thanks!

  4. jcrist
    closed this as completedin #772on Nov 15, 2024
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