Inspection errors on valid union syntax for Python < 3.10

[ofek]

Description

The following code:

from __future__ import annotations
from msgspec import Struct, inspect

class Bug(Struct):
    field: str | int = 0

inspect.type_info(Bug)

produces the following:

Traceback (most recent call last):
  File "C:\Users\ofek\Desktop\rf\m.py", line 7, in <module>
    inspect.type_info(Bug)
  File "C:\Users\ofek\AppData\Local\Programs\Python\Python39\lib\site-packages\msgspec\inspect.py", line 629, in type_info
    return multi_type_info([type])[0]
  File "C:\Users\ofek\AppData\Local\Programs\Python\Python39\lib\site-packages\msgspec\inspect.py", line 598, in multi_type_info
    return _Translator(types).run()
  File "C:\Users\ofek\AppData\Local\Programs\Python\Python39\lib\site-packages\msgspec\inspect.py", line 744, in run
    MsgpackDecoder(Tuple[self.types])
  File "C:\Users\ofek\AppData\Local\Programs\Python\Python39\lib\site-packages\msgspec\_utils.py", line 130, in get_class_annotations
    value = typing._eval_type(value, cls_locals, cls_globals)
  File "C:\Users\ofek\AppData\Local\Programs\Python\Python39\lib\typing.py", line 292, in _eval_type
    return t._evaluate(globalns, localns, recursive_guard)
  File "C:\Users\ofek\AppData\Local\Programs\Python\Python39\lib\typing.py", line 554, in _evaluate
    eval(self.__forward_code__, globalns, localns),
  File "<string>", line 1, in <module>
TypeError: unsupported operand type(s) for |: 'type' and 'type'

The following code is a workaround but limits what users are allowed to do when providing a library that extends msgspec:

from __future__ import annotations
from typing import Union
from msgspec import Struct, inspect

class Bug(Struct):
    field: Union[str, int] = 0

inspect.type_info(Bug)

[chuckwondo]

Using a pipe for type unions was introduced in Python 3.10, so anybody running an earlier version of Python shouldn't be surprised by this. If you want to "extend" msgspec and use the pipe syntax, then your extension must require python >= 3.10. If you want to support use of earlier versions of Python, then you must use the Union syntax, not the pipe syntax.

[ofek]

Doesn't from __future__ import annotations allow for that syntax though?

[ofek]

It looks like the reason the syntax error goes away is only due to the lazy computation. Thanks Chuck!

[chuckwondo]

Doesn't from __future__ import annotations allow for that syntax though?

No, that only allows for postponed evaluation of annotations.

[ofek]

Yes thanks, I added a note for users earlier today! https://ofek.dev/msgspec-click/usage/#caveats

[ashb]

Pydantic handles this via https://github.com/alexmojaki/eval_type_backport -- is there any appetite for doing the same in msgspec? Happy to PR it if that it'd be supported.

@jcrist What would your appetite for it be?

[jcrist]

Thanks for the comment - I'd be open to doing something like that. I've opened #771 to track this, lets continue discussion there :).