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Cannot set min_length for optional string filed #804
Description
Description
Error while creating an object with optional filed with min_length meta.
from typing import Annotated
import msgspec
class Author(msgspec.Struct):
name: Annotated[str, msgspec.Meta(min_length=1)]
biography: Annotated[None | str, msgspec.Meta(min_length=10)]
element = {"name": "Me", "biography": None}
author = msgspec.convert(element, type=Author)Traceback (most recent call last):
File "", line 198, in _run_module_as_main
File "", line 88, in _run_code
File "/path/report.py", line 13, in
author = msgspec.convert(element, type=Author)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: Can only set min_length on a str, bytes, or collection type - type typing.Annotated[None | str, msgspec.Meta(min_length=10)] is invalid
msgspec version 0.19.0
Activity
oek1ng commentedon Jan 24, 2025
Nonedoes not have a length, is that an issue? Try to useAnnonated[str, msgspec.Meta(min_length=10)] | NoneSiyet commentedon Apr 8, 2026
Resolved in the community fork msgspec-arise: issue #4, released in v0.20.2.
jcrist commentedon Apr 8, 2026
Not supporting this was intentional.
We only support applying annotations directly to the type they apply to for clarity.
Since
Optional[int]is the same asint | None, it's a slippery slope to applying annotations to a union and expecting them to apply to all union members.Annotated[int | str | None, Meta(ge=1)]for example. I'd rather just keep things simple and clear - annotations must be applied to the type directly, and not the union.Support constraints on Optional (union with None) types