⬆️ Update mypy to 1.14.1 - #12970

LGTM

Maybe we should extract the part related to plugin installation into a separate PR, but doesn't seem critical to me

Ok, so in an attempt to get this merged, I've moved the pydantic.mypy plugin changes to a separate PR #14081. This makes it easier to review which changes are causing what, as several reviewers had questions about what exactly is needed and what relates to what.

For this PR, the goal is simply to update to mypy 1.14.1. When doing so, we're getting 2 similar errors:

fastapi\encoders.py:244: error: Argument 1 to "asdict" has incompatible type "DataclassInstance | type[DataclassInstance]"; expected "DataclassInstance"  [arg-type]

What happens here is that first, we're calling dataclasses.is_dataclass(obj).

Due to the typing of this method in their dataclasses.pyi, mypy will go and type obj as DataclassInstance | type[DataclassInstance]. This then doesn't align with the signature of the following dataclasses.asdict(obj) statement, which expects a DataclassInstance.

So, these are the possible fixes for this:

• add an ignore statement
• add a type() call before calling is_dataclass, as @tamird suggests. This isn't actually necessary, because the implementation of dataclasses.is_dataclass will call type() when necessary. However, by doing this, we trick mypy into assuming that obj is NOT a type[DataclassInstance] and then it won't error on the following asdict call
• inbetween the original is_dataclass check and the asdict call, add the following: assert not isinstance(obj, type). This again will tell mypy not to assume the obj is a type. To me, this is the most direct/clear solution, but I think it's a matter of taste.

Will run this by Tiangolo to get his opinion.

Nice, thanks! 🚀

And thanks @svlandeg for the analysis, discussion and work on this! 🙌