Warn if a class inherits from Generic before BaseModel

[alexmojaki]

Change Summary

Emit a warning when writing:

class Model(Generic[T], BaseModel):

instead of:

class Model(BaseModel, Generic[T]):

More generally, BaseModel must appear in the __mro__ before Generic, so indirect subclasses are also checked. This means that __class_getitem__ will use BaseModel instead of Generic and Model[T] will return a model class rather than a _GenericAlias.

Related issue number

Closes #7845

Waiting for a reply to #6994 (comment) before continuing here, but thought I should push what I have so far. The failing tests might be of interest.

Checklist

• The pull request title is a good summary of the changes - it will be used in the changelog
• Unit tests for the changes exist
• Tests pass on CI
• Documentation reflects the changes where applicable
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

Selected Reviewer: @dmontagu

[sydney-runkle]

@alexmojaki,

Thanks for the contribution. I'll discuss this issue with the team tomorrow morning + follow up with you afterwards 😄.

[sydney-runkle]

@alexmojaki,

Feel free to move ahead with this. We would like to warn if a class inherits from Generic before BaseModel.

[alexmojaki]

please review

[sydney-runkle]

Nice! Thanks 🌟