Don't override __init__ method in MyPy plugin if it already exists

[patrick91]

Change Summary

This PR updates the MyPy plugin to not override the __init__ method if there's already one in the model.

Here's an example of the current issue:

https://replit.com/@patrick91/AngryThoseMath#main.py

running mypy main.py returns the following:

main.py:13: error: Too many positional arguments for "Person"
main.py:13: error: Missing named argument "name" for "Person"
main.py:13: error: Missing named argument "birth_year" for "Person"
main.py:15: note: Revealed type is "def (*, id: Any, name: Any, birth_year: Any, **kwargs: Any) -> main.Person"
main.py:16: note: Revealed type is "def (__pydantic_self__: main.Person, *, id: Any, name: Any, birth_year: Any, **kwargs: Any)"

while the code works since we have a custom constructor:

from pydantic import BaseModel


class Person(BaseModel):
    id: int
    name: str
    birth_year: int

    def __init__(self, id: int) -> None:
        super().__init__(id=id, name="Patrick", birth_year=1991)


print(Person(1))

reveal_type(Person)
reveal_type(Person.__init__)

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes where applicable
• changes/<pull request or issue id>-<github username>.md file added describing change
  (see changes/README.md for details)
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

[patrick91]

coverage fails because we don't hit these lines anymore: https://smokeshow.helpmanual.io/5k4z1l3e0n1q425g2r40/d_2705a9c05af28909_mypy_py.html#t644

I'm not sure if they are needed anymore

[patrick91]

please review

[samuelcolvin]

this is great, thank you so much.

[samuelcolvin]

Looks like the coverage has dropped, hence this not merging, see here.

Looks like the code which is no-longer covered can be removed, but if you're not sure, maybe safer to add no cover.

Please update.

[patrick91]

done!

please review