Add mypy static checking for default and default_factory

[klaa97]

Change Summary

This PR enhances the mypy Pydantic plugin to dynamically change the return type of the Field function, based on the default and default_factory arguments.

Moreover, it adds an error in case the default and default_factory arguments are both specified.

The main goal is to enhance static checks, since they allow for a way more robust code and avoid runtime errors.

Suggestions on the mypy plugin APIs usage are welcome (documentation on that is not very big 😄)

Related issue number

Fixes #3429

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

[klaa97]

please review

[Bobronium]

Thank you, that's a great feature! I have some small concerns, but otherwise LGTM 🚀!

[samuelcolvin]

LGTM, just a few changes.

[samuelcolvin]

please update.

[klaa97]

please review

[samuelcolvin]

LGTM

[samuelcolvin]

Thanks so much for this.

[klaa97]

Thank you for pydantic!

This PR is quite old and the mypy tests apparently changed in master, I will look into this later tonight and see if they are fixable 🙏

[samuelcolvin]

Thanks @klaa97, if we need to remove some older mypy that's fine, but we'll need to be explicit about which versions we support and which we drop.

[samuelcolvin]

please update (just adding this so I keep track of which PRs I need to review).

[klaa97]

please review

I should have fixed the tests, now it works also for mypy 0.910 (which had a few different way of handling things 😞 ). It also works with mypy 0.971, which is not currently tested in the CI.

To check what is going to change validation-side in the plug-in, I think the best way is to check the plugin-fail.txt changes. Thank you!

[klaa97]

Fixing coverage... Done!

[samuelcolvin]

It also works with mypy 0.971, which is not currently tested in the CI.

It is tested, it's run in the main test job of CI.

[samuelcolvin]

thanks so much.