Add support for removing TypeVarDef in mypy 0.920

[christianbundy]

Change Summary

Problem: While using the Pydantic plugin with Mypy 0.920 there is an error about TypeVarDef not being defined.

Solution: Add support for Mypy 0.920 without breaking compat with Mypy 0.910.

See-also: python/mypy#9951

Related issue number

Fixes #3528

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

[PrettyWood]

Great thanks! 👍

[christianbundy]

Looks like maybe this is failing tests? :/

I think the behavior of TypeVarType changed between 0.910 and master, so I'm unsure how we could support both.

[PrettyWood]

Oh didn't see it was not in official release. I went too fast.
Well then I guess it's like toml vs tomli, it will need the next mypy release and we'll make this update.
You can probably close this PR and open a new one later with update mypy to ... and all the necessary changes

[christianbundy]

Just pushed a commit that's passing mypy@master, but I'm still getting three errors on mypy@0.910:

pydantic/mypy.py:354: error: Too many arguments for "TypeVarType"  [call-arg]
pydantic/mypy.py:354: error: Argument 1 to "TypeVarType" has incompatible type "str"; expected "TypeVarDef"  [arg-type]
pydantic/mypy.py:654: error: List item 0 has incompatible type "TypeVarType"; expected "TypeVarLikeDef"  [list-item]

When the new version of Mypy comes out will we need to make a major version bump for this breaking change? I'm worried that this new usage of TypeVarType works on new versions of Mypy, but old versions of Mypy seem to have different behavior.

[hauntsaninja]

I believe all plugins have access to mypy version, which you could use to gate logic on.

You could also e.g. do the equivalent of:

def _get_tv_tvdef(*args):
    try:
        from mypy.types import TypeVarDef, TypeVarType
        tvdef = TypeVarDef(*args)
        tv = TypeVarType(tvdef)
        return tv, tvdef
    except ImportError:
        from mypy.types import TypeVarType
        tv = TypeVarType(*args)
        return tv, tv

self_type, tvar_def = _get_tv_tvdef(...)

Also, in case it's useful, this is the issue where mypy announces breaking changes to plugins: python/mypy#6617 (comment)

[hsheth2]

Now that mypy 0.920 has been released, I'm seeing this issue with pydantic 1.8.2:

setup.cfg:24: error: Error importing plugin "pydantic.mypy": cannot import name TypeVarDef

[christianbundy]

Just pushed another commit so that this works correctly in both 0.910 and 0.920 -- please let me know if there's anything else.

[hughhan1]

Hey @samuelcolvin, do you have an estimate on when the next release of pydantic would be that would include this change? Thanks!

[matteosantama]

@hughhan1 pre-release candidate v1.9.0a1, which incorporates this change, was released two days ago and it looks like there are some bug reports rolling in. So things are progressing, but I suspect all these new bugs need to be smoothed out before v1.9.0 can be officially released!