Use positional-only self in BaseModel constructor, so no field name can ever conflict with it.

[ariebovenberg]

Change Summary

As discussed in #7948, there is an edge case in the BaseModel constructor where using __pydantic_self__ would give a TypeError instead of an expected ValidationError. This edge case can be solved by using positional-only arguments — as soon as Python 3.7 support is dropped (see #7188)

Related issue number

Fixes #7948

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

Notes

• I didn't make the changes for the v1 module. I assume you'd rather not make any changes to it, right?
• CI for Python 3.7 will fail until support is officially dropped. Then, I'll rebase the changes and it'll be ready for review.

Selected Reviewer: @lig

[sydney-runkle]

@ariebovenberg,

I'm hoping to get the PR dropping Python 3.7 support across the line tomorrow, then we can move forward with this :). Thanks!

[codspeed-hq]

CodSpeed Performance Report

Merging #8072 will not alter performance

_{Comparing ariebovenberg:basemodel-self (4b3110a) with main (eb90b59)}

Summary

✅ 10 untouched benchmarks

[ariebovenberg]

@PrettyWood: it should be ready for review now, although I don't know what the one "release notes" check is about 🤔

There was an interesting wrinkle with the mypy plugin, which interestingly doesn't support positional-only arguments internally. I've added a comment explaining the workaround.

[ariebovenberg]

Forgot the exact phrase, so here it is: please review

[sydney-runkle]

@ariebovenberg,

Looks fantastic. Thanks for adding the simple test and the additional mypy docs.

Thanks @PrettyWood for the review as well :)

[sydney-runkle]

Thanks for adding this explanation 👍