8000 Adds `assert_never` helper by sobolevn · Pull Request #3294 · pydantic/pydantic · GitHub
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Adds assert_never helper #3294

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Merged
merged 3 commits into from
Aug 9, 2022
Merged

Adds assert_never helper #3294

merged 3 commits into from
Aug 9, 2022

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sobolevn
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@sobolevn sobolevn commented Oct 5, 2021 •
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Change Summary

I am working on mypy type narrowing bug. And our CI identified that this repo is also affected.
python/mypy#11273 (comment)

What bug am I talking about?
We were narrowing if conditions incorrectly. In your case:

    @staticmethod
    def _coerce_items(items: Union['AbstractSetIntStr', 'MappingIntStrAny']) -> 'MappingIntStrAny':
        if isinstance(items, Mapping):
            pass
        elif isinstance(items, AbstractSet):
            items = dict.fromkeys(items, ...)
        else:
            raise TypeError(f'Unexpected type of exclude value {items.__class__}')
        return items

In else case items were not narrowed to be <nothing>, because Mapping and Set cases are already covered. It was still something like MappingIntStrAny. This is why it was possible to use items.__class__.

But, after new mypy version will be released (and if this fix is merged), this line would raise an error:

pydantic/utils.py:554: error: <nothing> has no attribute "__class__"  [attr-defined]

Because NoReturn does not have __class__ attribute defined.

So, here's my solution on how to fix it: intoduce assert_never helper, docs: https://mypy.readthedocs.io/en/latest/literal_types.html?highlight=assert_never#exhaustive-checks

And type-ignore msg argument.

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

I think, that this is a pretty trivial change to go into changelog / news. If not, feel free to ping me to add it 🙂

PrettyWood
PrettyWood reviewed Oct 26, 2021
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Please update

@github-actions github-actions bot added the awaiting author revision awaiting changes from the PR author label Oct 26, 2021
@samuelcolvin samuelcolvin merged commit 10508f7 into pydantic:master Aug 9, 2022
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Thanks so much @sobolevn.

@sobolevn
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sobolevn commented Aug 9, 2022 •
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Awesome! Thank you for pydantic! 🎉

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