8000 Support correct `__post_init__` signatures for dataclasses · Issue #15498 · python/mypy · GitHub
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sobolevn opened this issue Jun 23, 2023 · 0 comments · Fixed by #15503
Closed

Support correct __post_init__ signatures for dataclasses #15498

sobolevn opened this issue Jun 23, 2023 · 0 comments · Fixed by #15503
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feature topic-dataclasses

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Feature

Right now mypy does not support proper __post_init__ signatures.
For example:

import dataclasses

@dataclasses.dataclass
class My:
    x: int
    y: dataclasses.InitVar[str] = 'a'

For now, mypy will allow a lot of incorrect code to slip in:

  • def __post_init__(self, y: str) -> str: ..., which needs to return None
  • def __post_init__(self, y: int) -> None: ..., which has incorrect type for y
  • def __post_init__(self) -> None: ..., which has no y
  • def __post_init__(self, x: int, y: str) -> None: ..., which has extra x param

Pitch

We can easily support this feature and make sure that __post_init__ signature is always correct for a dataclass.
@sobolevn sobolevn self-assigned this Jun 23, 2023
This was referenced Jun 23, 2023
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sobolevn added a commit that referenced this issue Jun 26, 2023
@sobolevn @pre-commit-ci @ilevkivskyi
Support better __post_init__ method signature for dataclasses (#1…
9511daa 
…5503)

Now we use a similar approach to
#14849
First, we generate a private name to store in a metadata (with `-`, so -
no conflicts, ever).
Next, we check override to be compatible: we take the currect signature
and compare it to the ideal one we have.

Simple and it works :)

Closes #15498
Closes #9254

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Ivan Levkivskyi <levkivskyi@gmail.com>
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Support better __post_init__ method signature for dataclasses python/mypy
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