python / python/mypy

(🐞) call-overload error when calling overloaded constructors which involve exhaustive Literal cases (e.g. for bool type argument)

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Description

Bug Report

Mypy raises call-overload when attempting to call an overloaded superclass constructor, if the overload hinges on literal True and False values and the call passes a bool value.

This is related to issue https://github.com/python/mypy/issues/8330

To Reproduce

class A:

    x: bool

    @overload
    def __init__(self, x: Literal[True]) -> None:
        ...
    @overload
    def __init__(self, x: Literal[False] = False) -> None:
        ...

    def __init__(self, x: bool = False) -> None:
        super().__init__()
        self.x = x

class B(A):

    @overload
    def __init__(self, x: Literal[True]) -> None:
        ...
    @overload
    def __init__(self, x: Literal[False] = False) -> None:
        ...

    def __init__(self, x: bool = False) -> None:
        super().__init__(x) # Mypy error: call-overload

# No overload variant of "__init__" of "A" matches argument type "bool"
# Possible overload variants:
#     def __init__(self, x: Literal[True]) -> None
#     def __init__(self, x: Literal[False] = ...) -> NoneMypy

class C(A):

    @overload
    def __init__(self, x: Literal[True]) -> None:
        ...
    @overload
    def __init__(self, x: Literal[False] = False) -> None:
        ...

    def __init__(self, x: bool = False) -> None:
        if x:
            super().__init__(x)  # OK
        else:
            super().__init__(x)  # OK

Please note: this is intended as a minimal example, not as an interesting example. I am happyt to produce interesting examples if they provide additional motivation to tackle this issue.

Expected Behavior

Because the bool type is fully covered by the Literal[True] and Literal[False] cases, it should be possible for Mypy to infer that the call super().__init__(x) is correct in the B constructor.

Actual Behavior

No overload variant of "__init__" of "A" matches argument type "bool"
Possible overload variants:
    def __init__(self, x: Literal[True]) -> None
    def __init__(self, x: Literal[False] = ...) -> NoneMypy

Your Environment

  • Mypy version used: mypy 1.7.1 (compiled: yes)
  • Mypy command-line flags: --strict
  • Python version used: 3.12.0 64-bit

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start by reproducing the minimal example with mypy 1.7.1 and --strict, focusing on the super().init(x) call in class B and comparing it with class C. Trace the overload checking involved in exhaustive Literal[True] and Literal[False] cases; done means the B constructor no longer produces call-overload and the regression is covered by a test.

Written by the indexing model from the issue text.

Assessment

Tech stack
python
Domain
devtools
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
35/100

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