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classes with descriptor properties in Unions #5570

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srittau opened this issue Sep 4, 2018 · 2 comments
Open

classes with descriptor properties in Unions #5570

srittau opened this issue Sep 4, 2018 · 2 comments
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bug mypy got something wrong false-positive mypy gave an error on correct code priority-1-normal topic-descriptors Properties, class vs. instance attributes topic-union-types

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@srittau
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srittau commented Sep 4, 2018

Please consider:

from typing import Any, Union

class getter:
    def __get__(self, instance: "X1", owner: Any) -> str:
        return ""

class X1:
    prop = getter()

class X2:
    def __init__(self) -> None:
        self.prop = ""

def foo(x: Union[X1, X2]) -> None:
    x.prop

mypy 0.620 (Python 3.6.6, default options) will warn:

foo.py:15: error: Argument 1 to "__get__" of "getter" has incompatible type "Union[X1, X2]"; expected "X1"

Changing the annotation of instance to Any will work around this problem.
@ilevkivskyi ilevkivskyi added bug mypy got something wrong priority-1-normal topic-union-types false-positive mypy gave an error on correct code labels Sep 4, 2018
@ilevkivskyi
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ilevkivskyi commented Oct 14, 2019

Note there is even more subtle manifestation of this bug if overloads are involved:

class getter:
    @overload
    def __get__(self, instance: None, owner: Any) -> getter: ...
    @overload
    def __get__(self, instance: object, owner: Any) -> str: ...

class X1:
    prop = getter()
class X2:
    prop = getter()

def foo(x: Type[Union[X1, X2]]) -> None:
    reveal_type(x.prop)

This shows str instead of getter because a wrong type is passed for instance (same cause as in the original example). The fix may involve some refactoring for how attributes on unions are accessed.

@ilevkivskyi ilevkivskyi mentioned this issue Oct 14, 2019
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@clokep clokep mentioned this issue Oct 19, 2021
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@daisylb
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daisylb commented Feb 3, 2022

FWIW, I'm running into this exact same issue on 0.931, because django-stubs uses the instance: None trick on Fields to model descriptors that return themselves when not accessed on an instance.

Another short reproducible example, based on the django-stubs code: 
from typing import Type, Union, Generic, TypeVar, overload

T = TypeVar("T")


class Model: ...

_T = TypeVar("_T", bound="Field")
_GT = TypeVar("_GT", covariant=True)

class Field(Generic[_GT]):
    @overload
    def __get__(self: _T, instance: None, owner) -> _T: ...
    # Model instance access
    @overload
    def __get__(self, instance: Model, owner) -> _GT: ...
    # non-Model instances
    @overload
    def __get__(self: _T, instance, owner) -> _T: ...


class A:
    attr: str


class B(Model):
    attr: Field[str] = Field()

b_inst: B
a_or_b_inst: Union[A, B]

reveal_type(B.attr) # test.Field[builtins.str]
reveal_type(b_inst.attr) # builtins.str*
reveal_type(a_or_b_inst.attr) # Union[builtins.str, test.Field[builtins.str]]
                              # (should just be builtins.str)

@JelleZijlstra JelleZijlstra added the topic-descriptors Properties, class vs. instance attributes label Mar 19, 2022
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bug mypy got something wrong false-positive mypy gave an error on correct code priority-1-normal topic-descriptors Properties, class vs. instance attributes topic-union-types
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