Reachability check seems to consider isinstance(obj, Hashable) as always True

Running mypy --warn-unreachable (version 0.920) against the following snippet gives a false "unreachable statement" error.

from collections.abc import Hashable
from typing import List, Tuple, Union

def wrap_hash(obj: Union[Tuple[int], List[int]]):
    if isinstance(obj, Hashable):
        return hash(obj)
    return hash(id(obj))  # error: Statement is unreachable  [unreachable]

wrap_hash((0,)) # reaches 1st return
wrap_hash([0])  # reaches 2nd return

We can even exchange the Union annotation for a single unhashable type like List or Dict, and still get the same unreachable error. Looks like isinstance(obj, Hashable) is just considered True for any type.

Is this a mypy bug? Can we add some hints into the example code so that mypy would figure out correct reachability?

@JelleZijlstra

The problem is that __hash__ is defined on object as a real method.

reveal_type(object.__hash__)  # N: Revealed type is "def (self: builtins.object) -> builtins.int"

We had this problem and discussion several times before.
Maybe we can try to change it to None | def (self: builtins.object) -> builtins.int?
CC @JelleZijlstra

We already have __hash__: None on list in typeshed. Why would object have to change?

I don't know how we can represent this idea in types, but: some object subtypes are hashable (def (self: builtins.object) -> builtins.int) and some are not (None). But, object defines that all subclasses are hashable by defining __hash__ as def (self: builtins.object) -> builtins.int.

But, on the other hand, we know that object itself is hashable. Only its subtypes can be unhashable.

But mypy already correctly errors when a subclass of a class that fulfills a protocol doesn't fulfill it:

from typing import Protocol

class X(Protocol):
    def foo(self, a: int) -> str:
        pass
    
class A:
    def foo(self, a: int) -> str:
        return ""
        
class B(A):
    def foo(self) -> str:  # type: ignore[override]
        return ""
        
a: X = A()
b: X = B()  # E: Incompatible types in assignment (expression has type "B", variable has type "X")

Why is Hashable different?

Looks like we have a bug in mypy 🤔
For some reason mypy treats Hashable as plain object.

1 NameExpr(Hashable [typing.Hashable])
TypeInfo(
  Name(typing.Hashable)
  Bases(builtins.object)
  Mro(typing.Hashable, builtins.object)
  Names(
    __hash__)
  DeclaredMetaclass(abc.ABCMeta)
  MetaclassType(abc.ABCMeta))
2 def () -> builtins.object

I will look into it.

I hope that #11802 will fix it! 🙏

Recognize `Hashable` as a real protocol (#11802)

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Reachability check seems to consider isinstance(obj, Hashable) as always True #11799

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Reachability check seems to consider isinstance(obj, Hashable) as always True #11799

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sobolevn commented on Dec 20, 2021

JelleZijlstra commented on Dec 20, 2021

sobolevn commented on Dec 20, 2021

JelleZijlstra commented on Dec 20, 2021

sobolevn commented on Dec 20, 2021

sobolevn commented on Dec 20, 2021

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