Assigning an iterator to the type hint `collections.abc.Hashable` gets error even though an iterator is hashable

[hyperkai]

*Memo:

• Python 3.14.0
• mypy 1.18.2
• mypy test.py
• Windows 11

An iterator is hashable as shown below:

print(hash(iter([0, 1, 2])))
# 127553071047

But assigning an iterator to the type hint collections.abc.Hashable gets the error as shown below even though an iterator is hashable:

from collections.abc import Hashable

v: Hashable = iter([0, 1, 2])
# Error

error: Incompatible types in assignment (expression has type "Iterator[int]", variable has type "Hashable")

[TeamSpen210]

The specific implementation iterator you chose there does happen to be hashable, but nothing about iterators requires that. The standard library explicitly doesn't document what the implementation of classes' iterators are specifically. In this case the hash also is not actually very useful. It's just the object implementation, hashing by identity, no relation to the underlying list.

Type checkers in general can't really check hashability properly, since the root class (object) is hashable, but as you go down the hierarchy different classes can add and remove the abillity. This breaks the typing rules.

[bzoracler]

This isn't specifically related to hashing:

from typing import Protocol

class SupportsHash(Protocol):
    def __hash__(self) -> int: ...

class SupportsStr(Protocol):
    def __str__(self) -> str: ...

v1: SupportsHash = iter((1,))  # E: Incompatible types in assignment (expression has type "Iterator[int]", variable has type "SupportsHash")
v2: SupportsStr = iter((1,))  # E: Incompatible types in assignment (expression has type "Iterator[int]", variable has type "SupportsStr")

mypy is probably rejecting this because there are items on the Hashable protocol (i.e. the __hash__ method) but not specifically on Iterator or Iterator's parents, even if the item would normally be inherited from object. I think mypy reporting an error here is fine from the POV of a design decision, but since items produced from iter() on standard library types are (probably all?) hashable, it might be that the annotations on typeshed need to be revised.