dict default key type should be Hashable, not Any

[randolf-scholz]

Python dictionaries raise a runtime-error when fed a non-hashable object. Therefore, the default key-type of a blank dict type hint should be Hashable not Any

• dict ⇝ dict[Hashable, Any]

[erictraut]

Unspecified type arguments default to Any for all generic types. This is well established in PEP 484.

While it is allowed to leave type arguments unspecified, it is not recommended. In other words, you generally shouldn't use a "bare" dict in a type annotation. If your intent is dict[Hashable, Any], then you should spell that out.

There is a draft PEP that proposes to add support for default type arguments, but even with this new facility, it wouldn't make sense to default the first type parameter of dict to Hashable because the type parameters for dict are invariant.

[jeremyn]

Just curious, but when would this matter? mypy doesn't report any problem on line 3 of the following code for me:

d: dict = {}
for k in d:
    hash(k)