Confusion with "immutable" and "hashable" in Design and History FAQ

[plusquant]

The page https://docs.python.org/3/faq/design.html#why-are-there-separate-tuple-and-list-data-types says at the end of its last paragraph:

Only immutable elements can be used as dictionary keys...

Which is not quite correct, since dictionaries only require its keys to be hashable (see https://docs.python.org/3/library/stdtypes.html#mapping-types-dict). Immutability doesn't necessarily imply hashability.

Thanks.

Linked PRs

• gh-134114: Clarify FAQ note about dictonary keys #134118

[skirpichev]

Immutability doesn't necessarily imply hashability.

Immutability is more strong requirement, it does.

[ryan-duve]

Is there an immutable element that can not be used as a dictionary key? Or a mutable element that can be used as a dictionary key?

[plusquant]

According to https://docs.python.org/3/glossary.html#term-hashable

Most of Python’s immutable built-in objects are hashable...

It means that there are some kind of immutable built-in objects that aren't hashable.

Additionally, you can make a class that doesn't implement __eq__ and/or __hash__ (by definition, it must have both in order to make the objects hashable) that does produce objects with immutable values. As a result, the objects will be immutable but still unusable as dict keys.

[skirpichev]

It means that there are some kind of immutable built-in objects that aren't hashable.
[...] Additionally, you can make a class that doesn't define __eq__ and/or __hash__

I would say, it's just an implementation detail. I.e. yes, you could break default hashability (per default, derived from the object class types are hashable) - by setting __hash__ = None. But there is no good reasons to do this for immutable objects.

Is there an immutable element that can not be used as a dictionary key?

Sure, you can create an artificial type:

class BadObj:
    __hash__ = None

[Locked-chess-official]

Is there an immutable element that can not be used as a dictionary key? Or a mutable element that can be used as a dictionary key?是否存在不可用作字典键的不可变元素？或者存在可用作字典键的可变元素？

Yes.

class CannotBeChangedClass:
    slots = ('value',)
    def __new__(cls, value):
        a=super().__new__(cls)
        object.__setattr__(a,'value',value)
        return a

    def __setattr__(self, item, value):
        raise AttributeError

    def __delattr__(self, item):
        raise AttributeError

    def __eq__(self, other):
        return self.value == other.value

This class is unhashable (have no hash) so it cannot be the key of dict.

class CanBeChangedClass:
    def __init__(self, value):
        self.value = value

This class is changedable, but if a class has no "eq", the hash is default to use the hash of id, so it is hashable.