Asking “is hashable” about a Python value

Python 3.x

Use collections.abc.Hashable or typing.Hashable.

>>> import typing
>>> isinstance({}, typing.Hashable)
False
>>> isinstance(0, typing.Hashable)
True

Note: both are the same one, the latter is simply an alias of the former. Also note that collections.Hashable was removed in Python 3.10+ (deprecated since 3.7).

Python 2.6+ (an original answer)

Since Python 2.6 you can use the abstract base class collections.Hashable:

>>> import collections
>>> isinstance({}, collections.Hashable)
False
>>> isinstance(0, collections.Hashable)
True

This approach is also mentioned briefly in the documentation for __hash__.

Doing so means that not only will instances of the class raise an appropriate TypeError when a program attempts to retrieve their hash value, but they will also be correctly identified as unhashable when checking isinstance(obj, collections.Hashable) (unlike classes which define their own __hash__() to explicitly raise TypeError).