python3

define a function _is_hashable by the following information

We define any object as hashable if it (a1) has a __hash__ attribute, and (a2) the attributes value is not None; also (b) if the object is iterable (has an __iter__ attribute) then every value iterated over is also hashable (by this same definition). Also note that str is hashable as a special case: if we apply the above definition it will create infinite recursion because every value that we iterate over in a str is a str that we can iterate over! Raise a ValueError exception if any value is not hashable.

I try to use '__hash__' not in type(object).__dict__ but cannot work