What makes a user-defined class unhashable?

Question

The docs say that a class is hashable as long as it defines __hash__ method and __eq__ method. However:

class X(list):   # read-only interface of `tuple` and `list` should be the same, so reuse tuple.__hash__   __hash__ = tuple.__hash__  x1 = X() s = {x1} # TypeError: unhashable type: 'X' 

What makes X unhashable?

Note that I must have identical lists (in terms of regular equality) to be hashed to the same value; otherwise, I will violate this requirement on hash functions:

The only required property is that objects which compare equal have the same hash value

The docs do warn that a hashable object shouldn't be modified during its lifetime, and of course I don't modify instances of X after creation. Of course, the interpreter won't check that anyway.

Answer 1

Simply setting the __hash__ method to that of the tuple class is not enough. You haven't actually told it how to hash any differently. tuples are hashable because they are immutable. If you really wanted to make you specific example work, it might be like this:

class X2(list):     def __hash__(self):         return hash(tuple(self)) 

In this case you are actually defining how to hash your custom list subclass. You just have to define exactly how it can generate a hash. You can hash on whatever you want, as opposed to using the tuple's hashing method:

def __hash__(self):     return hash("foobar"*len(self))