Why doesn't Python hash lists using ID?

Question

When using a dictionary in Python, the following is impossible:

d = {}
d[[1,2,3]] = 4

since \'list\' is an unhashable type. Ho

Answer 1

It is a requirement that if a == b, then hash(a) == hash(b). Using the id can break this, because the ID will not change if you mutate the list. Then you might have two lists that have equal contents, but have different hashes.

Another way to look at it is, yes, you could do it, but it would mean that you could not retrieve the dict value with another list with the same contents. You could only retrieve it by using the exact same list object as the key.