List unhashable, but tuple hashable?

Mainly, because tuples are immutable. Assume the following works:

>>> l = [1, 2, 3]
>>> t = (1, 2, 3)
>>> x = {l: 'a list', t: 'a tuple'}

Now, what happens when you do l.append(4)? You’ve modified the key in your dictionary! From afar! If you’re familiar with how hashing algorithms work, this should frighten you. Tuples, on the other hand, are absolutely immutable. t += (1,) might look like it’s modifying the tuple, but really it’s not: it simply creating a new tuple, leaving your dictionary key unchanged.