List unhashable, but tuple hashable?

Question

In How to hash lists? I was told that I should convert to a tuple first, e.g. [1,2,3,4,5] to (1,2,3,4,5).

So the first cannot be hashed, bu

Answer 1

You could totally make that work, but I bet you wouldn't like the effects.

from functools import reduce
from operator import xor

class List(list):
    def __hash__(self):
        return reduce(xor, self)

Now let's see what happens:

>>> l = List([23,42,99])
>>> hash(l)
94
>>> d = {l: "Hello"}
>>> d[l]
'Hello'
>>> l.append(7)
>>> d
{[23, 42, 99, 7]: 'Hello'}
>>> l
[23, 42, 99, 7]
>>> d[l]
Traceback (most recent call last):
  File "", line 1, in 
KeyError: [23, 42, 99, 7]

---

edit: So I thought about this some more. You could make the above example work, if you return the list's id as its hash value:

class List(list):
    def __hash__(self):
        return id(self)

In that case, d[l] will give you 'Hello', but neither d[[23,42,99,7]] nor d[List([23,42,99,7])] will (because you're creating a new [Ll]ist.