Why can't I use a list as a dict key in python?

python list dictionary tuples hashable
wim picture wim · Aug 31, 2011 · Viewed 113.4k times · Source

I'm a bit confused about what can/can't be used as a key for a python dict. 

dicked = {}
dicked[None] = 'foo'     # None ok
dicked[(1,3)] = 'baz'    # tuple ok
import sys
dicked[sys] = 'bar'      # wow, even a module is ok !
dicked[(1,[3])] = 'qux'  # oops, not allowed

So a tuple is an immutable type but if I hide a list inside of it, then it can't be a key.. couldn't I just as easily hide a list inside a module?

I had some vague idea that that the key has to be "hashable" but I'm just going to admit my own ignorance about the technical details; I don't know what's really going on here. What would go wrong if you tried to use lists as keys, with the hash as, say, their memory location?

Answer

user395760 picture user395760 · Aug 31, 2011

There's a good article on the topic in the Python wiki: Why Lists Can't Be Dictionary Keys. As explained there:

What would go wrong if you tried to use lists as keys, with the hash as, say, their memory location?

It can be done without really breaking any of the requirements, but it leads to unexpected behavior. Lists are generally treated as if their value was derived from their content's values, for instance when checking (in-)equality. Many would - understandably - expect that you can use any list [1, 2] to get the same key, where you'd have to keep around exactly the same list object. But lookup by value breaks as soon as a list used as key is modified, and for lookup by identity requires you to keep around exactly the same list - which isn't requires for any other common list operation (at least none I can think of).

Other objects such as modules and object make a much bigger deal out of their object identity anyway (when was the last time you had two distinct module objects called sys?), and are compared by that anyway. Therefore, it's less surprising - or even expected - that they, when used as dict keys, compare by identity in that case as well.

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