How do I make a list a key value in a dictionary?

Question

I need to make a dictionary where you can reference [[1,2],[3,4]] --> ([1,2]:0, [2,3]:0) I've tried different ways but I can't use a list in a dictionary. So i tried using tuples, but its still the same. Any help is appreciated!

• Serdalis: If you are trying to use a list as a key I think you may have the wrong implementation for your algorithm.
• Felix Kling: So i tried using tuples, but its still the same Please show us your code!
• Mark Amery: Why do you want to do this? If the content of the lists isn't going to change, and you want to use the content as the key in your dict, then convert the list to tuple and use that as a key, as others have suggested. On the other hand, if you're going to be changing the content of the list but still want to be able to map the list to the same value in the dictionary, then call the id() builtin function on your list and use the result of that as your key. These are two very different use cases and you haven't given enough information for us to determine which approach solves your problem.

Answer 1

You need to use tuples:

dict.fromkeys((tuple(i) for i in [[1,2],[3,4]]), 0)

or (for python2.7+)

{tuple(i): 0 for i in [[1,2], [3,4]]}

Edit:

Reading the comments, OP probably want to count occurrences of a list:

>>> collections.Counter(tuple(i) for i in [[1,2], [1,2], [3,4]])
Counter({(1, 2): 2, (3, 4): 1})

• user1828072: Is there any way to change the 0? Like, if you have a list of [[1,2],[1,2],[2,3]], I need it to return [[1:2]:2, [2:3]:1]?
• JBernardo: @user1828072 Of course, just write 1 as the last argument instead of 0
• JBernardo: @user1828072 Oh, if you need to count the number of times a list is inside another, you'll probably need collections.Counter
• mgilson: In the first version, I prefer dict((tuple(i),0) for i in [[1,2],[3,4]]) rather than using the classmethod -- But that's just a matter of taste I suppose. (It's also closer to the syntax used to create the dict-comp in py2.7)
• user1828072: As you can probably tell, im quite new at this! I mean, the last number '0', instead of it being 0, can it return the number of occurances? For example: y = [[1,2],[1,2],[2,3]] ; there are two [1:2], is there a way to use a variable to return something like:[1,2]:2 instead? Thanks a lot!

Answer 2

Lists can't be used as dictionary keys since they aren't hashable (probably because they can be mutated so coming up with a reasonable hash function is impossible). tuple however poses no problem:

d = {(1,2):0, (3,4):0}

Note that in your example, you seem to imply that you're trying to build a dictionary like this:

((1,2):0, (3,4):0)

That won't work. You need curly brackets to make a dictionary.

Answer 3

([1,2]:0, [2,3]:0) is not a dictionary. I think you meant to use: {(1,2):0, (2,3):1}

• mgilson: It looks like we had the same idea at the same time -- however, your second example won't work either since you're using lists as keys which won't fly.
• brian buck: Right, it looks like we also noticed that at the same time too! I edited it to reflect. Thanks!