CODEWITHSUNDEEP
python
Lostsoul
Lostsoul

Reputation: 25999

How can I assign a list in a tuple in a dictionary?

I'm new to python and experimenting a bit but having trouble making a list into a tuple to use as a dictionary key. Here's an example, which should make it more clear:

dict_of_lists_values = {}
dict_of_lists_values[('dog', 'cat')] = 10
dict_of_lists_values[('dog1', 'cat1')] = 10
dict_of_lists_values[('dog1', 'cat2')] = 10
dict_of_lists_values
{('dog', 'cat'): 10, ('dog1', 'cat2'): 10, ('dog1', 'cat1'): 10}

This works perfectly, and allows me to have a two values I can use as keys in a dictionary. When I try to apply this to a list, I get an error: TypeError: unhashable type: 'list'

dict_of_lists_values = {}
a = [22, 39, 0]
b = [15, 38, 12]
dict[(a, b)] = 'please work'

Based on my previous experiment, I think if I convert the list into a string it would work but I want it as it as a list not a string.

Is this possible?

Upvotes: 2

Views: 338

Answers (3)

cheeken
cheeken

Reputation: 34655

No. It's not possible to use list types for dictionary keys. However, you could extend list, make it hashable, and then use that new type. (Though it's a bit cumbersome.)

class hlist(list):
    def __hash__(self):
        # Hash it somehow; here, I convert it to a hashable tuple ... and then hash it
        return hash(tuple(self))

l1 = hlist([1,2,3])
l2 = hlist([4,5,6])

d = {
    l1:"Hi.",
    l2:"Hello!"
}

Please note Sven's comment below. Mutable keys are dangerous because their hash becomes stale if they are modified.

Upvotes: 3

D K
D K

Reputation: 5760

Dictionaries in Python can only have immutable/hashable keys.

Strings, numbers, and tuples are immutable, so they can be used as dictionary keys. Instances have a unique __hash__(), so they can also be used. But lists are mutable, so they cannot be used as keys.

Upvotes: 1

Wooble
Wooble

Reputation: 89907

Call tuple() on a list to create a tuple with the elements in the list.

Upvotes: 4

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