Hashing arrays in Python

69
votes

Is it possible to hash lists?

For example, I know that hashes of tuples are possible:

>>> hash((1,2,3,4,5,6))
-319527650

But is it possible to hash a list?

>>> hash([1,2,3,4,5,6])
hash_value

Possible Solution:

Very in depth explanation to the hashing of lists, here.

3
pythonarrayshash
Do you mean obtaining a hash value from an array, or do you mean some kind of array-like data structure which uses hashing internally? - juanchopanza

3 Answers

67
votes

Just try it:

>>> hash((1,2,3))
2528502973977326415
>>> hash([1,2,3])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'list'
>>> hash(frozenset((1,2,3)))
-7699079583225461316
>>> hash(set((1,2,3)))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'set'

So you can get hash of tuple and frozenset since the are immutable, and you can't do it for list and set because they are mutable.

25
votes

If you really need to use a list as a dictionary key, try converting it to a string first.
my_list = str(my_list)

8
votes

Python doesn't allow you to use mutable data as keys in dictionaries, because changes after insertion would make the object un-findable. You can use tuples as keys.