TypeError : Unhashable type

Question

I am trying to get a list of list of tuples : something like [ [(1,0),(2,0),(3,0)],[(1,1),(2,1),(3,1)....]] I used this statement

set([(a,b)for         


        

Answer 1

TLDR:

- You can't hash a list, a set, nor a dict to put that into sets

- You can hash a tuple to put it into a set.

Example:

>>> {1, 2, [3, 4]}
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'list'

>>> {1, 2, (3, 4)}
set([1, 2, (3, 4)])

Note that hashing is somehow recursive and the above holds true for nested items:

>>> {1, 2, 3, (4, [2, 3])}
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'list'

Dict keys also are hashable, so the above holds for dict keys too.

Answer 2

... and so you should do something like this:

set(tuple ((a,b) for a in range(3)) for b in range(3))

... and if needed convert back to list

Answer 3

A list is unhashable because its contents can change over its lifetime. You can update an item contained in the list at any time.

A list doesn't use a hash for indexing, so it isn't restricted to hashable items.

Answer 4

You are creating a set via set(...) call, and set needs hashable items. You can't have set of lists. Because list's arent hashable.

[[(a,b) for a in range(3)] for b in range(3)] is a list. It's not a hashable type. The __hash__ you saw in dir(...) isn't a method, it's just None.

A list comprehension returns a list, you don't need to explicitly use list there, just use:

>>> [[(a,b) for a in range(3)] for b in range(3)]
[[(0, 0), (1, 0), (2, 0)], [(0, 1), (1, 1), (2, 1)], [(0, 2), (1, 2), (2, 2)]]

Try those:

>>> a = {1, 2, 3}
>>> b= [1, 2, 3]
>>> type(a)
<class 'set'>
>>> type(b)
<class 'list'>
>>> {1, 2, []}
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'list'
>>> print([].__hash__)
None
>>> [[],[],[]] #list of lists
[[], [], []]
>>> {[], [], []} #set of lists
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'list'

Answer 5

You'll find that instances of list do not provide a __hash__ --rather, that attribute of each list is actually None (try print [].__hash__). Thus, list is unhashable.

The reason your code works with list and not set is because set constructs a single set of items without duplicates, whereas a list can contain arbitrary data.

Answer 6

The real reason because set does not work is the fact, that it uses the hash function to distinguish different values. This means that sets only allows hashable objects. Why a list is not hashable is already pointed out.