python - 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 a in range(3)]for b in range(3))

But it gives me an error

TypeError: unhashable type: 'list'

I have 2 questions for the Python Guru's:

a) When I look at the Python definition of Hashable -

"An object is hashable if it has a hash value which never changes during its lifetime (it needs a hash() method)"

when I used dir function on the expression above

dir([(a,b)for a in range(3)]for b in range(3))

it seems to say the __hash__ is there. So, why do I get the error?

I was able to get [[(0, 0), (1, 0), (2, 0)], [(0, 1), (1, 1), (2, 1)], [(0, 2), (1, 2), (2, 2)]] by using the list command :

list(list((a,b) for a in range(3)) for bin range(3))

b)list and set both takes Iterable as parameter. How come one works(list) and another doesn't (set)?

Answer 1

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'