TypeError : Unhashable type

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)?