Sets in Python

[sapsi]

Hello,
I recently tried using the set function in Python and was surprised to
find that

a=[ 1, 2,3, [1,2] ]

doesn't work with 'set', throwing TyperError (unhashable exception). I
found out that this is because lists can't be hashed.

So,this implies 'a' cannot be a set in python which i think is quite
unfortunate, after all 'a' does look like a mathematical set.

My question is,
1) Why can't lists be hashed?
and
2) This is not related, but is there i neat way (without pop and list
comprehension) to convert a set into a list? I say neat because i'm
guessing using list comprehension might turn out be slow and there
might be other methods which are faster.

Thank you for your time
SM