Unable to convert list into set, raises “unhashable type: 'list' ” error

Question

So I\'m trying to find all sub-lists of a list and here is what I have now. I\'m new to Python and I don\'t understand why \" Q3_ans=set(ans)\" raises an error. I\'ve tried

Answer 1

Here's the essence of the problem:

>>> set([[["a"],["b"],["a","b"]]])
Traceback (most recent call last):
  File "", line 1, in 
TypeError: unhashable type: 'list'

So, what does that mean?

set([iterable])

Return a new set ... object whose elements are taken from iterable. The elements of a set must be hashable.

hashable

An object is hashable if it has a hash value which never changes during its lifetime (it needs a __hash__() method), and can be compared to other objects (it needs an __eq__() method). Hashable objects which compare equal must have the same hash value.

Hashability makes an object usable as a dictionary key and a set member, because these data structures use the hash value internally.

All of Python’s immutable built-in objects are hashable; mutable containers (such as lists or dictionaries) are not.

The keywords here are mutable

Mutable objects can change their value but keep their id().

and immutable

An object with a fixed value. Immutable objects include numbers, strings and tuples. Such an object cannot be altered. A new object has to be created if a different value has to be stored. They play an important role in places where a constant hash value is needed, for example as a key in a dictionary.

So, you can't use a list as set element. A tuple would work:

>>> set([(("a"),("b"),("a","b"))])
{('a', 'b', ('a', 'b'))}