List all subsets of a set

A set of 3 items has 8 subsets, including the empty set.

"""
List the subsets of a set.
A Python set can hold only hashable objects, but a Python set is not hashable.
Therefore a Python set cannot contain a Python set.
The subsets function is therefore forced to return a list instead of set.
A Python list can contain a list.
"""

import sys


def subsets(s):
    "Return a list of the subsets of s."
    assert type(s) == list

    if not s:            #if s is empty,
        return [set()]   #The empty set is the only subset of the empty set.

    powerSet = []

    for subset in subsets(s[1:]):
        powerSet.append(subset)
        powerSet.append(subset | {s[0]})   #union of two sets

    return powerSet


s = [0, 1, 2]   #or try s = [0, 1, 2, 3]

for i, subset in enumerate(subsets(s), start = 1):
    print(i, subset)

sys.exit(0)

1 set()
2 {0}
3 {1}
4 {0, 1}
5 {2}
6 {0, 2}
7 {1, 2}
8 {0, 1, 2}

Things to try

1. The function subsets returns a list. Create this list with a list comprehension.

       extras = [set(), {s[0]}]
       return [subset | extra for subset in subsets(s[1:]) for extra in extras]
   

2. Prove that a set is not hashable.

   s = set()
   
   try:
       h = hash(s)
   except TypeError as error:
       print("A set is not hashable:", error)
   else:
       print("A set is hashable.")
   

   A set is not hashable: unhashable type: 'set'