Every Subset by Bitmask

Problem

Given a list of distinct items, return every subset of it. Order the subsets by counting: read a subset as a binary number in which bit i is 1 when items[i] is included, and list the subsets from that number 0 up to the largest. Inside each subset keep the items in their original order. The list holds at most 15 items, and the empty list still has one subset.

Examples

Input:  items = ["a", "b"]
Output: [[], ['a'], ['b'], ['a', 'b']]
Why:    the numbers 0, 1, 2 and 3 in binary are 00, 01, 10 and 11

Input:  items = [1, 2, 3]
Output: [[], [1], [2], [1, 2], [3], [1, 3], [2, 3], [1, 2, 3]]
Why:    items[2] joins from number 4 onwards, when bit 2 turns on

Input:  items = []
Output: [[]]
Why:    edge case, the empty set is the only subset

Hints

0 / 3

1. Hint 1reveal
2. Hint 2locked
3. Hint 3locked

def all_subsets(items):
    n, subsets = len(items), []
    for mask in range(1 << n):                # every n-bit pattern is one subset
        subsets.append([items[i] for i in range(n) if mask >> i & 1])
    return subsets

print(all_subsets(["a", "b"]))  # -> [[], ['a'], ['b'], ['a', 'b']]
print(all_subsets([1, 2, 3]))   # -> [[], [1], [2], [1, 2], [3], [1, 3], [2, 3], [1, 2, 3]]
print(all_subsets([]))          # -> [[]]