Python backtracking

Build an answer one choice at a time; when a choice leads nowhere, undo it and try the next. Recursion keeps track of where you are.

Check as early as you can whether a partial answer can still work, so dead ends are left quickly.

def subsets(items, chosen=[]):
    if not items:
        print(chosen)
        return
    subsets(items[1:], chosen + [items[0]])
    subsets(items[1:], chosen)

subsets([1, 2])

Watch for: Changing a shared list without undoing the change afterwards leaves the next choice with the wrong start.