Using generators, recursion, and decorators to return all even subsets of a sequence

generating_subsets.py

#!/usr/bin/env python3

"""
Problem inspired by "Even Subsets" found here:
https://cs61a.org/assets/slides/28-Scheme_Lists_full.pdf

Playing around with recursion, generators, and decorators...of course, this
is completely for my own edification. I would not recommend writing code like
this in an applied setting... :)
"""


from typing import Iterable, Generator, List, Callable


def is_even(g: Generator[List, None, None]) -> Callable:
    def wrapper(*args):
        for subset in g(*args):
            if sum(subset) % 2 == 0:
                yield subset
    return wrapper


@is_even
def subset_gen(s: Iterable[int]) -> Generator[List[int], None, None]:
    """
    Generator that returns non-empty subsets of iterable `s` as lists, recursively

    >>> t0 = all_subsets([])
    >>> next(t0)
    Traceback (most recent call last):
    StopIteration
    >>> t1 = all_subsets((1,))
    >>> for e in t1:
    ...     print(e)
    [1]
    >>> t2 = all_subsets((1, 2))
    >>> for e in t2:
    ...     print(e)
    [1]
    [1, 2]
    [2]
    >>> t2 = all_subsets([k+1 for k in range(3)])
    >>> for e in t2:
    ...     print(e)
    [1]
    [1, 2]
    [2]
    [1, 2, 3]
    [2, 3]
    [1, 3]
    [3]
    """
    # Encapsulate the recursion otherwise any decorator is applied to recursive
    # calls, too, since the look up finds function name in global frame, wherein
    # it's being decorated!
    #
    # See the following issue for more info:
    # https://stackoverflow.com/questions/13915122/how-to-decorate-a-generator-in-python
    def gen(s: Iterable[int]) -> Generator[List[int], None, None]:
        if s:
            first, rest = list(s[:1]), list(s[1:])
            yield first
            for subset in gen(rest):
                yield first + subset
                yield subset
    return gen(s)


def main():
    s = [3, 4, 5, 7]  # Test case for decorated generator
    for even_subset in subset_gen(s):
        print(even_subset)


if __name__ == '__main__':
    main()