Python 3 filter() + enumerate() = filtered_enum() !!

_iter_value_t = TypeVar('_iter_value_t')

def filtered_enum(things: Iterable[_iter_value_t], mask: Iterable[bool], start: int = 0) -> Iterable[Tuple[int, _iter_value_t]]:
    return ((i, v) for (i, (v, mb)) in enumerate(zip(things, mask), start) if mb)