from collections import Iterable

def flatten(items):
    """Yield items from any nested iterable; see REF."""
    for x in items:
        if isinstance(x, Iterable) and not isinstance(x, (str, bytes)):
            yield from flatten(x)
        else:
            yield x

arr = [[1, [2]], (3, 4, {5, 6}, 7), 8, "9"]
list(flatten(arr))