Python: flatten nested lists with indices

Question

Given a list of arbitrairly deep nested lists of arbitrary size, I would like an flat, depth-first iterator over all elements in the tree, but with path indicies as well such th

Answer 1

Recursion is good approach for flattening deeply nested lists. Your implementation is also well done. I would suggest modifying it with this similar recipe as follows:

Code

from collections import Iterable


def indexed_flatten(items):
    """Yield items from any nested iterable."""
    for i, item in enumerate(items):
        if isinstance(item, Iterable) and not isinstance(item, (str, bytes)):
            for item_, idx in indexed_flatten(item):
                yield item_, (i,) + idx
        else:
            yield item, (i,)


lst = [[[1, 2, 3], [4, 5]], [6], [7, [8, 9]], 10]
list(indexed_flatten(lst))

Output

[(1, (0, 0, 0)),
 (2, (0, 0, 1)),
 (3, (0, 0, 2)),
 (4, (0, 1, 0)),
 (5, (0, 1, 1)),
 (6, (1, 0)),
 (7, (2, 0)),
 (8, (2, 1, 0)),
 (9, (2, 1, 1)),
 (10, (3,))]

This robustly works with many item types, e.g. [[[1, 2, 3], {4, 5}], [6], (7, [8, "9"]), 10].