def flatten(l):
    for el in l:
        if not el or not isinstance(el[0], list):
            yield el
        else:
            yield from flatten(el)
 
def nested_to_listoflist(l):
    return [el for el in flatten(l)]
    # or return list(flatten(l))
 
 
nested_list = [[[1, 2, 3], [[5, 6, 7], [8, 9, 10, 11, 23]]], [4], [[12, 13, 14], [[15, 16], [[17, 18], [19, 20]]]], [21, 22, 25, 26]]
unnested_list = nested_to_listoflist(nested_list)
print(unnested_list)

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