Flatten nested list with unlimited depth

flatten.py

from collections.abc import Iterable

def flatten(items, ltypes=Iterable):
    """flatten(sequence) -> list

    Returns a single, flat list which contains all elements retrieved from the sequence
    """
    if not isinstance(items, list):
        yield items
    stack = [iter(items)]
    while stack:
        for item in stack[-1]:
            if isinstance(item, ltypes):
                stack.append(iter(item))
                break
            yield item
        else:
            stack.pop()


def flatten2(input, ltypes=(list,tuple)):
    ltype = type(input)
    output = []
    stack = []
    stack.extend(reversed(input))
    while stack:
        top = stack.pop()
        if isinstance(top, ltypes):
            stack.extend(reversed(top))
        else:
            output.append(top)
 
    return ltype(output)


def flatten3(l, ltypes=(list, tuple)):
    ltype = type(l)
    l = list(l)
    i = 0
    while i < len(l):
        while isinstance(l[i], ltypes):
            if not l[i]:
                l.pop(i)
                i -= 1
                break
            else:
                l[i:i + 1] = l[i]
        i += 1
    return ltype(l)