Recursively flatten a nested iterable

flatten.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-
import collections
def flatten(lst):
    """Flatten a iterable.

    >>> from __future__ import print_function
    >>> lst = [[1, 2], [], [1, [[2, 3], 3]], 4, 5]
    >>> print(flatten(lst))
    [1, 2, 1, 2, 3, 3, 4, 5]
    """
    assert(isinstance(lst, collections.Iterable))
    if not len(lst):
        return lst
    head, *tail = lst
    if isinstance(head, collections.Iterable):
        return flatten(head) + flatten(tail)
    return [head] + flatten(tail)

if __name__ == '__main__':
    import doctest
    doctest.testmod()