https://qiita.com/shiracamus/items/fb85943ed34d5ec09c4f

flatten.py

#!/usr/bin/env python
# -*- coding:utf-8 -*-

def flatten(data, depth=-1):
    """
    flatten(data) -> list
    flatten(data, depth) -> list

    Return flatted data of list or tupple as list.

    >>> data = [[1, 2], [3, [4, 5, [6]]]]
    >>> flatten(data)
    [1, 2, 3, 4, 5, 6]
    >>> flatten(data, 0)
    [[1, 2], [3, [4, 5, [6]]]]
    >>> flatten(data, 1)
    [1, 2, 3, [4, 5, [6]]]
    >>> flatten(data, 2)
    [1, 2, 3, 4, 5, [6]]
    >>> flatten(data, 3)
    [1, 2, 3, 4, 5, 6]
    """
    return [element
            for item in data
            for element in (flatten(item, depth - 1)
                            if depth != 0 and hasattr(item, '__iter__')
                            else [item])
            ]