>>> def flatten(lol):

... for item in lol:

... if isinstance(item, list):

... for subitem in flatten(item):

... yield subitem

... else:

... yield item

...

>>> lol = [1, 2, [3,4,5], [6,[7,8,9], []]]

>>> flatten(lol)

>>> list(flatten(lol))

[1, 2, 3, 4, 5, 6, 7, 8, 9]