Here’s a nice short function for flattening irregular lists in Python that I found on Stackoverflow a few weeks back. It returns a generator so should work fine for very big lists.

def flatten_list(object_list):
    """
    Flattens a list of objects.
    """
    for element in object_list:
        if isinstance(element, collections.Iterable) and not \
                isinstance(element, basestring):
            for sublist in flatten_list(element):
                yield sublist
        else:
            yield element

I tested it with lists like the following and it did the job:

[None]
[]
[[], []]
['a', 'b', 'c']
[['a'], ['b', 'c']]
[['a', 'b'], ['c'], 'd']
['a']
[['a']]<br>