Flatten arbitrary nested lists in Python

flatten_nested_lists.py

def flatten(lst):
    for item in lst:
        # if item is a sublist
        if isinstance(item, (list,tuple)):
            # flatten sublist
            for j in flatten(item):
                yield j
        else:
            yield item


# list(flatten([[1,2,3], [[4], [5], [6]], [7,8,9]]))