#!/usr/bin/python3

mylist_regular = [[1, 2, [3]], 4]
mylist_recursive = [[1, 2, [3]], 4]

# flatten a list of lists of integers, recursively
# lst : list of integers
# returns: a flattened list
def flatten_recursive(lst):
    # return the current element if it's not a list, else call routine again with current list
    # initial value is empty list
    return sum(([x] if not isinstance(x, list) else flatten_recursive(x) for x in lst), [])


# flatten a list of lists of integers, recursively
# lst : list of integers, unpacked in positional params
# returns: the next interger from the list
def flatten_regular(*lst):
    for x in lst:
        print(x)
        if hasattr(x, '__iter__'):
            # if this is another list (it's iterable), unpack it and
            for y in flatten_regular(*x):
                # yield each value upon invocation
                yield y
        else:
            # if it's not iterable (not a list), yield the intermediate value
            yield x


print(flatten_recursive(mylist_recursive))
# convert the result back into a list
print(list(flatten_regular(mylist_regular)))