Recursively flatten an arbitrary number of nested iterators.

flatten.py

l = [1, 2, 3, [4, 5, 6, [9, 8, 7, [12, 11, 10], 13, 14], 15, 16], 17, 18]


def flatten(input):
    """ Flatten nested iterators. Looks specifically for lists, tuples, and sets, and returns them sorted"""

    # Prepare the ourput variable
    output = []

    # Look for wheter an input is a list, tuple, or set
    if isinstance(input, (list, tuple, set)):
        # Lookp over that iterator, and extend the output by its flattened return
        for item in input:
            output.extend(flatten(item))
    else:
        # Append a single NON-iterator item to the output
        output.append(input)

    return sorted(output)

if __name__ == '__main__':
    print flatten(l)