flatten.py

def flatten(intlist):
    """flatten an arbitrarily nested list of integers
    e.g.
    flatten([[1,2,[3]],4]) returns [1,2,3,4]
    """
    retlist = [ ]
    for n in intlist:
        if type(n) == list:
            retlist.extend(flatten(n))
        else:
            retlist.append(n)
    return retlist

if __name__ == '__main__':
    #forgoing external unit test to keep this all in one file

    #using a list as a key in a dictionary not possible
    tests = {
            "[ ]" : [ ],
            "[1,2,3,4]" : [1,2,3,4],
            "[0,0,0,0]" : [0,0,0,0],
            "[[1,2,[3]],4]" : [1,2,3,4],
            "[1, 3, [4, 6, 7], 9, 0]" : [1,3,4,6,7,9,0]
            }

    for k,v in tests.iteritems():
        test = eval(k)
        result = 'passed' if flatten(test) == v else 'failed'
        print('test {0} {1}, => {2}'.format(result, k, v))