bad recursion, still works

[iu2]

Hi,

I wrote this wrong recursive function that flattens a list:

def flatten(lst, acc=[]):
    #print 'acc =', acc, 'lst =', lst
    if type(lst) != list:
        acc.append(lst)
    else:
        for item in lst:
            flatten(item)
    return acc

a = [1, 2, [3, 4, 5], [6, [7, 8, [9, 10], 11], 12], 13, 14]
b = flatten(a)
print b

I was amazed to realize that it flattens the list alright. Why? 'acc'
should be an empty list on each invocation of flatten, but is seems to
accumulate anyway...