floufen

def fold_flatten(it):
    "Flatten any number of nested collections into one list"
    def _flattener(acc, a):
        try:
            return acc + list(fold_flatten(iter(a))) # Recursion
        except TypeError: # a is not iterable
            return acc + [a]
    return fold(_flattener, it, [])

fold_flatten([1, [2, [3, 4], 5]]) # => [1, 2, 3, 4, 5]