Wondering of a clean way to flatten a list (multiple levels)

flattener.py

#!/usr/bin/env python

def flatten(array):
    out = []
    def real_flatten(array):
        for item in array:
            if isinstance(item, (list, tuple, set)):
                real_flatten(item)
            else:
                out.append(item)
    real_flatten(array)
    return out

if __name__ == '__main__':
    abc = [4,5,6,[1,2],[3,[7, 8],9],10]
    print flatten(abc)

# Output: [4, 5, 6, 1, 2, 3, 7, 8, 9, 10]