Convertir une liste de liste en un tableau

def list_leaves(a_list, res):
#    print a_list, res
    if (type(a_list) is list) :
        for elem in a_list :
            list_leaves(elem, res)
    else:
#        print "Appending " , a_list
        res.append( a_list)

test = [ ['a', 'b', ['c', 'd']] , [[1, 2], 3, 4]]
result = ["START"]
list_leaves(test, result)
print(result)

test = [[1,2], [3, 4]]
result = ["START"]
list_leaves(test, result)
print(result)

johand@bata:~/src/CCM/PYTHON$ python leaves.py 
['START', 'a', 'b', 'c', 'd', 1, 2, 3, 4]
['START', 1, 2, 3, 4]