asdf.py

a = ('val', 1, 1)
b = ('list', 2, [a, a])
c = ('val', 3, 2)
l = [a,b,a,c]

class MySet():
    def __init__(self):
        self._data = set()
    def __contains__(self, value):
        return (value in self._data)
    def add(self, value):
        self._data.add(value)

def gen(l, known):
    for type, id, val in l:
        if id not in known:
            if type == 'list':
                for inner_val in gen(val, known):
                    yield inner_val
            yield val

known = MySet()
for x in gen(l, known):
    print x

# Hier erwarte ich jetzt die Ausgabe:
#  1
#  2
#  3