Python Linked List: prevent the list to become circular

linked_circle.py

def is_list_circular(l):
    slower = l
    faster = l.get_next()

    while True:
        # if faster pointer finds a None element
        # then the list is not circular
        if (faster is None) or (faster.get_next() is None):
            return False
        # if faster element catch up with the slower
        # then the list is circular
        elif (faster == slower) or (faster.get_next() == slower):
            return True
        # in any other case we just move slower by one
        # and faster by two steps
        else:
            slower = slower.get_next()
            faster = faster.get_next().get_next()

class ListElement(object):
    def __init__(self):
        self._next = None
    def add_next(self, next_node):
        self._next = next_node

        # check if the list, with the added element
        # is circular. If it is, then remove the just
        # added element.
        if is_list_circular(self):
            self._next = None
    def get_next(self):
        return self._next

# I create 4 separate ListElement
a = ListElement()
b = ListElement()
c = ListElement()
d = ListElement()

# I chain them and in particular I try to connect element d to a
a.add_next(b)
b.add_next(c)
c.add_next(d)
d.add_next(a)

# I expect the first three to have a next element
# but the fourth to have None
assert a.get_next() is not None
assert b.get_next() is not None
assert c.get_next() is not None
assert d.get_next() is None