Programming puzzle: Does this linked list have a loop? https://www.interviewcake.com/question/python/linked-list-cycles?__s=rf71cpqpqpezb7dgts5r

loop.py

#!/usr/bin/env python

# does this linked list have a cycle?
# https://www.interviewcake.com/question/python/linked-list-cycles?__s=rf71cpqpqpezb7dgts5r

class ListNode:
    next = None

def has_loop(l):
    seen = set()
    current = l
    while True:
        if current is None:
            # reached the end: no loop.
            return False
        if current in seen:
            # found a loop
            return True
        seen.add(current)
        current = current.next

# the list is not a loop
def testcase0():
    head = ListNode()
    head.next = ListNode()
    return head

# the list is an end-to-end loop
def testcase1():
    head = ListNode()
    head.next = ListNode()
    head.next.next = head
    return head

# the list loops back into the middle
def testcase2():
    head = ListNode()
    head.next = ListNode()
    head.next.next = ListNode()
    head.next.next.next = head.next
    return head

if __name__ == "__main__":
    tests = {
        testcase0(): False,
        testcase1(): True,
        testcase2(): True
    }

    for testcase, expected in tests.iteritems():
        assert has_loop(testcase) == expected

    print "all tests passed."