How do you find if there is a loop in a singly linked list?

looped_linked_list.py

""" Given a singly linked list, how do you find if there is a loop?
    assumptions:
    - `head` can be None or the first node in a linked list
"""

def is_loop(head):
  if head is None or head.next is None:
    return false
  else:
    first, second = head, head.next
    while second is not None:
      first = first.next
      second = second.next.next
      if first is second:
        return true 
    return false