Should add the check if no cycle is found:

  def detectCycle(self, head):
    cycle_length = 0
    slow, fast = head
    while fast and fast.next:
      fast = fast.next.next 
      slow = slow.next
      if slow == fast: 
        cycle_length = self.calculate_cycle_length(slow)
        break
    # HERE: should handle case where no cycle is found.
    if fast is None or fast.next is None:
        return None

    return self.find_start(head, cycle_length)