Phantom Data in Python BST Preorder

Question

I have a python function for finding the preorder of Binary Search Trees.

class Node:
  def __init__(self, val = 0, left = None, right = None):
    self.val = val
    self.right = right
    self.left = 

class Tree:
  def __init__(self, root=None):
    self.root = Node(root) if type(root) != Node else root

  def add(self, val):
    node = self.root
    found = False
    while not found:
      if node.val >= val:
        if node.left is None:
          node.left = Node(val)
          found = True
        else:
          node = node.left
      else:
        if node.right is None:
          node.right = Node(val)
          found = True
        else:
          node = node.right

def preorder(node, output = []):
  output += [node.val]
  if node.left is not None: 
    preorder(node.left, output)
  if node.right is not None:
    preorder(node.right, output)
  return output

This works fine on the first call, but every time I run it after the data from previous calls shows up. For example:

 preorder(tree.root)
[3, 3, 3, 3, 2, 1, 5, 4, 5]
 preorder(tree.root)
[3, 3, 3, 3, 2, 1, 5, 4, 5, 3, 3, 3, 3, 2, 1, 5, 4, 5]
 preorder(tree.root)
[3, 3, 3, 3, 2, 1, 5, 4, 5, 3, 3, 3, 3, 2, 1, 5, 4, 5, 3, 3, 3, 3, 2, 1, 5, 4, 5]

This is easily remedied by adding a empty list to the input parameters, for example:

 preorder(tree.root, [])
[3, 3, 3, 3, 2, 1, 5, 4, 5]
 preorder(tree.root, [])
[3, 3, 3, 3, 2, 1, 5, 4, 5]

I have my solution to the problem, but my question is why is this occurring? The preorder function doesn't seem to be changing the state of the tree, and I would assume that output would be set to the default value of [] unless a different list is given. Any information would be appreciated. Thank you!