Python tree depth-first traversal, pre-order, middle-order, and post-order

There are three types of tree depth-first traversal : pre-order, middle-order, and post-order.
1. Pre-order priority traversal:

class TreeNode():
	def  __init__(self,x):
		self.val = x
		self.left = None
		self.right = None
class Solution(object):
    def preorder(self, root):
        if root is None:
            return ''
        print(root.val)
        if root.left:
            self.preorder(root.left)
        if root.right:
            self.preorder(root.right)

2. Mid-order traversal:

def midorder(self,root):
	if not root:
		return ''
	if root.left:
		self.midorder(root.left)
	print(root.val)
	if root.right:
		self.midorder(root.right)
		

3. Post-order traversal:

def midorder(self,root):
	if not root:
		return ''
	if root.left:
		self.midorder(root.left)
	if root.right:
		self.midorder(root.right)
	print(root.val)