Traversing a tree means visiting every node. For a binary tree there are three classic recursive orders, which differ only in when the node itself is visited relative to its children:

Pre-order: the node, then left subtree, then right. (Good for copying a tree.)
In-order: left subtree, then the node, then right. (On a search tree, this visits values in sorted order.)
Post-order: left, right, then the node. (Good for deleting or for calculations that need the children first.)

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

def preorder(node):
    if node is None:
        return []
    return [node.value] + preorder(node.left) + preorder(node.right)

def inorder(node):
    if node is None:
        return []
    return inorder(node.left) + [node.value] + inorder(node.right)

def postorder(node):
    if node is None:
        return []
    return postorder(node.left) + postorder(node.right) + [node.value]

tree = Node(4, Node(2, Node(1), Node(3)), Node(6, Node(5), Node(7)))
print(preorder(tree))
print(inorder(tree))
print(postorder(tree))

The base case is an empty subtree, None, which contributes nothing.