import queue class ExpressionTree: def __init__(self): self.root = None def pre_order(self, current_node):

Here's the completed code: ``` import queue class ExpressionTree: def __init__(self): self.root = None def pre_order(self, current_node): # Check if current_node exists if current_node: # Print the value of the current_node print(current_node.data) # Call pre_order recursively on the left subtree self.pre_order(current_node.left_child) # Call pre_order recursively on the right subtree self.pre_order(current_node.right_child) et = ExpressionTree() et.pre_order(et.root) ``` The code defines an ExpressionTree class with a pre_order method that performs a pre-order traversal of the tree. It takes a current_node parameter that specifies the root of the subtree to traverse. It starts by checking whether the current node exists. If it does, it prints the data of the current node, which is the root of the subtree. Then, it calls the pre_order method recursively on the left child of the current node, which traverses the left subtree. Finally, it calls the pre_order method recursively on the right child of the current node, which traverses the right subtree. This process continues until all nodes in the tree have been visited. Finally, it creates an ExpressionTree object and calls the pre_order method with the root of the tree as the parameter to perform a pre-order traversal of the entire tree.