Python Program to Construct a Tree and Perform Tree Operations

class Tree:
    def __init__(self, data=None, parent=None):
        self.key = data
        self.children = []
        self.parent = parent
 
    def set_root(self, data):
        self.key = data
 
    def add(self, node):
        self.children.append(node)
 
    def search(self, key):
        if self.key == key:
            return self
        for child in self.children:
            temp = child.search(key)
            if temp is not None:
                return temp
        return None
 
    def remove(self):
        parent = self.parent
        index = parent.children.index(self)
        parent.children.remove(self)
        for child in reversed(self.children):
            parent.children.insert(index, child)
            child.parent = parent
 
    def bfs_display(self):
        queue = [self]
        while queue != []:
            popped = queue.pop(0)
            for child in popped.children:
                queue.append(child)
            print(popped.key, end=' ')
 
 
tree = None
 
print('Menu (this assumes no duplicate keys)')
print('add <data> at root')
print('add <data> below <data>')
print('remove <data>')
print('display')
print('quit')
 
while True:
    do = input('What would you like to do? ').split()
 
    operation = do[0].strip().lower()
    if operation == 'add':
        data = int(do[1])
        new_node = Tree(data)
        suboperation = do[2].strip().lower() 
        if suboperation == 'at':
            tree = new_node
        elif suboperation == 'below':
            position = do[3].strip().lower()
            key = int(position)
            ref_node = None
            if tree is not None:
                ref_node = tree.search(key)
            if ref_node is None:
                print('No such key.')
                continue
            new_node.parent = ref_node
            ref_node.add(new_node)
 
    elif operation == 'remove':
        data = int(do[1])
        to_remove = tree.search(data)
        if tree == to_remove:
            if tree.children == []:
                tree = None
            else:
                leaf = tree.children[0]
                while leaf.children != []:
                    leaf = leaf.children[0]
                leaf.parent.children.remove(leaf)
                leaf.parent = None
                leaf.children = tree.children
                tree = leaf
        else:
            to_remove.remove()
 
    elif operation == 'display':
        if tree is not None:
            print('BFS traversal display: ', end='')
            tree.bfs_display()
            print()
        else:
            print('Tree is empty.')
 
    elif operation == 'quit':
        break