Python Program to Display Tree Nodes using BFS Traversal

class Tree:
    def __init__(self, data=None):
        self.key = data
        self.children = []
 
    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 bfs(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('bfs')
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
            ref_node.add(new_node)
 
    elif operation == 'bfs':
        if tree is None:
            print('Tree is empty.')
        else:
            print('BFS traversal: ', end='')
            tree.bfs()
            print()
 
    elif operation == 'quit':
        break