Python Trie的前序遍历

我需要对一个trie进行一次预排序遍历，我做了一系列的尝试，但没有得到任何结果。这里是node和trie类，trie类中的最后一个方法不起作用。我尝试使用递归来处理这个问题，但是，我不认为我使用了正确的代码。请帮帮我

    class Node_Q3:

    def __init__(self, letter):
        self.letter = letter
        self.complete_word = False
        self.children = {}
        
        
class Trie_Q3:

        self.root = Node_Q3('')
        self.nodes = {}
        if type(word_list) is list:
            for i in word_list:
                self.insert(i)
            
        elif word_list == None:
            pass
        else:
            self.insert(word)
            
    
    
    def insert(self, word):


        node = self.root
        word = word.lower()

        for char in word:
            if char in node.children:
                node = node.children[char]
            else:
                new_node = Node_Q3(char)
                node.children[char] = new_node
                node = new_node
        
        self.nodes[word] = node
        node.complete_word = True

        
    def lookup(self, word):

        if not self.root.children:
            return False
        node = self.root
        word = word.lower()
        for letter in word:
            if letter in node.children.keys():
                node = node.children[letter]
            else:
                return False
        return node.complete_word         


    def preorder_traversal(self):

        lst = []
        val = self.root.letter
        if val == None: 
            val=''
            
        for c in self.root.children.values():
            lst.extend(.preorder_traversal()) 
            
        lst = [val+x for x in lst]
        return lst
                ```