Python: Custom implementation

# assume only lower case characters
class Trie():
    class Node():
        def __init__(self, value):
            self.value = value
            self.children = [None] * 26
            self.is_word = False

    def build_node(self, word):
        node = self.root
        for letter in word:
            letter_idx = ord(letter) - ord('a')
            if node[letter_idx] == None:
                node = Node(letter)
            
        node.is_word = True

    def __init__(self, words):
        self.root = Node("")
         for word in words:
             self.build_node(word)

    def is_word(self, word):
        for letter in word:
            letter_idx = ord(letter) - ord('a')
            if not node[letter_idx]:
                return False
            node = node[letter_idx]
         return node.is_word