trieAddWord.py

# add a word to an existing trie, recursively
def addWord(word, trie):
    
    # non-base case
    if word:
        letter, rest = word[0], word[1:]
        if letter not in trie:
            trie[letter] = {}
        addWord(rest, trie[letter])
        
    # base case
    else:
        trie['$'] = None

trie = {}

addWord("apple", trie)
addWord("app", trie)
addWord("appo", trie)
addWord("api", trie)

trie == {'a': {'p': {'i': {'$': None}, 'p': {'l': {'e': {'$': None}}, 'o': {'$': None}, '$': None}}}} # True

lgtm!