donnemartin/interactive-coding-challenges · error · KeyError

word does not exist

Error message

word does not exist

What it means

Raised by Trie.remove when the word is not stored as a terminated word in the trie. find returns a node only if the full path exists AND the final node has terminates=True; otherwise remove raises KeyError. Prefixes of stored words also trigger this (e.g. removing 'ab' when only 'abc' is stored).

Source

Thrown at graphs_trees/trie/trie.py:47

    def insert(self, word):
        if word is None:
            raise TypeError('word cannot be None')
        node = self.root
        parent = None
        for char in word:
            if char in node.children:
                node = node.children[char]
            else:
                node.children[char] = Node(char, parent=node)
                node = node.children[char]
        node.terminates = True

    def remove(self, word):
        if word is None:
            raise TypeError('word cannot be None')
        node = self.find(word)
        if node is None:
            raise KeyError('word does not exist')
        node.terminates = False
        parent = node.parent
        while parent is not None:
            # As we are propagating the delete up the 
            # parents, if this node has children, stop
            # here to prevent orphaning its children.
            # Or
            # if this node is a terminating node that is
            # not the terminating node of the input word, 
            # stop to prevent removing the associated word.
            if node.children or node.terminates:
                return
            del parent.children[node.key]
            node = parent
            parent = parent.parent

    def list_words(self):
        result = []

View on GitHub (pinned to 358f2cc604)

Solutions

  1. Check with trie.find(word) first and only remove if it returns a node
  2. Track deletion state upstream to avoid double-remove
  3. Catch KeyError and treat 'already absent' as success (idempotent delete)

Example fix

# before
trie.remove(word)  # KeyError if absent

# after
if trie.find(word) is not None:
    trie.remove(word)
Defensive patterns

Strategy: try-catch

Validate before calling

if trie.find(word) is not None:
    trie.remove(word)

Type guard

def word_in_trie(trie, word) -> bool:
    return word is not None and trie.find(word) is not None

Try / catch

try:
    trie.remove(word)
except KeyError:
    pass  # word already absent

Prevention

When it happens

Trigger: Calling remove on a never-inserted word; removing a word twice; removing a proper prefix of a stored word.

Common situations: Duplicate delete requests in request handling; deleting from stale word lists; assuming prefixes count as words.

Related errors

AI-assisted analysis of donnemartin/interactive-coding-challenges@358f2cc604 (2026-08-28). Data as JSON: /api/errors/a66272f1c566a40b. Report an issue: GitHub.