class Trie(object):
    ...
    def all_prefixes(self):
        results = set()
        if self.flag:
            results.add(self.value)
        if not self.children: return results
        return reduce(lambda a, b: a | b, 
                     [node.all_prefixes() for 
                      node in self.children.values()]) | results

    def autocomplete(self, prefix):
        node = self
        for char in prefix:
            if char not in node.children:
                return set()
            node = node.children[char]
        return node.all_prefixes()