Map Sum Pairs — Python

# Time:  O(n), n is the length of key
# Space: O(t), t is the number of nodes in trie

import collections


class MapSum(object):

    def __init__(self):
        """
        Initialize your data structure here.
        """
        _trie = lambda: collections.defaultdict(_trie)
        self.__root = _trie()


    def insert(self, key, val):
        """
        :type key: str
        :type val: int
        :rtype: void
        """
        # Time: O(n)
        curr = self.__root
        for c in key:
            curr = curr[c]
        delta = val
        if "_end" in curr:
            delta -= curr["_end"]

        curr = self.__root
        for c in key:
            curr = curr[c]
            if "_count" in curr:
                curr["_count"] += delta
            else:
                curr["_count"] = delta
        curr["_end"] = val


    def sum(self, prefix):
        """
        :type prefix: str
        :rtype: int
        """
        # Time: O(n)
        curr = self.__root
        for c in prefix:
            if c not in curr:
                return 0
            curr = curr[c]
        return curr["_count"]