def __init__(self, default=lambda: None, path_constructor=list):
        """
        default: A function returning the value to use for new nodes.
        path_constructor: A function which constructs a path from a list. The
            path type must support addition (concatenation).
        """
        self.value = default()
        self._children = {}
        self._default = default
        self._path_constructor = path_constructor

        default: A function returning the value to use for new nodes.
        path_constructor: A function which constructs a path from a list. The
            path type must support addition (concatenation).
        
featuretools/utils/trie.py