a different variant of: https://gist.github.com/hrldcpr/2012250

simple_tree.py

import collections


class SimpleTree(collections.defaultdict):
    """
    >>> tree = SimpleTree()

    >>> tree.add(keys=("1",), value="value 1")

    >>> tree.add(keys=("2a", "2b"), value="value 2.1")
    >>> tree.add(keys=("2a", "2b"), value="value 2.2")

    >>> tree.split_add("3a.3b.3c", "value 3.1")
    >>> tree.split_add("3a.3b.3c", "value 3.2")
    >>> tree.split_add("3a.3b.3c", "value 3.3")

    >>> tree.pprint(indent=2, print_depth=True)
    0   <key>  1
    1 <value>    value 1
    0   <key>  2a
    1   <key>    2b
    2 <value>      value 2.1
    2 <value>      value 2.2
    0   <key>  3a
    1   <key>    3b
    2   <key>      3c
    3 <value>        value 3.1
    3 <value>        value 3.2
    3 <value>        value 3.3

    >>> tree.split_add("4a-4b-4c-4d", "value 4.1", split_by="-")
    >>> tree.split_add("4a-4b-4c-4d", "value 4.2", split_by="-")
    >>> tree.split_add("4a-4b-4c-4d", "value 4.3", split_by="-")
    >>> tree.split_add("4a-4b-4c-4d", "value 4.4", split_by="-")

    >>> for keys, values in tree.iter_values():
    ...     print(keys, values)
    ['1'] ['value 1']
    ['2a', '2b'] ['value 2.1', 'value 2.2']
    ['3a', '3b', '3c'] ['value 3.1', 'value 3.2', 'value 3.3']
    ['4a', '4b', '4c', '4d'] ['value 4.1', 'value 4.2', 'value 4.3', 'value 4.4']
    """

    def __init__(self):
        super().__init__(dict)
        self.__type = type(self)

    def __missing__(self, key):
        value = self[key] = self.__type()  # retain local pointer to value
        return value  # faster to return than dict lookup

    def add(self, keys, value=None, child=None):
        first_key = keys[0]

        if child is not None:
            obj = child[first_key]
        else:
            obj = self[first_key]

        if len(keys) == 1:
            if first_key in obj:
                obj[first_key].append(value)
            else:
                obj[first_key] = [value]
        else:
            self.add(keys[1:], value=value, child=obj)

    def split_add(self, keys, value, split_by="."):
        keys = keys.split(split_by)
        self.add(keys, value)

    def __iter__(self, keys=None, obj=None, depth=1):
        if keys is None:
            keys = []

        if obj is None:
            obj = self

        for key, values in sorted(obj.items()):
            if isinstance(values, self.__type):
                yield (keys, key, True)
                yield from self.__iter__(keys + [key], obj=values, depth=depth + 1)
            else:
                for value in values:
                    yield (keys, value, False)

    def iter_values(self, keys=None, obj=None, depth=1):
        if keys is None:
            keys = []

        if obj is None:
            obj = self

        for key, values in sorted(obj.items()):
            if isinstance(values, self.__type):
                yield from self.iter_values(keys + [key], obj=values, depth=depth + 1)
            else:
                yield (keys, values)

    def pprint(self, indent=4, print_depth=False):
        for keys, obj, is_key in self:
            depth = len(keys)
            indent_str = " " * (depth * indent)
            obj_type = "<key>" if is_key else "<value>"
            txt = "%7s %s %s" % (obj_type, indent_str, obj)

            if print_depth:
                print(depth, txt)
            else:
                print(txt)


if __name__ == '__main__':
    import doctest
    print(doctest.testmod())