deepdict.py

In [3]: from collections import UserDict, defaultdict

In [4]: class A(UserDict):
   ...:     def __getitem__(self, key):
   ...:         if key not in self:
   ...:             self[key] = defaultdict(lambda: A())
   ...:         return super().__getitem__(key)
   ...:

In [6]: a = A()

In [7]: a['x']['y']['z'] = 1

In [8]: a
Out[8]: {'x': defaultdict(<function A.__getitem__.<locals>.<lambda> at 0x109fa1ae8>, {'y': {'z': 1}})}

In [9]: a['a']['b']['c']['d']
Out[9]: {}

In [10]: a
Out[10]: {'x': defaultdict(<function A.__getitem__.<locals>.<lambda> at 0x109fa1ae8>, {'y': {'z': 1}}), 'a': defaultdict(<function A.__getitem__.<locals>.<lambda> at 0x109fa12f0>, {'b': {'c': defaultdict(<function A.__getitem__.<locals>.<lambda> at 0x10a114158>, {'d': {}})}})}
   
   
   # or... you can just do def A(): return defaultdict(A)