NoneDict.py

class Singleton(type):
    _instances = {}
    def __call__(cls, *args, **kwargs):
        if cls not in cls._instances:
            cls._instances[cls] = super(Singleton, cls).__call__(*args, **kwargs)
        return cls._instances[cls]

class NoneDict(dict, metaclass=Singleton):
    def __getitem__(self, key):
            return NoneDict()

class NoneSubsDict(dict):
    def __getitem__(self, key):
        if (val := self.get(key)) is not None:
            return NoneSubsDict(val) if isinstance(val, dict) else val
        return NoneDict()
        

dct = NoneSubsDict({ 'a' : { 'b' : { 'c' : 'd' }}})
dct['a']['b']['c'] # 'd'
dct['a']['b']['5'] # NoneDict
dct['a']['5']['c'] # NoneDict
dct['5']['b']['c'] # NoneDict