But this is potentially dangerous. E.g.: dict.setObjectForKey(@"b", @"a"); assert(dict.containsObjectForKey[@"a"]); A bit tangential related to the ar

But this is potentially dangerous. E.g.: dict.setObjectForKey(@"b", @"a"); assert(dict.containsObjectForKey[@"a"]); A bit tangential related to the article: In Python, I often found that it is helpful to also have a separate "None"-like type despite None, which I called NotSpecified. Often in functions you have sth like: def func(opt: int = None): if opt is None: opt = get_default_opt() ... This works fine when you now that opt should not be None here. However, if None is allowed for opt and causes some other specific behavior, I now do: def func(opt: Optional[int] = NotSpecified): if opt is NotSpecified: opt = get_default_opt() ... This is also useful for dicts: my_dict.get("foo", NotSpecified)