Type Hints to extend built-in structures

Working on revisions to a book. Fun stuff. See https://www.packtpub.com/product/python-3-object-oriented-programming/9781849511261 I may have the privilege of working with Dusty.

I've been using mypy for all my 2nd edition changes, but not in --strict mode. 

I've decided to ramp things up, and switch to strict type checking for all of the examples and case studies.

This lead me to stumble over

class MyThing(dict):
    def some_extra_method(self):
        pass

I absolutely could not get this to work for hours and hours.

I more-or-less gave up on it, until I started a similar example for a later chapter.

class ListWithFeatures(list):
    def feature(self):
        pass

This is almost the same, but, somehow, I understood it better.  As written, it is rejected by mypy. What I meant was this.

class ListWithFeatures(List[MyThing]):
    @overload
    def __init__(self) -> None: ...
    @overload
    def __init__(self, source: Iterable[MyThing]) -> None: ...
    def __init__(self, source: Optional[Iterable[MyThing]]) -> None:
        if source:
	    super().__init__(source)
        else:
            super().__init__()
    def feature(self) -> float:
        return sum(thing.some_extra_method())/len(self)

I don't know why, but this was easier for me to visualize the problem.. It clarified my understanding profoundly.

We don't simply extend list or dict.  We should extend them because list is an alias for List[Any], and when being strict, we need to avoid Any. Aha.