[Python-ideas] Optional static typing -- the crossroads

[Greg Ewing]

Andrew Barnert wrote:
> The obvious way that already works (with MyPy, and also with ABCs for isinstance checking):
> 
>     class HashableIterable(Iterable, Hashable): pass
> 
>     def spam(a: HashableIterable):
>         pass

That way might be obvious, but it's wrong. It says that spam()
only takes an instance of the specific class HashableIterable
or a subclass thereof. It won't accept anything else, even if
it implements Iterable and Hashable perfectly well.

> 
> But, there's no reason typing.py couldn't add a wrapper that does this automatically:
> 
>     class Multiple:
>         @staticmethod
>         def __getitem__(*types);
>             return type(types[0])(
>                 '_'.join(t.__name__ for t in types), types, {})

Automating it won't make it any more right. There needs to
be a distinct concept such as Intersection() as a counterpart
to Union().

-- 
Greg