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

[Andrew Barnert]

On Aug 17, 2014, at 2:33, Greg Ewing <greg.ewing at canterbury.ac.nz> wrote:

> Stefan Behnel wrote:
>> However, it does not support protocols, so it still needs
>> something that allows us to say Iterable(int) in some way.
> 
> Just had a thought -- does mypy provide a way to express
> a type that supports more than one protocol? E.g. can you
> say that something must be both Iterable and Hashable?

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

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, {})

    def spam(a: Multiple[Iterable, Hashable]):
        pass

And, on analogy with the proposal for | as a shortcut for Union, the base class could add:

    def __and__(self, other):
        return Multiple[self, other]

    def spam(a: Iterable & Hashable):
        pass