[Python-Dev] PEP 484 update: add Type[T]

[Guido van Rossum]

There's a more fundamental PEP 484 update that I'd like to add. The
discussion is in https://github.com/python/typing/issues/107.

Currently we don't have a way to talk about arguments and variables
whose type is itself a type or class. The only annotation you can use
for this is 'type' which says "this argument/variable is a type
object" (or a class). But it's often useful to be able to say "this is
a class and it must be a subclass of X".

In fact this was proposed in the original rounds of discussion about
PEP 484, but at the time it felt too far removed from practice to know
quite how it should be used, so I just put it off. But it's been one
of the features that's been requested most by the early adopters of
PEP 484 at Dropbox. So I'd like to add it now.

At runtime this shouldn't do much; Type would be just a generic class
of one parameter that records its one type parameter. The real magic
would happen in the type checker, which will be able to use types
involving Type. It should also be possible to use this with type
variables, so we could write e.g.

T = TypeVar('T', bound=int)
def factory(c: Type[T]) -> T:
    <implementation>

This would define factory() as a function whose argument must be a
subclass of int and returning an instance of that subclass. (The
bound= option to TypeVar() is already described in PEP 484, although
mypy hasn't implemented it yet.)

(If I screwed up this example, hopefully Jukka will correct me. :-)

Again, I'd like this to go out with 3.5.2, because it requires adding
something to typing.py (and again, that's allowed because PEP 484 is
provisional -- see PEP 411 for an explanation).

-- 
--Guido van Rossum (python.org/~guido)