[Python-Dev] Remove typing from the stdlib

[Lukasz Langa]

> On Nov 7, 2017, at 9:39 AM, Brett Cannon <brett at python.org> wrote:
> 
> On Tue, 7 Nov 2017 at 03:34 Paul Moore <p.f.moore at gmail.com <mailto:p.f.moore at gmail.com>> wrote:
> 
> Because type annotations are a development-time feature, they should
> *not* require a dependency in the final deployment (apart from Python
> itself). However, because they are a language syntax feature they are
> of necessity written in the application source. And type specification
> of anything more complex than basic types (for example, List[int])
> requires classes defined in the typing module. Therefore, typing must
> be in the stdlib so that use of type annotations by the developer
> doesn't impose a runtime dependency on the end user.
> 
> That's not necessarily true if Lukasz's PEP lands and annotations become strings. The dependency would only need to be installed if someone chose to introspect the annotations and then "instantiate" them into actual objects. And that only comes up if someone does it from outside by a 3rd-party, who would then need to install the type annotation dependencies themselves.

PEP 563 is the first step there but it's not enough. For this idea to work, `typing.TYPE_CHECKING` would need to be moved to the Python runtime, so that you can do

if TYPE_CHECKING:
    from typing import *

More importantly, you would put type aliases, type variables and new-types in this `if` block, too.

But even if all this is true, there's still two remaining features that require runtime availability:

1. `cast()`; and
2. creating generic classes by subclassing `Generic[T]` or any of its subclasses. And soon enough, Protocols.

I hope I'm not forgetting any other cases. For `cast()` I have an idea how to move it to a variable annotation. For generic classes, there's no magic, you need `typing` at runtime. Fortunately, that latter case is (I hope?) relatively unlikely.

All the above is summarized here:
https://github.com/python/typing/issues/496 <https://github.com/python/typing/issues/496>

- Ł
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