[Python-Dev] Remove typing from the stdlib (original) (raw)

Lukasz Langa lukasz at langa.pl
Tue Nov 7 13:55:35 EST 2017

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>

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