[Python-ideas] Proposal: Use mypy syntax for function annotations

[Bob Ippolito]

On Thu, Aug 14, 2014 at 5:14 PM, Steven D'Aprano <steve at pearwood.info>
wrote:

> On Thu, Aug 14, 2014 at 01:54:20PM -0700, Bob Ippolito wrote:
> > * Deprecation of using function annotations for any other purpose. They
> > can't really be composed, and ideally type checkers can work primarily at
> > the syntax level and not have to evaluate the module with a full Python
> > interpreter in order to extract the annotations, so it's best to keep the
> > syntax uniform.
>
> -1
>
> I don't think this one is justified. At the very least, I think the
> decision to deprecate or not should be deferred until at least 3.7. It's
> enough to say that:
>
> - you can use function annotations for type checking; or
>
> - you can use function annotations for something else;
>
> but not both at the same time. I don't think it is a big burden to have
> mypy and other linters support an "opt-out" decorator, say, so that
> projects can use annotations for something else without confusing the
> linter.
>

Certainly using annotations for purposes other than type checking would be
permissible, but discouraged. No enforcement of any kind is proposed, just
language to go in the documentation. It would be non-trivial for the Python
compiler or runtime to enforce it anyway!

For that sort of annotation, a decorator probably isn't best, some
module-level syntax would likely be more appropriate.


> As I understand it, the current behaviour of mypy is that you have to
> import typing in the module before it will type check the module, so
> that already gives you a way to skip type annotations on a per-module
> basis: just don't import typing.
>

The current implementation of mypy will typecheck any module that uses
annotations or imports from typing.

-bob
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