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

[Brett Cannon]

On Wed Aug 20 2014 at 9:29:34 AM Paul Moore <p.f.moore at gmail.com> wrote:

> On 14 August 2014 00:30, Guido van Rossum <guido at python.org> wrote:
> > We certainly *could* do that. However, I haven't seen sufficient other
> uses
> > of annotations. If there is only one use for annotations (going forward),
> > annotations would be unambiguous. If we allow different types of
> > annotations, there would have to be a way to tell whether a particular
> > annotation is intended as a type annotation or not. Currently mypy
> ignores
> > all modules that don't import typing.py (using any form of import
> > statement), and we could continue this convention. But it would mean that
> > something like this would still require the typing import in order to be
> > checked by mypy:
> >
> > import typing
> >
> > def gcd(int a, int b) -> int:
> >     <tralala>
>
> Sorry, I'm slowly going through this thread, so my apologies if this
> has been covered later, but it seems to me that projects (and in
> particular libraries) that want to target 2.7 as well as 3.x will be
> forced to avoid this feature. And avoid any dependencies that use it.
>
> Specifically, the annotation syntax is purely Python 3.x, so without
> some form of translation or import hook, 2.7 won't accept it. And
> adding a dependency (even if it's only a very lightweight typing.py
> plus a hook installed somewhere/somehow) for something that only has
> value as part of a developer-level type check doesn't seem like a good
> solution.
>
> So, I'd like someone to explain (maybe by pointing me to relevant mypy
> docs, I haven't investigated them yet) how I should modify my existing
> code that supports 2.7 and 3.x so that it uses the new functionality
> *without* extra runtime dependencies that only deliver build/test-time
> benefits (that's the problem I always had with
> setuptools/pkg_resources, and I'd not like to see it repeated here).
>

I suspect the answer is "you don't". Just like everything else that is
syntactically exclusive to Python 3, it's a perk you get with Python 3-only
code that you simply can't get if you want to maintain
backwards-compatibility.
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