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[Python-ideas] PEP 563: Postponed Evaluation of Annotations, first draft

Ethan Smith ethan at ethanhs.me
Mon Sep 11 20:33:51 EDT 2017 
On Mon, Sep 11, 2017 at 1:21 PM, Yury Selivanov <yselivanov.ml at gmail.com>
wrote:

> On Mon, Sep 11, 2017 at 3:25 PM, Lukasz Langa <lukasz at langa.pl> wrote:
> [..]
> > This PEP is proposing delaying evaluation until annotations are accessed
> but
> > gives user code the power to decide whether the string form is enough, or
> > maybe an AST would be enough, or actual evaluation with get_type_hints()
> or
> > eval() is necessary.
>
> I'm one of those who used annotations for other purposes than type
> hints. And even if annotations became strings in Python 3.7 *without
> future import*, fixing my libraries would be easy -- just add an
> eval().
>
> That said, the PEP doesn't cover an alternative solution:
>
> 1. Add another special attribute to functions: __annotations_text__.
>
> 2. __annotations__ becomes a dynamic Mapping, which evaluates stuff
> from __annotations_text__ *lazily*.
>
> 3. Recommend linters and IDEs to support "# pragma: annotations", as a
> way to say that the Python files follows the new Python 3.7
> annotations semantics.
>
>
I really like this proposal! I agree the linters should understand the
semantics of if TYPE_CHECKING, Python 2.7-3.6 will continue to need it.


> That would maintain full backwards compatibility with all existing
> Python libraries and would not require a future import.
>
> Yury
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