[Python-Dev] PEP 563: Postponed Evaluation of Annotations

Ivan Levkivskyi levkivskyi at gmail.com
Thu Nov 2 15:32:06 EDT 2017

On 1 November 2017 at 23:48, Lukasz Langa <lukasz at langa.pl> wrote:

> Runtime annotation resolution and class decorators
> --------------------------------------------------
> Metaclasses and class decorators that need to resolve annotations for
> the current class will fail for annotations that use the name of the
> current class.  Example::
>     def class_decorator(cls):
>         annotations = get_type_hints(cls)  # raises NameError on 'C'
>         print(f'Annotations for {cls}: {annotations}')
>         return cls
>     @class_decorator
>     class C:
>         singleton: 'C' = None
> This was already true before this PEP.  The class decorator acts on
> the class before it's assigned a name in the current definition scope.
Just a random idea: maybe this can be resolved by just updating the localns
before calling get_type_hints() like this:

  localns = locals().copy()
  localns.update({cls.__name__: cls})

In general I like how the PEP is written now. I maybe would add examples
for this

> the cases listed above might be worked around by placing the usage
> in a if TYPE_CHECKING: block.
> ...
> For named tuples, using the new class definition syntax introduced in
Python 3.6 solves the issue.

actually showing something like

      Alias = List[Tuple[int, SomeClass]]

  class NT(NamedTuple):
      one: SomeClass
      other: Alias

  class SomeClass:

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