[Python-Dev] PEP 563: Postponed Evaluation of Annotations (original) (raw)

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 classdecorator(cls): annotations = gettypehints(cls) # raises NameError on 'C' print(f'Annotations for {cls}: {annotations}') return cls @classdecorator 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 TYPECHECKING: block. ... For named tuples, using the new class definition syntax introduced in Python 3.6 solves the issue.

actually showing something like

if TYPE_CHECKING: Alias = List[Tuple[int, SomeClass]]

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

class SomeClass: ...

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