[Python-Dev] PEP 484 update proposal: annotating decorated declarations

Koos Zevenhoven k7hoven at gmail.com
Fri Jun 2 16:07:01 EDT 2017

On Fri, Jun 2, 2017 at 8:57 PM, Guido van Rossum <guido at python.org> wrote:
> On Fri, Jun 2, 2017 at 9:41 AM, Koos Zevenhoven <k7hoven at gmail.com> wrote:
>> I still don't understand what would happen with __annotations__. If
>> the decorator returns a non-function, one would expect the annotations
>> to be in the __annotations__ attribute of the enclosing class or
>> module. If it returns a function, they would be in the __annotations__
>> attribute of the function. And I'm talking about the runtime behavior
>> in Python as explained in PEP484 and PEP526. I would expect these
>> declarations to behave according to the same principles as other ways
>> to annotate variables/functions. If there is no runtime behavior, a
>> comment-based syntax might be more appropriate. Or have I missed
>> something?
> So when returning a function, the runtime version of the decorator can
> easily update the function's __annotations__. But when returning a
> non-function, the decorator would have a hard time updating
> of the containing class/module without "cheating" (e.g. sys._getframe()).
> think the latter is similar to e.g. attributes defined with @property --
> those don't end up in __annotations__ either. I think this is an
> deficiency.

I suppose it is, especially because there seems to be nothing that prevents
you from getting runtime annotations in the enclosing class/module

number: int

def number():
    return 42

But for functions one could have (
 the context manager example):

def session(url: str) -> ContextManager[DatabaseSession]: ...

def session(url: str) -> Iterator[DatabaseSession]:
    s = DatabaseSession(url)
        yield s

This makes it clear that the function is declared elsewhere. But the
`predeclared` decorator would need tricks like sys._getframe(1) to set
session.__annotations__ according to the predeclaration.

-- Koos

+ Koos Zevenhoven + http://twitter.com/k7hoven +
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