Function Annotation got NameError: name 'xxx' is not defined

[ichabod801]

You can do the circular references within a function because the code within the function is not executed until you call the function. At that point the relevant names have been defined.

I don't think you can get around this. The only thing I can think of is to define a dummy version of Cls2, then Cls1 with the Cls2 annotation, then Cls2 with the Cls1 annotation. But with the way names work in Python, I expect that the annotation in Cls1 will still point to the evaluated dummy Cls2.

Another idea is to have Cls1 and Cls2 both inherit from Cls0, and use Cls0 as the annotation. Not as precise as you want, but it's something.

Doing some more research, it appears PEP 583 calls for delayed evaluation of annotations. This was implemented in 3.7 and can be imported from __future__ as annotations. Otherwise PEP 484 recommends using a string of the expression, which should be evaluated once the module is fully loaded.