Proposal: inheritance + annotations

After some time annotating non-trivial codebases, I've found something that feels like a nuisance and a bad practice. Essentially, when overriding methods in subclasses, having to re-specify the whole type signatures in subclasses is:

• type consuming (I'm writing/annotating a subclass and have to copy potentially complicated types for a lot of parameters)
• error prone
• not a source new information (which is the benefit of annotations)
• a violation of the DRY principle (if I choose to change the type of an argument in the superclass, I have to edit all the subclasses).

One cases where this trivially happens is when defining standard python magic methods; I want my __str__ methods checked, but it feels silly to tell the typechecker every time that "yes, this method returns a string". It also happens in more complicated definitions, for example https://github.com/dmoisset/django/blob/typing-requests/django/core/files/uploadhandler.py#L83 and https://github.com/dmoisset/django/blob/typing-requests/django/core/files/uploadhandler.py#L83 vs line 170 of the same file.

Currently, the PEP says «For a checked function, the default annotation for arguments and for the return type is Any». I'd like to introduce some exception to this rule for methods inheriting an annotated method.

I know that there's a potential problem with this that is when inheriting standard classes (especially but not limited to object) which already have annotated clases, may force undesired annotations. My idea here is to introduce some kind of marker (perhaps a @typing.inherit_type_hints decorator?) that can be used at class and method levels and means "inherit annotations from the parent class unless specifically overriden by this class". In this way it's opt-in and also tells both the human reader and the static analyzer where to lookup the annotations.

The implementation of this decorator could be trivial (another identity decorator, just used as a marker for static analysis tools) , but also could actually copy the __anotations__ field to make the inherited information also available to runtime tools.

What do you think of this?

首先审阅提案以及 django/core/files/uploadhandler.py 中引用的示例，然后考虑可选的 inherit_type_hints 标记将如何与重写的方法和标准类交互。要视为完成，需要达成一个一致的继承模型，包括注解是否在运行时可用，而不仅仅是实现草案。