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?

Inizia esaminando la proposta e gli esempi referenziati in django/core/files/uploadhandler.py, quindi considera come un marker inherit_type_hints opt-in interagirebbe con i metodi sovrascritti e le classi standard. Per considerarlo completato sarebbe necessario concordare un modello di ereditarietà, compreso stabilire se le annotazioni sono disponibili a runtime, non solo uno schema di implementazione.