[Typing] Add type hint to sender in widget messages?

[rodrigogiraoserrao]

The class Message defines the attribute sender as being of the type MessageTarget, which is a fairly generic protocol. Fine.

When writing custom messages, wouldn't it make sense to add an explicit type hint to mark the class variable sender as being of the type of the widget within which the message is being defined?

E.g., won't this make sense:

class MyWidget(Widget):
   class MyMessage(Message):

        sender: MyWidget  # Isn't this something sensible to do?

        def __init__(self, sender: MyWidget, ...):
            ...

By adding sender: MyWidget in the body of the class, IDEs and static checkers will let me make use of .sender as of being the type I already know it is.

[willmcgugan]

Great in principle. But I suspect that will break typing somewhere. Feels like a LSP violation.

[davep]

Perhaps the way to go here would be to make Message a generic, so the sub-class can be declared including the type expected for the sender?

[rodrigogiraoserrao]

Assuming LSP means Liskov's Substitution Principle, I can't see how that could be true.
A message is defined inside a widget namespace, implying that the sender will be an instance of that type of widget or a subclass of another. If B is a subclass of A and the sender is typed as A, it is fine if the message is sent from B.

[rodrigogiraoserrao]

Reading this on the mypy docs makes me wonder whether I'm getting things the other way around.
I'll wait for a eureka moment or for someone to enlighten me.

[rodrigogiraoserrao]

Work on #1831 seems to suggest there is no issue with LSP.
I request we change this into an enhancement task and add it to the backlog.

[willmcgugan]

Ok, feel free to add that.

[rodrigogiraoserrao]

This became irrelevant as of #1940.

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