Class cannot subclass "Screen", "Message", etc.

[PabloLec]

Hi,

MyPy is raising the following exceptions:

Class cannot subclass "ListView" (has type "Any")  [misc]
Class cannot subclass "Screen" (has type "Any")  [misc]
Class cannot subclass "Message" (has type "Any")  [misc]
Class cannot subclass "Screen" (has type "Any")  [misc]
Class cannot subclass "Horizontal" (has type "Any")  [misc]

In my code, I'm subclassing in the most regular way, e.g for Screen :

class ModalScreen(Screen):
    ...

And get the previous exceptions.
It started happening when upgrading textual from 0.20.1 to 0.21.0.
Any idea why ? I'm clueless about this MyPy error and would be curious to know its reason.

[davep]

I would expect mypy to perhaps give a warning about inheriting from Screen with more recent versions of Textual as there is a new feature -- still to be properly documented -- for typing a screen and being able to return a result from a screen when it's dismissed. See #2321 for more details on that.

If you're not intending to return a result from your modal screen you can change the code you show above to be like this:

class ModalScreen(Screen[None]):
    ...

As a slight aside to this, you might want to note that ModalScreen is now a type in Textual.

More generally though, I'm not clear on why you'd be seeing the warnings you're showing there. I'm running mypy v1.2.0 here and testing it on a couple of my own projects that are heavy on inheriting from things in Textual I'm not seeing anything like that.

Do you think you could show a simple stand-alone bit of code that demonstrates these warnings, and also give details of your mypy version, plus your Python version, so I can test the same here?

[PabloLec]

Thanks, @davep, it did the trick! So, the type passed to Screen is its return type. And is it correct that the use of Generic makes it mandatory to type hint? Additionally, is the advantage of type hinting the ScreenResultType to provide a hint for the eventual return type of its callback?

By the way, I'm also using mypy 1.2, but with the --strict flag. I encountered this error for Screen and App. I'm not sure why I saw this error for Horizontal, etc. before, but I'm not seeing it today, which is weird but not textual related."

[davep]

Additionally, is the advantage of type hinting the ScreenResultType to provide a hint for the eventual return type of its callback?

Spot on, exactly this.

Yup, I always run with the --strict flag too.