[Feature Request] Pass custom parameters to `App`

(I know textual is still in active development and that this might be in the works, but just throwing it up here to help keep track)

There’s no way to pass in and store values inside a class that inherits from App, to store application-level information. For instance, the following code does not work:

class MyApp(App):
    def __init__(self, custom_arg, *args, **kwargs):
        self.custom_arg = custom_arg
        super().__init__(*args, **kwargs)
    async def on_mount(self):
        print(self.custom_arg)
MyApp(5).run()

The cause of this seems the be that in the run() function, a new instance of MyApp is created, which is the instance on which on_mount() is actually caused. This is a bit annoying and requires using global variables to store the parameters we want, which is not ideal.

[CyberSmash]

Because I came looking for this, The above code is slightly wrong. More correct is:

class MyApp(App):
    def __init__(self, *args, custom_arg, **kwargs):
        self.custom_arg = custom_arg
        super().__init__(*args, **kwargs)
    async def on_mount(self):
        print(self.custom_arg)
# This is the change, no () after MyApp
MyApp.run(custom_arg=5)

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