@on decorator behavior varies between no selector and selector=""

[traveller1011]

I find it interesting that the two method signatures below are not equivalent.

Method 1: passing a Falsy selector

    @on(Button.Pressed, selector="")
    async def handle_button_press_child(self, event: Button.Pressed) -> None:
        self.notify("HIT CHILD BUTTON EVENT")

Method 2: passing no selector

    @on(Button.Pressed)
    async def handle_button_press_child(self, event: Button.Pressed) -> None:
        self.notify("HIT CHILD BUTTON EVENT")

The difference

Passing a blank selector kills the event bubbling--- or otherwise results in behavior which i found "surprising". I would argue behavior should be the same in both cases, if only to safe future brain cycles answering for this oddity.

Working Example

• Comment/uncomment the decorator lines below to see the behavior difference.

from textual import on
from textual.containers import Container
from textual.app import App, ComposeResult
from textual.widgets import Button, Rule, Static


class ChildContainer(Container):
    DEFAULT_CSS = """
    ChildContainer {
        background: green;
    }
    """

    def compose(self):
        yield Static("Begin ChildContainer")
        yield Button.success("Button - ChildContainer")

    # @on(Button.Pressed, selector="")
    @on(Button.Pressed)
    async def handle_button_press(self, _: Button.Pressed) -> None:
        self.notify("Button Event --> ChildContainer", severity="information")


class ParentContainer(Container):
    DEFAULT_CSS = """
    ParentContainer {
        background: red;
    }
    """

    def compose(self):
        yield Static("Begin ParentContainer")
        yield Button.warning("Button - ParentContainer")
        yield Rule()
        yield ChildContainer()

    # @on(Button.Pressed, selector="")
    @on(Button.Pressed)
    async def handle_button_press(self, _: Button.Pressed) -> None:
        self.notify("Button Event --> ParentContainer", severity="warning")


class EventBubbleApp(App):

    def compose(self) -> ComposeResult:
        yield Static("Begin EventBubbleApp")
        yield ParentContainer()

    # @on(Button.Pressed, selector="")
    @on(Button.Pressed)
    async def handle_button_press(self, _: Button.Pressed) -> None:
        self.notify("Button Event --> EventBubbleApp", severity="error")


if __name__ == "__main__":
    app = EventBubbleApp()
    app.run()

[willmcgugan]

An empty string is a valid selector, albeit one that doesn't match anything. The equivalent of no selector would be selector=None.

[traveller1011]

Well, I understand (now). But it did throw me for a loop this AM... a bit of "if a tree falls in the woods" vibes.

Anyhow, back to work, thanks Will.