reflex/docs/events/event_arguments.md

import reflex as rx

Event Arguments

In some use cases, you want to pass additional arguments to your event handlers. To do this you can bind an event trigger to a lambda, which can call your event handler with the arguments you want.

Try typing a color in an input below and clicking away from it to change the color of the input.

class ArgState(rx.State):
    colors: list[str] = ["rgba(222,44,12)", "white", "#007ac2"]

    def change_color(self, color: str, index: int):
        self.colors[index] = color

def event_arguments_example():
    return rx.hstack(
        rx.chakra.input(default_value=ArgState.colors[0], on_blur=lambda c: ArgState.change_color(c, 0), bg=ArgState.colors[0]),
        rx.chakra.input(default_value=ArgState.colors[1], on_blur=lambda c: ArgState.change_color(c, 1), bg=ArgState.colors[1]),
        rx.chakra.input(default_value=ArgState.colors[2], on_blur=lambda c: ArgState.change_color(c, 2), bg=ArgState.colors[2]),
    )

In this case, in we want to pass two arguments to the event handler change_color, the color and the index of the color to change.

The on_blur event trigger passes the text of the input as an argument to the lambda, and the lambda calls the change_color event handler with the text and the index of the input.

# Event Handler Parameters should provide type annotations.
Like state vars, be sure to provide the right type annotations for the parameters in an event handler.