reflex/docs/events/setters.md

import reflex as rx

Setters

Every base var has a built-in event handler to set it's value for convenience, called set_VARNAME.

Say you wanted to change the value of the select component. You could write your own event handler to do this:

options: list[str] = ["1", "2", "3", "4"]
class SetterState1(rx.State):
    selected: str = "1"

    def change(self, value):
        self.selected = value


def code_setter():
    return rx.vstack(
        rx.badge(SetterState1.selected, color_scheme="green"),
        rx.select(
            options,
            on_change= lambda value: SetterState1.change(value),
        )
    )

Or you could could use a built-in setter for conciseness.

options: list[str] = ["1", "2", "3", "4"]
class SetterState2(rx.State):
    selected: str = "1"

def code_setter_2():
    return rx.vstack(
        rx.badge(SetterState2.selected, color_scheme="green"),
        rx.select(
            options,
            on_change= SetterState2.set_selected,
        )
    )

In this example, the setter for selected is set_selected. Both of these examples are equivalent.

Setters are a great way to make your code more concise. But if you want to do something more complicated, you can always write your own function in the state.