How to Prevent Reactive Feedback Loops in Shiny for Python When Syncing a Preset and Text Input

Prevent feedback loops when syncing reactive inputs in Shiny for Python. Use a req guard to ignore auto updates and keep preset and text input aligned.

When you wire up two reactive inputs in Shiny for Python so they keep each other in sync, it’s easy to create a feedback loop. A typical case: a preset select controls a text input, while a manual change in the text input should flip the preset to a sentinel value like “changed”. The trouble starts when a programmatic update of the text input is treated as a manual edit and immediately forces the preset back to “changed”.

Reproducing the issue

The following minimal app shows the unintended loop. Selecting a preset updates the text input unless the preset is “changed”. Any change to the text input then sets the preset to “changed” again, including programmatic updates initiated by the first reactive effect.

from shiny import App, ui, reactive
ui_shell = ui.page_fillable(
    ui.layout_sidebar(
        ui.sidebar(
            ui.input_select("preset_choice", "preset_choice", choices=["A", "B", "C", "changed"]),
            ui.input_text("option_field", "option_field", value=""),
        )
    )
)
def backend(inputs, outputs, sess):
    @reactive.effect
    @reactive.event(inputs.preset_choice)
    def sync_from_preset():
        if inputs.preset_choice() != "changed":
            ui.update_text("option_field", value=str(inputs.preset_choice()))
    @reactive.effect
    @reactive.event(inputs.option_field)
    def mark_changed():
        ui.update_select("preset_choice", selected="changed")
app = App(ui_shell, backend)

What’s going on

The second reactive effect is bound to the text input. It fires whenever that input changes, regardless of whether the change came from a user typing or from ui.update_text(). As a result, a preset-driven update immediately flips the preset select back to “changed”, defeating the original intent.

A small guard that fixes the loop

Gate the preset update with req() so it only happens when the text input actually differs from the preset. If the change originated from the preset selection, both values are the same and the update is skipped.

from shiny import App, ui, reactive, req
ui_shell = ui.page_fillable(
    ui.layout_sidebar(
        ui.sidebar(
            ui.input_select("preset_choice", "preset_choice", choices=["A", "B", "C", "changed"]),
            ui.input_text("option_field", "option_field", value=""),
        )
    )
)
def backend(inputs, outputs, sess):
    @reactive.effect
    @reactive.event(inputs.preset_choice)
    def sync_from_preset():
        if inputs.preset_choice() != "changed":
            ui.update_text("option_field", value=str(inputs.preset_choice()))
    @reactive.effect
    @reactive.event(inputs.option_field)
    def mark_changed():
        req(inputs.option_field() != inputs.preset_choice())
        ui.update_select("preset_choice", selected="changed")
app = App(ui_shell, backend)

In this example, an equivalent if guard would also work.

Why this matters

Without a guard, you end up with reactive churn and confusing UI behavior. A simple inequality check prevents the preset from flipping when the text input was updated by the preset itself, preserving the intended distinction between user edits and programmatic synchronization.

Takeaways

When syncing inputs in Shiny for Python, assume that programmatic updates trigger the same reactive events as manual edits. Protect downstream updates with a condition such as req(a() != b()) so you only react when there’s a meaningful change. If you prefer, an equivalent conditional block achieves the same effect here.