Python callbacks#

Python callbacks (sometimes also called event handlers) are Python functions that you can attach to widgets. You can only use these callbacks in Bokeh server apps. For interactive callbacks that don’t require a Bokeh server, see JavaScript callbacks.

Bokeh’s Python callbacks are called when certain attributes on a Bokeh Model are changed. The function signature of event handlers is determined by how they are attached to widgets (whether by .on_change or .on_event, for example).

on_change callback triggers#

All widgets, for example, have an .on_change method that takes an attribute name and one or more event handlers as parameters. These handlers are expected to have the function signature, (attr, old, new), where attr refers to the changed attribute’s name, and old and new refer to the previous and updated values of the attribute.

def my_text_input_handler(attr, old, new):
    print("Previous label: " + old)
    print("Updated label: " + new)

text_input = TextInput(value="default", title="Label:")
text_input.on_change("value", my_text_input_handler)

For more information about the attributes to watch using .on_change, see the respective Model’s under bokeh.models in the reference guide.

on_event callback triggers#

Additionally, some widgets, including the Button, Dropdown, and CheckboxGroup, have an .on_event method that takes an event handler as its only parameter. For a plain Button, this handler is called without parameters. For the other widgets with .on_event, the handler is passed the new attribute value.

def my_radio_handler(new):
    print('Radio button option ' + str(new) + ' selected.')

radio_group = RadioGroup(labels=["Option 1", "Option 2", "Option 3"], active=0)
radio_group.on_event('button_click', my_radio_handler)

For more information about the attributes to watch using .on_event, see the respective entry for a widget under bokeh.models in the reference guide.

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