In [1]:
from __future__ import print_function # 2.7 compatability

from IPython.html import widgets # Widget definitions
from IPython.display import display # Used to display widgets in the notebook

Traitlet Events

As mentioned in Part 1, the widget attributes are IPython traitlets. Traitlets are eventful. To handle changes, the on_trait_change method of the widget can be used to register a callback. The docstring for on_trait_change can be seen below. Both the name and remove properties are optional.

In [2]:
Setup a handler to be called when a trait changes.

        This is used to setup dynamic notifications of trait changes.

        Static handlers can be created by creating methods on a HasTraits
        subclass with the naming convention '_[traitname]_changed'.  Thus,
        to create static handler for the trait 'a', create the method
        _a_changed(self, name, old, new) (fewer arguments can be used, see

        handler : callable
            A callable that is called when a trait changes.  Its
            signature can be handler(), handler(name), handler(name, new)
            or handler(name, old, new).
        name : list, str, None
            If None, the handler will apply to all traits.  If a list
            of str, handler will apply to all names in the list.  If a
            str, the handler will apply just to that name.
        remove : bool
            If False (the default), then install the handler.  If True
            then unintall it.

Mentioned in the doc string, the callback registered can have 4 possible signatures:

  • callback()
  • callback(trait_name)
  • callback(trait_name, new_value)
  • callback(trait_name, old_value, new_value)

Using this method, an example of how to output an IntSliderWiget's value as it is changed can be seen below.

In [3]:
int_range = widgets.IntSliderWidget()

def on_value_change(name, value):

int_range.on_trait_change(on_value_change, 'value')

Specialized Events

Button Click Event

The ButtonWidget is a special widget, like the ContainerWidget and TabWidget, that isn't used to represent a data type. Instead the button widget is used to handle mouse clicks. The on_click method of the ButtonWidget can be used to register function to be called when the button is clicked. The docstring of the on_click can be seen below.

In [4]:
Register a callback to execute when the button is clicked.

        The callback will be called with one argument,
        the clicked button widget instance.

        remove : bool (optional)
            Set to true to remove the callback from the list of callbacks.

Button clicks are transmitted from the front-end to the back-end using custom messages. By using the on_click method, a button that prints a message when it has been clicked is shown below.

In [5]:
button = widgets.ButtonWidget(description="Click Me!")

def on_button_clicked(b):
    print("Button clicked.")

Button clicked.
Button clicked.
Button clicked.

Event handlers can also be used to create widgets. In the example below, clicking a button spawns another button with a description equal to how many times the parent button had been clicked at the time.

In [6]:
def new_button(clicked):
    button = widgets.ButtonWidget()
    button.clicks = 0
    clicked.clicks += 1
    button.description = "%d" % clicked.clicks
button = widgets.ButtonWidget(description = "Start")
button.clicks = 0