notebook/examples/Interactive Widgets/Widget Events.ipynb

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Widget Events¶

Special events¶

In [1]:

from __future__ import print_function

The ButtonWidget is not 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 doc string of the on_click can be seen below.

In [2]:

from IPython.html import widgets
print(widgets.ButtonWidget.on_click.__doc__)

Register a callback to execute when the button is clicked.

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

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

Example¶

Since button clicks are stateless, they 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 [3]:

from IPython.display import display
button = widgets.ButtonWidget(description="Click Me!")
display(button)

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

button.on_click(on_button_clicked)

Button clicked.

Button clicked.

Button clicked.

on_sumbit¶

The TextWidget also has a special on_submit event. The on_submit event fires when the user hits return.

In [4]:

text = widgets.TextWidget()
display(text)

def handle_submit(sender):
    print(text.value)

text.on_submit(handle_submit)

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Traitlet events¶

Widget properties are IPython traitlets and traitlets are eventful. To handle changes, the on_trait_change method of the widget can be used to register a callback. The doc string for on_trait_change can be seen below.

In [5]:

print(widgets.Widget.on_trait_change.__doc__)

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
        below).

        Parameters
        ----------
        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.
        

Signatures¶

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 [6]:

int_range = widgets.IntSliderWidget()
display(int_range)

def on_value_change(name, value):
    print(value)

int_range.on_trait_change(on_value_change, 'value')

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