Widget Events

Special events

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from __future__ import print_function

The Button is not used to represent a data type. Instead the button widget is used to handle mouse clicks. The on_click method of the Button 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.

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from IPython.html import widgets


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.

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

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



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

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text = widgets.Text()

def handle_submit(sender):


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.

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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 IntSlider's value as it is changed can be seen below.

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int_range = widgets.IntSlider()

def on_value_change(name, value):

int_range.on_trait_change(on_value_change, 'value')

Linking Widgets

Often, you may want to simply link widget attributes together. Synchronization of attributes can be done in a simpler way than by using bare traitlets events.

The first method is to use the link and directional_link functions from the traitlets module.

Linking traitlets attributes from the server side

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from IPython.utils import traitlets
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caption = widgets.Latex(value = 'The values of slider1, slider2 and slider3 are synchronized')
sliders1, slider2, slider3 = widgets.IntSlider(description='Slider 1'),\
                             widgets.IntSlider(description='Slider 2'),\
                             widgets.IntSlider(description='Slider 3')
l = traitlets.link((sliders1, 'value'), (slider2, 'value'), (slider3, 'value'))
display(caption, sliders1, slider2, slider3)
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caption = widgets.Latex(value = 'Changes in source values are reflected in target1 and target2')
source, target1, target2 = widgets.IntSlider(description='Source'),\
                           widgets.IntSlider(description='Target 1'),\
                           widgets.IntSlider(description='Target 2')
traitlets.dlink((source, 'value'), (target1, 'value'), (target2, 'value'))
display(caption, source, target1, target2)

Function traitlets.link returns a Link object. The link can be broken by calling the unlink method.

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# l.unlink()

Linking widgets attributes from the client side

When synchronizing traitlets attributes, you may experience a lag because of the latency dues to the rountrip to the server side. You can also directly link widgets attributes, either in a unidirectional or a bidirectional fashion using the link widgets.

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caption = widgets.Latex(value = 'The values of range1, range2 and range3 are synchronized')
range1, range2, range3 = widgets.IntSlider(description='Range 1'),\
                         widgets.IntSlider(description='Range 2'),\
                         widgets.IntSlider(description='Range 3')
l = widgets.jslink((range1, 'value'), (range2, 'value'), (range3, 'value'))
display(caption, range1, range2, range3)
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caption = widgets.Latex(value = 'Changes in source_range values are reflected in target_range1 and target_range2')
source_range, target_range1, target_range2 = widgets.IntSlider(description='Source range'),\
                                             widgets.IntSlider(description='Target range 1'),\
                                             widgets.IntSlider(description='Target range 2')
widgets.jsdlink((source_range, 'value'), (target_range1, 'value'), (target_range2, 'value'))
display(caption, source_range, target_range1, target_range2)

Function widgets.jslink returns a Link widget. The link can be broken by calling the unlink method.

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# l.unlink()