Ipywidgets: print statement of ipywidgets not work in JupyterLab

[Honghe]

Phenomenon:

• print statement not work in JupyterLab(can not see any output), but work in Jupyter Notebook

Environment:

• Ubuntu 16.04
• Chrome v66
• JupyterLab 0.32.1
• Python 3.6.5

Code:

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

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

button.on_click(on_button_clicked)

[camillescott]

I'm also running into the same problem, similar environment.

[jasongrout]

You'll need to use the output capturing mechanism in ipywidgets:

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

output = widgets.Output()

@output.capture()
def on_button_clicked(b):
    print("Button clicked.")

button.on_click(on_button_clicked)
display(output)

[jasongrout]

Another advantage of this method is that it lets you put that output where you want, not just in that cell.

[ankostis]

I remember there is jupyter document describing the necessity for such breaking change, but google doesn't help me...
Since this is a landing-issue, @jasongrout can you provide some architectural link on what is going?

[jasongrout]

https://github.com/jupyterlab/jupyterlab/issues/3151#issuecomment-339476572 and https://github.com/jupyterlab/jupyterlab/issues/1254 give some more context on this.

[jasongrout]

Closing as answered.

[chimaobiv]

@jasongrout, can @output.capture be used inside a class? And how or where do you place this?

[zwelz3]

@jasongrout, can @output.capture be used inside a class? And how or where do you place this?

Stumbled across this. Here is a naive way...

```import ipywidgets as widgets
from IPython.display import display

class ExampleButton(widgets.Button):
output = widgets.Output()

@output.capture()
def on_button_clicked(b):
    print("Button clicked.")

button = ExampleButton(description="Click Me!")
display(button)

button.on_click(ExampleButton.on_button_clicked)
display(ExampleButton.output)