Output widget's clear_output not clearing when using append_stdout

I stumbled upon a question on Stack Overflow where the first line printed in the output widget doesn’t get cleared by the first call to clear_output.

import ipywidgets as widgets

def clear_output():
    change_output_button = widgets.Button(description="Change output?")
    the_output = widgets.Output()
    clear_output_widget = widgets.VBox([change_output_button, the_output])
    clear_output_widget.click_count = 0

    def button_clicked(_button):
        clear_output_widget.click_count += 1
        the_output.clear_output()
        the_output.append_stdout(f"button clicked {clear_output_widget.click_count} times.")

    change_output_button.on_click(button_clicked)

    return clear_output_widget`

Replacing append_stdout with the context manager fixes the issue:

    def button_clicked(_button):
        clear_output_widget.click_count += 1
        the_output.clear_output()
        with the_output:
            print(f"button clicked {clear_output_widget.click_count} times.")

but I am unsure of the reasons behind the erratic behaviour. I tried to look at the source of append_stdout but I haven’t found it so far. Happy to submit a PR once I get to the bottom of this issue.

[wonsjb]

Hi!

I did have the same issue. But turned out that I could not even use the context manager, as I was trying to clear / update the Output from a thread, so that my notebook would still be available, and the output would update with data from rest calls. And that does not work very well. I assume that the with out: is using some static variable that don’t play well with threading.

I worked around it by using a out = widgets.HTML() and calling out.value = f"<pref>{my_message}</pre>" in the thread function.

[pnsvk]

workaround for this is posted on another thread: https://github.com/jupyter-widgets/ipywidgets/issues/3260#issuecomment-907715980