Sam Hooke

NiceGUI: tkinter error when updating pyplot

TL;DR: if you get the error can't delete Tcl command when updating a pyplot within NiceGUI, add close=False to ui.pyplot().

Problem §

NiceGUI provides a simple example of a static plot with pyplot.

I wanted to be able to update the plot, so taking that static example, I split it out into two steps:

  1. Create the plot as my_plot.
  2. Update my_plot by populating it with data.
# Create plot
with ui.pyplot(
        figsize=(3, 2),
    ) as my_plot:
    pass

# Update plot
with my_plot:
    x = np.linspace(0.0, 5.0)
    y = np.cos(2 * np.pi * x) * np.exp(-x)
    plt.plot(x, y, '-')

ui.update(my_plot)

However, this gave a long Python traceback with the final error line of:

_tkinter.TclError: can't delete Tcl command

Solution §

The solution is to add close=False inside the call to ui.pyplot()1.

# Create plot
with ui.pyplot(
        figsize=(3, 2),
        close=False,
    ) as my_plot:
    pass

This is actually documented in the static example linked above, in the usage of the optional close argument, but is very easy to miss. See the following quote (emphasis added):

close: whether the figure should be closed after exiting the context; set to False if you want to update it later (default: True)

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