What prevents a figure showing before notebook cell completion?

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
35/100
Issue type
Bug
Clarity
Needs clarification
Activity status
Stale
Tech stack
jupyter, jupyter-notebook, python
Domain
data-visualization, frontend

Research direction

The issue names no source files or tests. Start by reproducing the single-cell and split-cell examples with the %matplotlib widget backend, then compare the fig.canvas.draw() behavior with the Qt backend; done means the cause and an appropriate rendering behavior are established.

Written by the indexing model from the issue text.

Description

My issue boils down to:

"Why does the figure not show immediately, but instead waits until the cell has completed running?"

%matplotlib widget
import matplotlib.pyplot as plt
from time import sleep
fig = plt.figure()
plt.plot([1,2,3])
sleep(2)
Figure only showing at end of sleep/cell

This behaviour is preventing fun uses, like dynamic plotting in a single cell:

%matplotlib widget
from matplotlib import pyplot as plt
from time import sleep
import numpy as np
fig = plt.figure()

for i in range(10):
    x, y = np.random.random(2)
    plt.scatter(x, y)
    fig.canvas.draw()
    sleep(0.1)

The previous code works if one splits it into two cells, placing the figure() call in the first one:

# Cell 1
%matplotlib widget
from matplotlib import pyplot as plt
from time import sleep
import numpy as np
fig = plt.figure()

# Cell 2
for i in range(10):
    x, y = np.random.random(2)
    plt.scatter(x, y)
    fig.canvas.draw()
    sleep(0.1)
Dynamic plotting if using two cells

Edit: I also note that in the qt backend, there figure and plot are shown immediately, in contrast to the ipympl behaviour.

Dominant language
Jupyter Notebook
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