What prevents a figure showing before notebook cell completion?

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)

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)

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

この issue ではソースファイルもテストも指定されていません。まず、%matplotlib widget バックエンドを使って単一セルと分割セルの例を再現し、次に Qt バックエンドでの fig.canvas.draw() の動作と比較してください。原因と適切なレンダリング動作が確立されれば完了です。