Matplotlib figure drawn twice

Question

The function in the code snippet below should create a simple matplotlib figure and return it.

import matplotlib.pyplot as plt
import numpy as np


def random_figure():
    fig, axes = plt.subplots(2, 1, squeeze=True)
    for ax in axes:
        ax.plot(*np.random.rand(10, 2))
    return fig

What I don't understand is that the figure it is drawn more often than expected:

If I store the return value with my_fig = random_figure(), it is still drawn.
If I just run random_figure(), it is shown twice:

Is this a setting in matplotlib I'm unaware of?

I'm using jupyter notebook in vscode, if that's relevant.

PS, my expectation is that it behave like the function

def random_number():
    return np.random.rand()

which outputs nothing when the return value is stored with my_num = random_number().

Is there any additional configuration in your jupyter notebbok, like `%matplotlib inline`? — nonDucor, Jun 24 '22 at 17:23
vscode's `settings.json` has a few jupyter-settings, but nothing that seems related — ElRudi, Jun 24 '22 at 17:33
Ok I tried a bit more and `%matplotlib inline` has no effect, but `%matplotlib widget` indeed returns the expected number of plots. So I guess that's it. Still confused as to why the initial behaviour would make sense — ElRudi, Jun 24 '22 at 17:53

Wayne · Answer 1 · 2022-06-24T21:10:50.457

You are observing it printing twice because of the REPL-like design of the final referenced item of each Jupyter cell, see first two sentences here. In Jupyter this limits it to showing it once:

import matplotlib.pyplot as plt
import numpy as np


def random_figure():
    fig, axes = plt.subplots(2, 1, squeeze=True)
    for ax in axes:
        ax.plot(*np.random.rand(10, 2))
    return fig
random_figure();

Remember the end of a cell is special because of the REPL design for it. That is why you can just put a variable on the final line of a cell and get the value back without needing print(my_variable).
The code block above works because the last things the cell sees is ; which returns nothing.
With what you had, it returns the plot and so it shows it yet again after it's made it.

With my_fig = random_figure() you don't see it twice because what is returned by random_figure() gets consumed by the assignment.

The trick with adding the ; is often used to suppress object codes of plots (things that like <Figure size 432x288 with 0 Axes>') and sometimes more from showing up in addition to your plot in the output area, see example here, in particular Kynan's comment here explaining why it works.

Matplotlib figure drawn twice

1 Answers1