How to patch a matplotlib plot function

Question

I am doing a unit test (using pytest) of a plot function that calls a plot method via mplfiance (essentially a candlestick plot using matplotlib).

I would like to path the call of the plot function in my test so that it wont actually create the graph. I would just like to see if the parameters are correct when i call the function.

For example

def plot_something(param1, param2):
    # doing something with param1 and para2
    param1 += 1
    param2 *= 2
    # below is the actual plotting where I would like to patch
    make_plot(param1, param2)

def make_plot(param1, param2):
    # make plot here
    plot(param1, param2)

How can I create a test case to just make sure 1) both plot_something() and make_plot() are called. 2) the parameters are asserted 3) no actual graph is actually display on the screen?

I suppose patching is the way to go to replace make_plot() call with something in the test function. But I don't know how exactly.

Many thanks for your advice.

I didn't use the module myself yet, but does this help you: docs.python.org/3/library/unittest.mock.html? — Dr. V
– Dr. V, Commented May 24, 2020 at 20:24

Dr. V · Accepted Answer · 2020-05-24 20:40:02Z

2

This made me curious to test the mock module myself. Here is my code that seems to work fine:

from unittest import TestCase, main
from unittest.mock import Mock
import pylab

def plot_something():
    x = [1,2,3]
    y = [3,1,2]
    pylab.plot(x, y, "o-")
    pylab.show()


class TestPlot(TestCase):
    def test_plot_something(self):
        # save the original methods for later use (if required)
        pyplot, pyshow = pylab.plot, pylab.show

        pylab.plot, pylab.show = Mock(), Mock()
        plot_something()

        pylab.plot.assert_called_once_with([1,2,3], [3,1,2], "o-")
        pylab.show.assert_called_once_with()

        # restore the original ones again
        pylab.plot, pylab.show = pyplot, pyshow

    def test_plot_again_but_plot_really(self):
        # this actually plots
        plot_something()


if __name__ == "__main__":
    main()

So, here I even patched the pylab methods. It's of course even easier to intervene on the higher level (whether plot_someting is called).

answered May 24, 2020 at 20:40

Dr. V

1,9841 gold badge15 silver badges17 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

J_yang Over a year ago

Thanks. While I couldn't directly incorporate that, I took the idea and use @patch decorator for my testcase function.

Collectives™ on Stack Overflow

How to patch a matplotlib plot function

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related