Skip to main content

pytest plugin to help with testing figures output from Matplotlib

Project description


This is a plugin to facilitate image comparison for Matplotlib figures in pytest.

For each figure to test, the reference image is subtracted from the generated image, and the RMS of the residual is compared to a user-specified tolerance. If the residual is too large, the test will fail (this is implemented using helper functions from matplotlib.testing).

For more information on how to write tests to do this, see the Using section below.


This plugin is compatible with Python 3.6 and later, and requires pytest and matplotlib to be installed.

To install, you can do:

pip install pytest-mpl

You can check that the plugin is registered with pytest by doing:

pytest --version

which will show a list of plugins:

This is pytest version 2.7.1, imported from ...
setuptools registered plugins:
  pytest-mpl-0.1 at ...


With Baseline Images

To use, you simply need to mark the function where you want to compare images using @pytest.mark.mpl_image_compare, and make sure that the function returns a Matplotlib figure (or any figure object that has a savefig method):

import pytest
import matplotlib.pyplot as plt

def test_succeeds():
    fig = plt.figure()
    ax = fig.add_subplot(1,1,1)
    return fig

To generate the baseline images, run the tests with the --mpl-generate-path option with the name of the directory where the generated images should be placed:

pytest --mpl-generate-path=baseline

If the directory does not exist, it will be created. The directory will be interpreted as being relative to where you are running pytest. Once you are happy with the generated images, you should move them to a sub-directory called baseline relative to the test files (this name is configurable, see below). You can also generate the baseline image directly in the right directory.

With a Hash Library

Instead of comparing to baseline images, you can instead compare against a JSON library of SHA-256 hashes. This has the advantage of not having to check baseline images into the repository with the tests, or download them from a remote source.

The hash library can be generated with --mpl-generate-hash-library=path_to_file.json. The hash library to be used can either be specified via the --mpl-hash-library= command line argument, or via the hash_library= keyword argument to the @pytest.mark.mpl_image_compare decorator.

When generating a hash library, the tests will also be run as usual against the existing hash library specified by --mpl-hash-library or the keyword argument. However, generating baseline images will always result in the tests being skipped.

Hybrid Mode: Hashes and Images

It is possible to configure both hashes and baseline images. In this scenario only the hash comparison can determine the test result. If the hash comparison fails, the test will fail, however a comparison to the baseline image will be carried out so the actual difference can be seen. If the hash comparison passes, the comparison to the baseline image is skipped (unless results always is configured).

This is especially useful if the baseline images are external to the repository containing the tests, and are accessed via HTTP. In this situation, if the hashes match, the baseline images won’t be retrieved, saving time and bandwidth. Also, it allows the tests to be modified and the hashes updated to reflect the changes without having to modify the external images.

Running Tests

Once tests are written with baseline images, a hash library, or both to compare against, the tests can be run with:

pytest --mpl

and the tests will pass if the images are the same. If you omit the --mpl option, the tests will run but will only check that the code runs, without checking the output images.

Generating a Test Summary

By specifying the --mpl-generate-summary=html CLI argument, a HTML summary page will be generated showing the test result, log entry and generated result image. When in the (default) image comparison mode, the baseline image, diff image and RMS (if any), and tolerance of each test will also be shown. When in the hash comparison mode, the baseline hash and result hash will also be shown. When in hybrid mode, all of these are included.

When generating a HTML summary, the --mpl-results-always option is automatically applied (see section below). Therefore images for passing tests will also be shown.

html all

html filter

html result

As well as html, basic-html can be specified for an alternative HTML summary which does not rely on JavaScript or external resources. A json summary can also be saved. Multiple options can be specified comma-separated.



The RMS tolerance for the image comparison (which defaults to 2) can be specified in the mpl_image_compare decorator with the tolerance argument:

def test_image():

Savefig options

You can pass keyword arguments to savefig by using savefig_kwargs in the mpl_image_compare decorator:

def test_image():

Baseline images

The baseline directory (which defaults to baseline ) and the filename of the plot (which defaults to the name of the test with a .png suffix) can be customized with the baseline_dir and filename arguments in the mpl_image_compare decorator:

def test_image():

The baseline directory in the decorator above will be interpreted as being relative to the test file. Note that the baseline directory can also be a URL (which should start with http:// or https:// and end in a slash). If you want to specify mirrors, set baseline_dir to a comma-separated list of URLs (real commas in the URL should be encoded as %2C).

Finally, you can also set a custom baseline directory globally when running tests by running pytest with:

pytest --mpl --mpl-baseline-path=baseline_images

This directory will be interpreted as being relative to where pytest is run. However, if the --mpl-baseline-relative option is also included, this directory will be interpreted as being relative to the current test directory. In addition, if both this option and the baseline_dir option in the mpl_image_compare decorator are used, the one in the decorator takes precedence.

Results always

By default, result images are only saved for tests that fail. Passing --mpl-results-always to pytest will force result images to be saved for all tests, even for tests that pass.

When in hybrid mode, even if a test passes hash comparison, a comparison to the baseline image will also be carried out, with the baseline image and diff image (if image comparison fails) saved for all tests. This secondary comparison will not affect the success status of the test.

This option is useful for always comparing the result images against the baseline images, while only assessing the tests against the hash library. If you only update your baseline images after merging a PR, this option means that the generated summary will always show how the PR affects the baseline images, with the success status of each test (based on the hash library) also shown in the generated summary. This option is applied automatically when generating a HTML summary.

When the --mpl-results-always option is active, and some hash comparison tests are performed, a hash library containing all the result hashes will also be saved to the root of the results directory. The filename will be extracted from --mpl-generate-hash-library, --mpl-hash-library or hash_library= in that order.

Base style

By default, tests will be run using the Matplotlib ‘classic’ style (ignoring any locally defined RC parameters). This can be overridden by using the style argument:

def test_image():

Package version dependencies

Different versions of Matplotlib and FreeType may result in slightly different images. When testing on multiple platforms or as part of a pipeline, it is important to ensure that the versions of these packages match the versions used to generate the images used for comparison. It can be useful to pin versions of Matplotlib and FreeType so as to avoid automatic updates that fail tests.

Removing text

If you are running a test for which you are not interested in comparing the text labels, you can use the remove_text argument to the decorator:

def test_image():

This will make the test insensitive to changes in e.g. the freetype library.

Test failure example

If the images produced by the tests are correct, then the test will pass, but if they are not, the test will fail with a message similar to the following:

E               Exception: Error: Image files did not match.
E                 RMS Value: 142.2287807767823
E                 Expected:
E                   /var/folders/zy/t1l3sx310d3d6p0kyxqzlrnr0000gr/T/tmp4h4oxr7y/baseline-coords_overlay_auto_coord_meta.png
E                 Actual:
E                   /var/folders/zy/t1l3sx310d3d6p0kyxqzlrnr0000gr/T/tmp4h4oxr7y/coords_overlay_auto_coord_meta.png
E                 Difference:
E                   /var/folders/zy/t1l3sx310d3d6p0kyxqzlrnr0000gr/T/tmp4h4oxr7y/coords_overlay_auto_coord_meta-failed-diff.png
E                 Tolerance:
E                   10

The image paths included in the exception are then available for inspection:







In this case, the differences are very clear, while in some cases it may be necessary to use the difference image, or blink the expected and actual images, in order to see what changed.

The default tolerance is 2, which is very strict. In some cases, you may want to relax this to account for differences in fonts across different systems.

By default, the expected, actual and difference files are written to a temporary directory with a non-deterministic path. If you want to instead write them to a specific directory, you can use:

pytest --mpl --mpl-results-path=results

The results directory will then contain one sub-directory per test, and each sub-directory will contain the three files mentioned above. If you are using a continuous integration service, you can then use the option to upload artifacts to upload these results to somewhere where you can view them. For more information, see:

Running the tests for pytest-mpl

If you are contributing some changes and want to run the tests, first install the latest version of the plugin then do:

cd tests
pytest --mpl

The reason for having to install the plugin first is to ensure that the plugin is correctly loaded as part of the test suite.

Project details

Download files

Download the file for your platform. If you're not sure which to choose, learn more about installing packages.

Source Distribution

pytest-mpl-0.16.1.tar.gz (663.6 kB view hashes)

Uploaded source

Built Distribution

pytest_mpl-0.16.1-py3-none-any.whl (28.7 kB view hashes)

Uploaded py3

Supported by

AWS AWS Cloud computing and Security Sponsor Datadog Datadog Monitoring Fastly Fastly CDN Google Google Download Analytics Microsoft Microsoft PSF Sponsor Pingdom Pingdom Monitoring Sentry Sentry Error logging StatusPage StatusPage Status page