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pytest plugin to help with testing figures output from Matplotlib

Project description

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A plugin to faciliate image comparison for [Matplotlib]( figures in pytest (which
uses some of the Matplotlib image comparison functions behind the scenes).

Matplotlib includes a number of test utilities and decorators, but these are geared towards the [nose]( testing framework. Pytest-mpl makes it easy to compare figures produced by tests to reference images when using [pytest](

For each figure to test, the reference image is substracted 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 in Matplotlib).

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


This plugin is compatible with Python 2.6, 2.7, and 3.3 and later, and requires [pytest](, [matplotlib]( and
[nose]( to be installed (nose is required by Matplotlib).

To install, you can do:

pip install pytest-mpl

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

py.test --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 ...


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``

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

py.test --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 ``py.test``. 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) in the same directory as the tests (or you can generate them there
directly). You can then run the tests simply with:

py.test --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.


The ``@pytest.mark.mpl_image_compare`` marker can take an argument which is the
RMS tolerance (which defaults to 10):

def test_image():

You can also pass keyword arguments to ``savefig`` by using ``savefig_kwargs``:

def test_image():

Other options include the name of 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):

def test_image():

The baseline directory in the decorator above will be interpreted as being
relative to the test file.

Finally, you can also set a custom baseline directory globally when running
tests by running ``py.test`` with:

py.test --mpl --mpl-baseline-dir=baseline_images

This directory will be interpreted as being relative to where the tests are
run. 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

Running the tests

To run the tests, first install the plugin then do:

cd tests
py.test --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.

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