How should one test matplotlib plotting with tox and py.test?

Question

How should one test matplotlib plotting with tox and py.test?

My files are:

testtox /
  - testtox.py
  - tox.ini
  - setup.py

testtox.py contains:

import matplotlib.pyplot as plt


def plotting():
    """Plot.

    Examples
    --------
    >>> plt.ion()
    >>> plotting()

    """
    plt.plot([1, 2, 1, 3])

tox.ini:

[tox]
envlist = py27

[testenv:py27]
sitepackages = True
commands = py.test --doctest-modules testtox.py
deps = pytest

setup.py:

from setuptools import setup

setup(name='testtox', py_modules=['testtox'])

py.test on its own works ok:

$ py.test --doctest-modules testtox.py

In this case the plot window is briefly flashed.

tox produces an error on the DISPLAY variable:

$ tox
[...cut ...]
=================================================== FAILURES ====================================================
__________________________________________ [doctest] testtox.plotting ___________________________________________
005     """Plot.
006 
007     Examples
008     --------
009     >>> plt.ion()
010     >>> plotting()
UNEXPECTED EXCEPTION: RuntimeError(u'Invalid DISPLAY variable',)
Traceback (most recent call last):
[...]

  File "/usr/local/lib/python2.7/dist-packages/matplotlib/backends/backend_qt5.py", line 138, in _create_qApp
    raise RuntimeError('Invalid DISPLAY variable')

RuntimeError: Invalid DISPLAY variable

This error happened with the most recent version of tox (2.1.1). An older version did not produce the error.

Answer 1

Setting the DISPLAY environment variable in tox.ini seems to do the trick in this case:

[tox]
envlist = py27

[testenv:py27]
setenv = 
    DISPLAY = :0
sitepackages = True
commands = py.test --doctest-modules testtox.py
deps = pytest