DOC: add section on setting random number seeds

matplotlib · dstansby · Aug 30, 2017 · Aug 30, 2017 · Aug 30, 2017 · 3833ff0f0ffacdbfa08ca68559aa50d4904abfc9
commit 3833ff0f0ffacdbfa08ca68559aa50d4904abfc9
diff --git a/doc/devel/testing.rst b/doc/devel/testing.rst
@@ -139,6 +139,23 @@ execution (such as created figures or modified rc params). The pytest fixture
 :func:`~matplotlib.testing.conftest.mpl_test_settings` will automatically clean
 these up; there is no need to do anything further.

+Random data in tests
+--------------------
+
+Random data can is a very convenient way to generate data for examples,
+however the randomness is problematic for testing (as the tests
+must be deterministic!).  To work around this set the seed in each test.
+For numpy use::
+
+  import numpy as np
+  np.random.seed(19680801)
+
+and Python's random number generator::
+
+  import random
+  random.seed(19680801)
+
+The seed is John Hunter's birthday.

 Writing an image comparison test
 --------------------------------