The testsetup in Doc/library/decimal.rst is not enough for isolating the tests in respect to the other rst files.  Currently we have the following testsetup, without a testcleanup:

 .. testsetup:: *
 
    import decimal
    import math
    from decimal import *
    # make sure each group gets a fresh context
    setcontext(Context())

Without a testcleanup, the changes on the context will affect the other files that use the context (like Doc/library/statistics.rst).  We should better isolate the tests adding also a testcleanup:

.. testcleanup:: *

   # make sure other tests (outside this file) get a fresh context
   setcontext(Context())

I am opening a PR.

New changeset 2c0b5c664ba7d36dc09788d3778d5b33e3fa1bd0 by Stefan Krah (Marco Buttu) in branch 'master':
bpo-30055: add testcleanup to leave a fresh context (#1094)
https://github.com/python/cpython/commit/2c0b5c664ba7d36dc09788d3778d5b33e3fa1bd0

Thanks, looks good.  (I hope GitHub runs the doctests at Travis CI, locally most of them failed so I didn't bother to run the tests for this one.)

We are not executing the doctests on Travis CI, but I executed them locally.  That's why I realized there was a missed testcleanup.  The issue27200 wants to fix all doctests, and I actually completed the job, but three PRs are still opened, and I am waiting for review approval.  If you want to contribute to the reviews, look for 'bpo-27200' on the GitHub PRs.  I hope the doctest will be run on Travis as soon as possible.  Thanks, Marco

Ok great, maybe I'll take a look.