DocTestCase.tearDown destructively clears its DocTest instance's globs,
preventing the test from being run repeatedly (such as with trial
--until-failure).

There's a fix for this in zope.testing's version of doctest, which
resets the globs instead:

http://svn.zope.org/?view=rev&rev=39023

Addendum:  This appears to be a regression in r36809 (Python 2.4+).

Is repeating a test with the same TestCase instance ever safe?  It'd be
better to create a new instance and run that.

If any of the variables in test.globs are changed by the test (e.g.
appending to a list), then rerunning the test will not necessarily give
the same result.

While the zope change allows tests that have immutable globals or don't
change their globals to function, it also lets other tests "almost"
work, and could lead to new bugs that are difficult to track down.

> If any of the variables in test.globs are changed by the test (e.g. 
appending to a list), then rerunning the test will not necessarily give
the same result.

This is true, but modifying the globals such that subsequent runs of the
same test can break equally affects subsequent runs of any other tests
that use that module:  such a test is already broken (unsafe to run with
other tests) to begin with, independent of the DocTestCase.tearDown issue.

If I create a test case with a command like:

   test = DocFileSuite('foo.txt', globs={'somelist': [42]})

The doctest isn't doing anything wrong if it modifies somelist.

Furthermore, Glyph has said he thinks the current --until-failure
behaviour in trial is a mistake: http://glyf.livejournal.com/72505.html

Well, whether that code is wrong depends on whether your project policy
wants repeatable tests or not.  A repeatable and arguably more idiomatic
way of writing that example is to give DocFileSuite a setUp function
which initializes any special globals required by the test.

In any case, DocTestCase allowing non-repeatable tests (which i don't
think are common) is no reason to disallow repeatable tests, which e.g.
at least Zope considers important enough to motivate fixing this issue.
 (Zope is also the source of the code being fixed, if i'm not mistaken.)

FWIW, the python testsuite needs repeatable tests, when running in
"reference leaks" mode.

See also r62100, where a DocTestSuite construction had to be moved into
the repeated function.

For the record, this bug also breaks zope.testrunner's --repeat option, if you have any doctests in your test suite that rely on test.globs not going away.

I've reproduced this on 3.11 as well.

The patch here needs to be converted to a GitHub PR and then tested and reviewed.

The patch on issue9736 has a unit test as well, which should be included (because the pjd's patch doesn't have one).

I have created a PR that combines the patch here with the patch from issue9736.

New changeset 7ba7eae50803b11766421cb8aae1780058a57e2b by Daniël van Noord in branch 'main':
bpo-2604: Make doctest.DocTestCase reset globs in teardown (GH-31932)
https://github.com/python/cpython/commit/7ba7eae50803b11766421cb8aae1780058a57e2b

New changeset 3c6019035f16b673cf0f0be6918f7d5493e5690e by Miss Islington (bot) in branch '3.9':
bpo-2604: Make doctest.DocTestCase reset globs in teardown (GH-31932)
https://github.com/python/cpython/commit/3c6019035f16b673cf0f0be6918f7d5493e5690e

New changeset f163ad22d3321cb9bb4e6cbaac5a723444641565 by Miss Islington (bot) in branch '3.10':
bpo-2604: Make doctest.DocTestCase reset globs in teardown (GH-31932)
https://github.com/python/cpython/commit/f163ad22d3321cb9bb4e6cbaac5a723444641565

After 14 years this bug is finally fixed. Thanks everyone for the patches and discussion.

Thanks! 😄️