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.