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TestProgram uses defaultTestLoader to load tests. Calling discover() on a TestLoader sets _top_level_dir. Calling discover with no top_level_dir implictly sets _top_level_dir but only the first time: the second time it is called with a different start_dir the prior one is used.
This leads to errors like:
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ERROR: test_run_list_failed_import (testtools.tests.test_run.TestRun)
testtools.tests.test_run.TestRun.test_run_list_failed_import
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_StringException: Traceback (most recent call last):
 File "/home/robertc/personal/testtools/testtools/tests/test_run.py", line 175, in test_run_list_failed_import
 run.main, ['prog', 'discover', '-l', broken.package.base, '*.py'], out)
 File "/home/robertc/personal/testtools/testtools/testcase.py", line 420, in assertRaises
 self.assertThat(our_callable, matcher)
 File "/home/robertc/personal/testtools/testtools/testcase.py", line 431, in assertThat
 mismatch_error = self._matchHelper(matchee, matcher, message, verbose)
 File "/home/robertc/personal/testtools/testtools/testcase.py", line 481, in _matchHelper
 mismatch = matcher.match(matchee)
 File "/home/robertc/personal/testtools/testtools/matchers/_exception.py", line 108, in match
 mismatch = self.exception_matcher.match(exc_info)
 File "/home/robertc/personal/testtools/testtools/matchers/_higherorder.py", line 62, in match
 mismatch = matcher.match(matchee)
 File "/home/robertc/personal/testtools/testtools/testcase.py", line 412, in match
 reraise(*matchee)
 File "/home/robertc/personal/testtools/testtools/matchers/_exception.py", line 101, in match
 result = matchee()
 File "/home/robertc/personal/testtools/testtools/testcase.py", line 965, in __call__
 return self._callable_object(*self._args, **self._kwargs)
 File "/home/robertc/personal/testtools/testtools/run.py", line 413, in main
 stdout=stdout)
 File "/home/robertc/personal/testtools/testtools/run.py", line 208, in __init__
 self.parseArgs(argv)
 File "/home/robertc/personal/testtools/testtools/run.py", line 252, in parseArgs
 self._do_discovery(argv[2:])
 File "/home/robertc/personal/testtools/testtools/run.py", line 368, in _do_discovery
 loaded = loader.discover(self.start, self.pattern, self.top)
 File "/home/robertc/.virtualenvs/testtools-2.7/local/lib/python2.7/site-packages/unittest2/loader.py", line 353, in discover
 raise ImportError('Start directory is not importable: %r' % start_dir)
ImportError: Start directory is not importable: '/tmp/tmpac4uwI'
When a test tries to test the behaviour of TestProgram (or a subclass thereof).
The fix is straight forward: at the end of discover restore the value of _top_level_dir that it had at entry.