To avoid a conflict between tool/test.py and a potential directory tool/test, I have renamed test.py to testit.py and made a corresponding change to test_all.py . Before I checked the change in, all tests, invoked from testit.py or test_all.py, ran. After checking in the changed files I noticed that the tests no longer ran. That is, testit.py would start but find no tests. Eventually I found out that everything worked as long as test.py or test.pyc remained in the tool directory. (So copying testit.py to test.py makes you again able to run the tests.) So I had checked my changes, finding everything working, checked in, and cleaned away test.py* . Thus, I didn't notice that something had gone astray before committing the changes. Now there's the question what's wrong with test.py/testit.py . I suspected an import of "test" from within the module but found no such import. I'm still searching for the cause of the problem, but if someone has a quick idea, it would be fine if he/she responded on the list or even fixed the problem. Stefan