Unittest failed with sys.exit

I'm trying to run my tests with unittest. Here is my structure :

projectname/
    projectname/
        foo.py
        bar.py
    tests/
        test_foo.py
        test_bar.py

I run it with :

cd tests/
python -m unittest discover

But in one file, for example foo.py, I use a sys.exit(0), and unittest doesn't really like it :

$ python -m unittest discover
....E.
======================================================================
ERROR: test_foo (...)
----------------------------------------------------------------------
Traceback (most recent call last):
...
...
File "/home/.../projectname/foo.py", line 12, in write
    sys.exit(0)
SystemExit: 0

----------------------------------------------------------------------
Ran 6 tests in 0.018s

FAILED (errors=1)

The sys.exit() use is voluntary, I can't remove it. I know there is an option called exit for the unittest.main function :

if __name__ == "__main__":
    unittest.main(exit=False)

But I want to test all the files in the tests directory. Another way is to do :

if __name__ == '__main__':
    tests = unittest.TestLoader().discover('.')
    unittest.TextTestRunner(verbosity=1).run(tests)

It finds all the test_ files, but the sys.exit() makes unittest crash.