I recently tried to port one of my packages to Python 3.6 and unit tests that worked in Python 2.7, 3.3-3.5 began failing in 3.6.

I originally thought it was a problem with coverage, but it turns out it was not. The full thread is: https://bitbucket.org/ned/coveragepy/issues/545/coverage-fails-on-python-36-travis-build

The highlight is this unit test causes Python to exit with status code 120 (which fails a Travis build):

class ParseArgumentsTestCase(unittest.TestCase):
    def test_no_arguments(self):  # type: () -> None
        with self.assertRaises(SystemExit):
            # Suppress argparse stderr.
            class NullWriter:
                def write(self, s):  # type: (str) -> None
                    pass

            sys.stderr = NullWriter()
            parse_arguments()

Ned found this corresponding note in the Python 3.6 release notes:

Changed in version 3.6: If an error occurs in the cleanup after the Python interpreter has caught SystemExit (such as an error flushing buffered data in the standard streams), the exit status is changed to 120.

If this is indeed, correct behavior and Python 3.6 is catching something incorrect (I agree this is not necessarily the most elegant unit test) then I am happy to close this issue. Just wanted to be sure I at least reported it in case this was a real issue.

My guess would be that the problem that your NullWriter doesn't have a flush method.  But I'm not familiar with this change, so I'm just guessing.

Behaviour changed in #5319. Add author Martin.

David is right. The 120 code was added in Issue 5319, as a way of indicating a problem in the final stages of the interpreter exiting. The two conditions that trigger this are calling the flush() method on sys.stdout and sys.stderr. If you add a dummy flush() implementation, it no longer exits with 120:

>>> subprocess.call((sys.executable, "-c", """
... class NullWriter:
...     def write(self, s): pass
... import sys; sys.stderr = NullWriter()"""))
120
>>> subprocess.call((sys.executable, "-c", """
... class NullWriter:
...     def write(self, s): pass
...     def flush(self): pass
... import sys; sys.stderr = NullWriter()"""))
0

It does not seem to be explicitly documented what you can set sys.stderr to, but I always thought it is safest to use an io.TextIOBase implementation. I would suggest to derive your NullWriter class from TextIOBase; that way you get a default flush() implementation for free.

Other options are to use StringIO() if you are not expecting too much output, or open(os.devnull, "w"). See also Issue 28864 about adding a predefined class like NullWriter to Python.

Having said all that, perhaps it would be reasonable to tolerate a missing flush() method, and not treat this as an error.

Stepping back a bit, I also suggest restoring sys.stderr after the test. Otherwise, you risk missing an error message. Try contextlib.redirect_stderr().

> Having said all that, perhaps it would be reasonable to tolerate a missing flush() method, and not treat this as an error.

In all cases the flush() method is called unconditionally. I think it can be considered as mandatory part of the writer protocol.

This issue is not a bug in Python, but a bug in the author's code. I suggest to close the issue, since enough clues have been provided to fix the author's issue.

I'm completely fine with closing this issue. Thanks for the help everyone.

If someone else doesn't close it in a couple days, I'll do it.