This is a common cause for annoyance when people first start working with yt. What this does is make the unit tests run as if they were in the "yt" folder, so you need to say:
$ nosetests data_objects/tests/test_slice.py
in order to run the tests in test_slice.py.
With this change you would instead do:
$ nosetests yt/data_objects/tests/test_slice.py
to do the same thing. I think the former is very confusing for new developers and is something we need to teach people every time we onboard them.
Note that using module-style test specification works the same before and after this PR:
$ nosetests yt.data_objects.tests.test_slice
I think this was originally set because we used to have outdated tests in the tests directory at the root of the repo that have since been removed. I think the intention was to avoid running tests in the tests directory. Now that those tests aren't there anymore, we can get rid of the workaround.
The former is confusing for me, too. I didn't realize we could fix this. Thank you.