New issue 628: Tests shouldn't write files to disk within the yt source distribution. https://bitbucket.org/yt_analysis/yt/issue/628/tests-shouldnt-write-files-to... Nathan Goldbaum: Now that we distribute the tests with source distributions, we need to be careful when saving files to disk since the tests run from a location inside the installed yt package. Since many distributions install packages in a location where the user does not have write privileges (for example, Enthought Canopy, macports, and Ubuntu), this can cause test failures due to an OSError raised due to insufficient permissions. Instead, we should write to $TMPDIR, /tmp, use tempfile, or only save files to in-memory file objects.