@bblay the filename is **relative** to where the test was run from. So if you perform the following you can easily generate an ```IndexError```:
```
Linux> cd .../lib/iris/tests/unit/cube
Linux> python test_Cube.py
======================================================================
ERROR: test_checksum_ignores_masked_values (__main__.Test_xml)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "test_Cube.py", line 35, in test_checksum_ignores_masked_values
    self.assertCML(cube, self.get_result_path('mask.cml'))
  File "/net/home/h05/itwl/projects/git/iris/lib/iris/tests/unit/__init__.py", line 39, in get_result_path
    path = calling_filename.rsplit("iris", 1)[1].rsplit("unit", 1)[1]
IndexError: list index out of range

----------------------------------------------------------------------
Ran 1 test in 0.187s

FAILED (errors=1)
```