Ensure that the Grade field data stays in clean_data.
This prevents a bug that happens when you pick an invalid entry in the choice a grade field. If this bug was triggered the value of the survey_record.grade was used. Now however a proper ValidationError is raised.
Coverage: code coverage testing for Python
Coverage.py is a Python module that measures code coverage during test execution.
It uses the code analysis tools and tracing hooks provided in the Python standard
library to determine which lines are executable, and which have been executed.
For more information, see http://nedbatchelder.com/code/modules/coverage.html