Decorator that runs the decorated test in a fresh interpreter subprocess, in isolation, so that it does not share global or interpreter state with the rest of the test run. It can decorate a test method or a whole TestCase subclass. Decorated methods must take no extra arguments. A failure, error or skip in the subprocess is reported for the corresponding test, and individual subtests that fail or are skipped are reported individually. A reported failure or error shows the original subprocess traceback as the cause of the exception.