on.

       If the output doesn't match, then a DocTestFailure is raised:

         >>> test = DocTestParser().get_doctest('''
         ...      >>> x = 1
         ...      >>> x
         ...      2
         ...      ''', {}, 'foo', 'foo.py', 0)

         >>> try:
         ...    runner.run(test)
         ... except DocTestFailure as f:
         ...    failure = f

       DocTestFailure objects provide access to the test:

         >>> failure.test is test
         True

       As well as to the example:

         >>> failure.example.want
         '2\n'

       and the actual output:

         >>> failure.got
         '1\n'

       If a failure or error occurs, the globals are left intact:

         >>> del test.globs['__builtins__']
         >>> test.globs
         {'x': 1}

         >>> test = DocTestParser().get_doctest('''
         ...      >>> x = 2
         ...      >>> raise KeyError
         ...      ''', {}, 'foo', 'foo.py', 0)

         >>> runner.run(test)
         Traceback (most recent call last):
         ...
         doctest.UnexpectedException: <DocTest foo from foo.py:0 (2 examples)>

         >>> del test.globs['__builtins__']
         >>> test.globs
         {'x': 2}

       But the globals are cleared if there is no error:

         >>> test = DocTestParser().get_doctest('''
         ...      >>> x = 2
         ...      ''', {}, 'foo', 'foo.py', 0)

         >>> runner.run(test)
         TestResults(failed=0, attempted=1)

         >>> test.globs
         {}

       NTc