'If the specified modules have never been imported, and import
prevention is requested, L{ensureNotImported} makes sure they will not
be imported in the future.'

def test_ensureWhenNotImported(self):

    modules = {}
    self.patch(sys, 'modules', modules)
    ensureNotImported(['m1', 'm2'], 'A    message.', preventImports=['m1', 'm2', 'm3'])
    self.assertEqual(modules, {'m1': None, 'm2': None, 'm3': None})
