def test_sessionUIDGeneration(self):
        """
        L{site.getSession} generates L{Session} objects with distinct UIDs from
        a secure source of entropy.
        """
        site = server.Site(resource.Resource())
        # Ensure that we _would_ use the unpredictable random source if the
        # test didn't stub it.
        self.assertIdentical(site._entropy, os.urandom)

        def predictableEntropy(n):
            predictableEntropy.x += 1
            return (unichr(predictableEntropy.x) * n).encode("charmap")
        predictableEntropy.x = 0
        self.patch(site, "_entropy", predictableEntropy)
        a = self.getAutoExpiringSession(site)
        b = self.getAutoExpiringSession(site)
        self.assertEqual(a.uid, b"01" * 0x20)
        self.assertEqual(b.uid, b"02" * 0x20)
        # This functionality is silly (the value is no longer used in session
        # generation), but 'counter' was a public attribute since time
        # immemorial so we should make sure if anyone was using it to get site
        # metrics or something it keeps working.
        self.assertEqual(site.counter, 2)

        L{site.getSession} generates L{Session} objects with distinct UIDs from
        a secure source of entropy.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/web/test/test_web.py