def test_extraChainFilesAreAddedIfSupplied(self):
        """
        If C{extraCertChain} is set and all prerequisites are met, the
        specified chain certificates are added to C{Context}s that get
        created.
        """
        opts = sslverify.OpenSSLCertificateOptions(
            privateKey=self.sKey,
            certificate=self.sCert,
            extraCertChain=self.extraCertChain,
        )
        opts._contextFactory = FakeContext
        ctx = opts.getContext()
        self.assertEqual(self.sKey, ctx._privateKey)
        self.assertEqual(self.sCert, ctx._certificate)
        self.assertEqual(self.extraCertChain, ctx._extraCertChain)

        If C{extraCertChain} is set and all prerequisites are met, the
        specified chain certificates are added to C{Context}s that get
        created.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_sslverify.py