def test_format(self):
        """
        Formatting is translated such that text is rendered correctly, even
        though old-style logging doesn't use PEP 3101 formatting.
        """
        event = self.forwardAndVerify(
            dict(log_format="Hello, {who}!", who="world")
        )
        self.assertEqual(
            legacyLog.textFromEventDict(event),
            "Hello, world!"
        )

        Formatting is translated such that text is rendered correctly, even
        though old-style logging doesn't use PEP 3101 formatting.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/logger/test/test_legacy.py