Method check

Lib/test/test_codeccallbacks.py:1179–1182

(input, expected, msg)

            br'\x5': (b'x\\', 1),
        }
        def check(input, expected, msg):
            with self.assertWarns(DeprecationWarning) as cm:
                self.assertEqual(decode(input, 'test.mutating2'), (expected, len(input)))
            self.assertIn(msg, str(cm.warning))

        check(br'\x0n\z', '\u0404\n\\z', r'"\z" is an invalid escape sequence')
        check(br'\x0n\501', '\u0404\n\u0141', r'"\501" is an invalid octal escape sequence')