def test_contentLengthIsNativeString(self):
        """
        The C{'CONTENT_LENGTH'} key of the C{environ} C{dict} passed to the
        application is always a native string.
        """
        for contentLength in b"1234", u"1234":
            request, result = self.prepareRequest()
            request.requestHeaders.addRawHeader(b"Content-Length", contentLength)
            request.requestReceived()
            result.addCallback(self.environKeyEqual('CONTENT_LENGTH', '1234'))
            self.assertIsInstance(self.successResultOf(result), str)

        The C{'CONTENT_LENGTH'} key of the C{environ} C{dict} passed to the
        application is always a native string.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/web/test/test_wsgi.py