def getAllRawHeaders(self):
        """
        Return an iterator of key, value pairs of all headers contained in this
        object, as L{bytes}.  The keys are capitalized in canonical
        capitalization.
        """
        for k, v in self._rawHeaders.items():
            yield self._canonicalNameCaps(k), v

        Return an iterator of key, value pairs of all headers contained in this
        object, as L{bytes}.  The keys are capitalized in canonical
        capitalization.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/web/http_headers.py