def handleHeader(self, key, value):
        """
        Called every time a header is received. Stores the header information
        as key-value pairs in the C{headers} attribute.

        @type key: C{str}
        @param key: An HTTP header field name.

        @type value: C{str}
        @param value: An HTTP header field value.
        """
        key = key.lower()
        l = self.headers.setdefault(key, [])
        l.append(value)

        Called every time a header is received. Stores the header information
        as key-value pairs in the C{headers} attribute.

        @type key: C{str}
        @param key: An HTTP header field name.

        @type value: C{str}
        @param value: An HTTP header field value.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/web/client.py