def dataReceived(self, data):
        """
        Interpret the next chunk of bytes received.  Either deliver them to the
        data callback or invoke the finish callback if enough bytes have been
        received.

        @raise RuntimeError: If the finish callback has already been invoked
            during a previous call to this methood.
        """
        if self.dataCallback is None:
            raise RuntimeError(
                "_IdentityTransferDecoder cannot decode data after finishing")

        if self.contentLength is None:
            self.dataCallback(data)
        elif len(data) < self.contentLength:
            self.contentLength -= len(data)
            self.dataCallback(data)
        else:
            # Make the state consistent before invoking any code belonging to
            # anyone else in case noMoreData ends up being called beneath this
            # stack frame.
            contentLength = self.contentLength
            dataCallback = self.dataCallback
            finishCallback = self.finishCallback
            self.dataCallback = self.finishCallback = None
            self.contentLength = 0

            dataCallback(data[:contentLength])
            finishCallback(data[contentLength:])

        Interpret the next chunk of bytes received.  Either deliver them to the
        data callback or invoke the finish callback if enough bytes have been
        received.

        @raise RuntimeError: If the finish callback has already been invoked
            during a previous call to this methood.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/web/http.py