Callback for partial output.

def _partial(self):
        """Callback for partial output."""
        raw_stdout = self._process.readAllStandardOutput()
        stdout = handle_qbytearray(raw_stdout, self._get_encoding())

        if self._partial_stdout is None:
            self._partial_stdout = stdout
        else:
            self._partial_stdout += stdout

        self.sig_partial.emit(self, stdout, None)