Return a unique identifier for the background data

def _compute_bgid(self, bg=None):
        """Return a unique identifier for the background data"""
        if bg is None:
            bg = self._bgdata
        if isinstance(bg, qpimage.QPImage):
            # Single QPImage
            if "identifier" in bg:
                return bg["identifier"]
            else:
                data = [bg.amp, bg.pha]
                for key in sorted(list(bg.meta.keys())):
                    val = bg.meta[key]
                    data.append("{}={}".format(key, val))
                return hash_obj(data)
        elif (isinstance(bg, list) and
              isinstance(bg[0], qpimage.QPImage)):
            # List of QPImage
            data = []
            for bgii in bg:
                data.append(self._compute_bgid(bgii))
            return hash_obj(data)
        elif (isinstance(bg, SeriesData) and
              (len(bg) == 1 or
               len(bg) == len(self))):
            # DataSet
            return bg.identifier
        else:
            raise ValueError("Unknown background data type: {}".format(bg))