def pb2json(pb, useFieldNumber=False):
    ''' convert google.protobuf.descriptor instance to JSON string '''
    js = {}
    # fields = pb.DESCRIPTOR.fields #all fields
    fields = pb.ListFields()        #only filled (including extensions)
    for field,value in fields:
        if useFieldNumber:
            key = field.number
        else:
            key = field.name
        if field.type == FD.TYPE_MESSAGE:
            ftype = partial(pb2json, useFieldNumber=useFieldNumber)
        elif field.type in _ftype2js:
            ftype = _ftype2js[field.type]
        else:
            raise ParseError("Field %s.%s of type '%d' is not supported" % (pb.__class__.__name__, field.name, field.type, ))
        if field.label == FD.LABEL_REPEATED:
            js_value = []
            for v in value:
                js_value.append(ftype(v))
        else:
            js_value = ftype(value)
        js[key] = js_value
    return js
convert google.protobuf.descriptor instance to JSON string