def CopyToProto(self, proto):

    super(Descriptor, self).CopyToProto(proto)
If the file object is None use a StringIO object.
dataset/ETHPy150Open AppScale/appscale/AppServer/google/net/proto2/python/public/descriptor.py/Descriptor.CopyToProto--crossover with--dataset/ETHPy150Open kobotoolbox/kobocat/onadata/apps/viewer/xls_writer.py/XlsWriter.set_file