code: def _file_size(self, field):
          
          size = 0
          try:
              handle = open(self._files[field], "r")
              size = os.fstat(handle.fileno()).st_size
              handle.close()
          except:
              size = 0
          self._file_lengths[field] = size
          return self._file_lengths[field]
docs: Returns the file size for given file field.
  
          Args:
              field (str): File field
  
          Returns:
              int. File size