code: def _convert_field(self, key, value):
          
          convert = self.converters.get(key, bytes.decode)
          try:
              return convert(value)
          except ValueError:
              # Leave in default bytes
              return value
docs: Convert value using self.converters[key].
  
          If `key` is not present in self.converters, a standard unicode decoding
          will be attempted.  If the conversion (either key-specific or the
          default one) fails with a ValueError, the original bytes object will be
          returned.