def encode_field(self, field, value):
        """Encode a python field value to a JSON value.

        Args:
          field: A ProtoRPC field instance.
          value: A python value supported by field.

        Returns:
          A JSON serializable value appropriate for field.
        """
        if isinstance(field, messages.BytesField):
            if field.repeated:
                value = [base64.b64encode(byte) for byte in value]
            else:
                value = base64.b64encode(value)
        elif isinstance(field, message_types.DateTimeField):
            # DateTimeField stores its data as a RFC 3339 compliant string.
            if field.repeated:
                value = [i.isoformat() for i in value]
            else:
                value = value.isoformat()
        return value
Encode a python field value to a JSON value.

        Args:
          field: A ProtoRPC field instance.
          value: A python value supported by field.

        Returns:
          A JSON serializable value appropriate for field.