Get request Json value by field name

def __get_value(self, field_name):
        """ Get request Json value by field name """
        value = request.values.get(field_name)
        if value is None:
            if self.json_form_data is None:
                value = None
            elif field_name in self.json_form_data:
                value = self.json_form_data[field_name]
        return value