transform.py 文件源码

def __call__(self, value):
        if isinstance(value, ConditionBase):
            # Create a conditional expression string with placeholders
            # for the provided condition.
            built_expression = self._condition_builder.build_expression(
                value, is_key_condition=self._is_key_condition)

            self._placeholder_names.update(
                built_expression.attribute_name_placeholders)
            self._placeholder_values.update(
                built_expression.attribute_value_placeholders)

            return built_expression.condition_expression
        # Use the user provided value if it is not a ConditonBase object.
        return value