langchain/evaluation/qa/eval_chain.py/ContextQAEvalChain/_validate_input_vars
class ContextQAEvalChain:    def _validate_input_vars(cls, prompt: PromptTemplate) -> None:
        expected_input_vars = {"query", "context", "result"}
        if expected_input_vars != set(prompt.input_variables):
            raise ValueError(
                f"Input variables should be {expected_input_vars}, "
                f"but got {prompt.input_variables}"
            )