如何用Pytest断言由指定异常引发的原始异常？

问题背景 #

你按照Stack Overflow教程及官方文档编写断言异常的测试套件，但遇到一个场景：需要从想要断言的异常引出新异常，现在需要用pytest正确捕获并匹配原始异常的消息。以下是可复现代码：

待测试代码（src/main.py） #

# in src/main.py

# this is not exactly like that in the code, but it is what s equivalent

class ModelValidationError(Exception):
    def __init__(self, *_):
        super().__init__(
            f"Unable to initialize model"
        )


class InvalidParametersWarning(Warning):
    def __init__(self, *_):
        super().__init__(
            "The given parameters are invalid"
        )

def handle_model_validation_error(environment: str, exception: ValueError):
    """Raise the correct exception based on the environment we are in.

    To avoid frightening the users, we raise a warning in production
    environments and an exception in development environments.

    Parameters
    ----------
    environment : str
        in which application environment we are in.
    exception : ValueError

    Raises
    ------
    ModelValidationError
        For development environments. Is is raised from the ValueError.
    InvalidParametersWarning
        For production environments.
    """
    if environment == "development":
        raise ModelValidationError() from exception
    else:
        raise InvalidParametersWarning()

原测试代码（存在问题） #

import pytest

from src.main import handle_model_validation_error


def test_exception():
    # 想要匹配ValueError的消息"nope"，该如何实现？
    with pytest.raises(ValueError, match="nope"):
        handle_model_validation_error("development", ValueError("nope"))

简化等价示例 #

import pytest


def test_exception():
    # 想要匹配ValueError的消息"nope"，该如何实现？
    with pytest.raises(ValueError, match="nope"):
        raise Exception from ValueError("nope")

解决方案 #

当使用raise NewException() from OriginalException抛出链式异常时，pytest的pytest.raises无法直接捕获原始异常——因为实际抛出的是新异常。此时需要先捕获新异常，再通过其__cause__属性访问原始异常，进而验证消息。

方法一：捕获新异常后手动断言原始异常 #

修改测试代码，先捕获抛出的ModelValidationError，再检查它的__cause__属性：

import pytest
from src.main import handle_model_validation_error, ModelValidationError

def test_exception_with_cause():
    with pytest.raises(ModelValidationError) as exc_info:
        handle_model_validation_error("development", ValueError("nope"))
    
    # 验证原始异常的类型
    assert isinstance(exc_info.value.__cause__, ValueError)
    # 验证原始异常的消息
    assert exc_info.value.__cause__.args[0] == "nope"

方法二：匹配原始异常的消息内容 #

如果只需要验证消息包含指定字符串，也可以这样写：

def test_exception_match_cause_message():
    with pytest.raises(ModelValidationError) as exc_info:
        handle_model_validation_error("development", ValueError("nope"))
    
    assert exc_info.value.__cause__ is not None
    assert "nope" in str(exc_info.value.__cause__)

简化示例的修改版本 #

针对简化示例，测试代码修改如下：

import pytest

def test_exception():
    with pytest.raises(Exception) as exc_info:
        raise Exception from ValueError("nope")
    
    assert isinstance(exc_info.value.__cause__, ValueError)
    assert exc_info.value.__cause__.args[0] == "nope"

关键说明 #

• exc_info.value是pytest捕获到的新异常实例（如ModelValidationError）
• Python链式异常中，__cause__属性专门存储通过from关键字关联的原始异常
• 可以通过__cause__.args[0]直接获取原始异常的消息，或用str(__cause__)转为字符串后做模糊匹配

内容的提问来源于stack exchange，提问作者Marco Bresson