raise 与 raise ... from 的区别

红太狼 2022-04-03 12:50 207阅读 0赞

## 起步 ##

Python 的 `raise` 和 `raise from` 之间的区别是什么？

try:
        print(1 / 0)
    except Exception as exc:
        raise RuntimeError("Something bad happened")

输出：

Traceback (most recent call last):
      File "test4.py", line 2, in <module>
        print(1 / 0)
    ZeroDivisionError: division by zero
    
    During handling of the above exception, another exception occurred:
    
    Traceback (most recent call last):
      File "test4.py", line 4, in <module>
        raise RuntimeError("Something bad happened")
    RuntimeError: Something bad happened

而 `raise from` ：

try:
        print(1 / 0)
    except Exception as exc:
        raise RuntimeError("Something bad happened") from exc

输出：

Traceback (most recent call last):
      File "test4.py", line 2, in <module>
        print(1 / 0)
    ZeroDivisionError: division by zero
    
    The above exception was the direct cause of the following exception:
    
    Traceback (most recent call last):
      File "test4.py", line 4, in <module>
        raise RuntimeError("Something bad happened") from exc
    RuntimeError: Something bad happened

## 分析 ##

不同之处在于，`from` 会为异常对象设置 `__cause__` 属性表明异常的是由谁直接引起的。

处理异常时发生了新的异常，在不使用 `from` 时更倾向于新异常与正在处理的异常没有关联。而 `from` 则是能指出新异常是因旧异常直接引起的。这样的异常之间的关联有助于后续对异常的分析和排查。`from` 语法会有个限制，就是第二个表达式必须是另一个异常类或实例。

如果在异常处理程序或 `finally` 块中引发异常，默认情况下，异常机制会隐式工作会将先前的异常附加为新异常的 `__context__` 属性。

当然，也可以通过 `with_traceback()` 方法为异常设置上下文 `__context__` 属性，这也能在 `traceback` 更好的显示异常信息。

raise Exception("foo occurred").with_traceback(tracebackobj)

## 禁止异常关联 ##

from 还有个特别的用法：`raise ... from None` ，它通过设置 `__suppress_context__` 属性指定来明确禁止异常关联：

try:
        print(1 / 0)
    except Exception as exc:
        raise RuntimeError("Something bad happened") from None

输出：

Traceback (most recent call last):
      File "test4.py", line 4, in <module>
        raise RuntimeError("Something bad happened") from None
    RuntimeError: Something bad happened

## 总结 ##

在异常处理程序或 `finally` 块中引发异常，Python 会为异常设置上下文，可以手动通过 `with_traceback()` 设置其上下文，或者通过 `from` 来指定异常因谁引起的。这些手段都是为了得到更友好的异常回溯信息，打印清晰的异常上下文。若要忽略上下文，则可以通过 `raise ... from None` 来禁止自动显示异常上下文。

## 参考 ##

*  [https://docs.python.org/3/ref...][https_docs.python.org_3_ref...]
 *  [https://docs.python.org/3/lib...][https_docs.python.org_3_lib...]
 *  [PEP 3109 -- Raising Exceptions in Python 3000][]
 *  [PEP 3134 -- Exception Chaining and Embedded Tracebacks][]

来源：[https://segmentfault.com/a/1190000017332255][https_segmentfault.com_a_1190000017332255]

[https_docs.python.org_3_ref...]: https://docs.python.org/3/reference/simple_stmts.html#raise
[https_docs.python.org_3_lib...]: https://docs.python.org/3/library/exceptions.html#BaseException
[PEP 3109 -- Raising Exceptions in Python 3000]: https://www.python.org/dev/peps/pep-3109/
[PEP 3134 -- Exception Chaining and Embedded Tracebacks]: https://www.python.org/dev/peps/pep-3134/
[https_segmentfault.com_a_1190000017332255]: https://segmentfault.com/a/1190000017332255