Python：即使在try/except子句中捕获，异常仍然会被引发

Question

Python：即使在try/except子句中捕获，异常仍然会被引发

5

在我的代码中，当出现异常时，我想捕获异常并将一些关于异常的信息打印到屏幕上，然后在完成操作后结束脚本。我尝试使用类似以下代码的内容，但我不明白为什么我会得到跟踪错误。

执行以下代码时：

try:
    1 / 0
except ZeroDivisionError:
    print("Exception: ZeroDivisionError")
    raise Exception

控制台输出：

Exception: ZeroDivisionError
Traceback (most recent call last):
  File "<pyshell#19>", line 2, in <module>
    1 / 0
ZeroDivisionError: division by zero

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<pyshell#19>", line 5, in <module>
    raise Exception
Exception

我原以为如果捕获了ZeroDivisionError，它就不会再被触发，只有我在结尾处写的raise Exception才会显示，但是两者都在控制台中显示。

为什么两者都显示，如何修改代码使只有第二个显示，或者有更好的方法实现我的目标？

- user3873725

1个回答

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可以查看英文原文，
原文链接

- Martijn Pieters · Accepted Answer

这里的控制台显示了上下文信息。当在异常处理程序中抛出异常时，Python会将当前异常附加到__context__属性中，如果新的异常没有被处理，Python稍后会显示该上下文信息。如果您不想显示上下文信息，则需要提供一个原因；您可以使用raise ... from None提供一个空原因：

try:
    1 / 0
except ZeroDivisionError:
    print("Exception: ZeroDivisionError")
    raise Exception from None

引用自 raise 语句文档：

from 子句用于异常链接: 如果给定，第二个表达式必须是另一个异常类或实例，然后将其作为 __cause__ 属性（可写）附加到已引发的异常上。如果未处理该异常，则两个异常都将被打印[...]

如果在异常处理程序中引发异常，则会隐式地启用类似的机制：先前的异常随后作为新异常的 __context__ 属性附加[...]

而从异常文档中得知：

When raising (or re-raising) an exception in an except clause __context__ is automatically set to the last exception caught; if the new exception is not handled the traceback that is eventually displayed will include the originating exception(s) and the final exception.

When raising a new exception (rather than using a bare raise to re-raise the exception currently being handled), the implicit exception context can be supplemented with an explicit cause by using from with raise:
raise new_exc from original_exc
The expression following from must be an exception or None. It will be set as __cause__ on the raised exception. Setting __cause__ also implicitly sets the __suppress_context__ attribute to True, so that using raise new_exc from None effectively replaces the old exception with the new one for display purposes (e.g. converting KeyError to AttributeError), while leaving the old exception available in __context__ for introspection when debugging.

The default traceback display code shows these chained exceptions in addition to the traceback for the exception itself. An explicitly chained exception in __cause__ is always shown when present. An implicitly chained exception in __context__ is shown only if __cause__ is None and __suppress_context__ is false.