从python __exit__块中重新分配异常 [英] Re-assign exception from within a python __exit__ block

问题描述

在自定义游标类中的 __ exit __ 块内，我想捕获一个异常，因此我可以反过来抛出一个更具体的异常。什么是正确的方法？

From within an __exit__ block in a custom cursor class I want to catch an exception so I can in turn throw a more specific exception. What is the proper way to do this?

class Cursor:
    def __enter__(self):
        ...

    def __exit__(self, ex_type, ex_val, tb):
        if ex_type == VagueThirdPartyError:
            # get new more specific error based on error code in ex_val and
            # return that one in its place.
            return False # ?
        else:
            return False

在 __ exit __ block似乎是一个黑客，但也许我在想。

Raising the specific exception within the __exit__ block seems like a hack, but maybe I'm over thinking it.

推荐答案

正确的过程是在 __ exit __ 处理程序之内引发新异常。

The proper procedure is to raise the new exception inside of the __exit__ handler.

您应该不要引发了通过的异常;允许上下文管理器链接，在这种情况下，你应该只是从处理程序返回一个假的值。提出自己的例外情况是非常好的。

You should not raise the exception that was passed in though; to allow for context manager chaining, in that case you should just return a falsey value from the handler. Raising your own exceptions is however perfectly fine.

请注意，最好使用身份测试是来验证传入异常的类型：

Note that it is better to use the identity test is to verify the type of the passed-in exception:

def __exit__(self, ex_type, ex_val, tb):
    if ex_type is VagueThirdPartyError:
        if ex_val.args[0] == 'foobar':
            raise SpecificException('Foobarred!')

        # Not raising a new exception, but surpressing the current one:
        if ex_val.args[0] == 'eggs-and-ham':
            # ignore this exception
            return True

        if ex_val.args[0] == 'baz':
            # re-raise this exception
            return False

    # No else required, the function exits and `None` is  returned

您还可以使用 issubclass（ex_type，VagueThirdPartyError）允许特定异常的子类。

You could also use issubclass(ex_type, VagueThirdPartyError) to allow for subclasses of the specific exception.

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