Python 'raise' 没有参数：什么是 "the last exception that was active in the current scope"？

Question

Python 的文档说：

If no expressions are present, raise re-raises the last exception that was active in the current scope.

(Python 3: https://docs.python.org/3/reference/simple_stmts.html#raise; Python 2.7: https://docs.python.org/2.7/reference/simple_stmts.html#raise.)

不过，"last active"的观念似乎变了。见证以下代码示例：

#
from __future__ import print_function
import sys
print('Python version =', sys.version)

try:
    raise Exception('EXPECTED')
except:
    try:
        raise Exception('UNEXPECTED')
    except:
        pass
    raise # re-raises UNEXPECTED for Python 2, and re-raises EXPECTED for Python 3

结果是 Python 2:

我没想到的结果

Python version = 2.7.15 (v2.7.15:ca079a3ea3, Apr 30 2018, 16:30:26) [MSC v.1500 64 bit (AMD64)]
Traceback (most recent call last):
  File "./x", line 10, in <module>
    raise Exception('UNEXPECTED')
Exception: UNEXPECTED

但是 Python 3:

得到了预期的（我的）结果

Python version = 3.6.8 (default, Feb 14 2019, 22:09:48)
[GCC 7.4.0]
Traceback (most recent call last):
  File "./x", line 7, in <module>
    raise Exception('EXPECTED')
Exception: EXPECTED

和

Python version = 3.7.2 (tags/v3.7.2:9a3ffc0492, Dec 23 2018, 23:09:28) [MSC v.1916 64 bit (AMD64)]
Traceback (most recent call last):
  File "./x", line 7, in <module>
    raise Exception('EXPECTED')
Exception: EXPECTED

那么"the last ... active"是什么意思呢？是否有关于此重大更改的一些文档？或者这是一个 Python 2 错误？

更重要的是：在 Python 2 中实现此功能的最佳方法是什么？（最好让代码在 Python 3 中继续工作。）

注意如果把代码改成

#
from __future__ import print_function
import sys
print('Python version =', sys.version)

def f():
    try:
        raise Exception('UNEXPECTED')
    except:
        pass

try:
    raise Exception('EXPECTED')
except:
    f()
    raise # always raises EXPECTED

然后 Python 2 也开始工作了：

Python version = 2.7.15 (v2.7.15:ca079a3ea3, Apr 30 2018, 16:30:26) [MSC v.1500 64 bit (AMD64)]
Traceback (most recent call last):
  File "./x", line 13, in <module>
    raise Exception('EXPECTED')
Exception: EXPECTED

我正在考虑改用那个...

Answer 1

raise 使用与 sys.exc_info 相同的信息，记录 both behaviors。由于记录了您的变通方法利用的 per-frame 行为，这就是要走的路。

PEP 3110 对 except 语句进行了一些更改。我相信它包括这个，但唯一明确提到的是 as 存储的异常在离开 except.

时被丢弃

Answer 2

Python 2 的行为与其说是错误，不如说是 设计缺陷 。 Python 3.0 通过添加异常链接功能解决了这个问题。可以在 PEP 3134 -- Exception Chaining and Embedded Tracebacks 中找到最接近此更改的文档动机：

During the handling of one exception (exception A), it is possible that another exception (exception B) may occur. In today's Python (version 2.4), if this happens, exception B is propagated outward and exception A is lost.

这正是您在 2.7 中看到的：EXPECTED (A) 丢失，因为 UNEXPECTED (B) 出现并覆盖了它。使用 Python 3 中较新的异常链接功能，可以通过异常实例上的 __cause__ 和 __context__ 属性保留两个错误的完整上下文。

对于更直接的 cross-compatible 解决方法，我鼓励您手动保留引用，明确显示 re-raised 是哪个错误，并且像往常一样避免裸露的 except 语句（总是太宽泛):

try:
    raise Exception('EXPECTED')
except Exception as err_expected:
    try:
        raise Exception('UNEXPECTED')
    except Exception as err_unexpected:
        pass
    raise err_expected

如果您希望以 cross-compatible 的方式抑制 exception-chaining 功能，您可以通过在 re-raising.

之前设置 err_expected.__cause__ = None 来实现

Python 'raise' 没有参数：什么是 "the last exception that was active in the current scope"？

Python 'raise' without arguments: what is "the last exception that was active in the current scope"?

python

exception

raise

python-2.7

python-3.x