During handling of the above exception, another exception occurred是如何发生的？

调试python代码，常常看到这样的提示，During handling of the above exception, another exception occurred。这是如何发生的？

请看如下代码：

x = 2
y = 0

try:
    result = x / y
except ZeroDivisionError:
    raise ValueError('raise in exception clause')
    print("=== division by zero!")
else:
    print("result is", result)
finally:
    raise ValueError('raise in finally clause')
    print("executing finally clause")

ZeroDivisionError必然发生，然后代码进入except分支，在这个分支中，遇到了一个raise，后面的print得不到执行。由于有finally分支，在raise之前，需要执行finally分支的代码，不行的是，此时又遇到了raise，它后面的print也得不到执行。因此运行这段代码的效果，就是如下：

E:\py>python try.py
Traceback (most recent call last):
  File "try.py", line 8, in 
    result = x / y
ZeroDivisionError: division by zero

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "try.py", line 10, in 
    raise ValueError('raise in exception clause')
ValueError: raise in exception clause

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "try.py", line 15, in 
    raise ValueError('raise in finally clause')
ValueError: raise in finally clause

这就是During handling of the above exception, another exception occurred的由来！在处理异常的except分支或离开try的finally分支有raise，就会出现这样的提示。

到此，自然而然我们会想到另一个问题，在这种情况下，如果这段代码整体有try护着，抛出来的异常时哪个呢？请看下面的测试代码：

def testA():
    x = 2
    y = 0
    
    try:
        result = x / y
    except ZeroDivisionError:
        raise ValueError('raise in exception clause')
        print("=== division by zero!")
    else:
        print("result is", result)
    finally:
        raise ValueError('raise in finally clause')
        print("executing finally clause")
        

try:
    testA()
except Exception as e:
    print(repr(e))

运行结果：

E:\py>python try.py
ValueError('raise in finally clause')

看出来了吧，抛出来的只有最后那一个exception！

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