Python 3异常出现未知原因删除封闭作用域中的变量

Question

Python 3异常出现未知原因删除封闭作用域中的变量

18

我有以下代码：

def foo():
    e = None
    try:
        raise Exception('I wish you would except me for who I am.')
    except Exception as e:
        print(e)
    print(e)

foo()

在 Python 2.7 中，这会像预期的那样运行并打印：

I wish you would except me for who I am.
I wish you would except me for who I am.

然而，在Python 3.x中，第一行被打印，但第二行没有被打印。它似乎删除了封闭作用域中的变量，使我从最后一个打印语句得到以下回溯：

Traceback (most recent call last):
  File "python", line 9, in <module>
  File "python", line 7, in foo
UnboundLocalError: local variable 'e' referenced before assignment

几乎就像在except块之后插入了一个del e语句一样。这种行为是否有原因？如果Python开发人员希望except块具有自己的局部作用域，并且不会泄漏到周围作用域，我可以理解它，但为什么必须删除先前分配的外部作用域变量呢？

- Shashank

1个回答

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- thefourtheye · Accepted Answer

引用 try 的文档，

When an exception has been assigned using as target, it is cleared at the end of the except clause. This is as if
except E as N:
   foo
was translated to
except E as N:
    try:
        foo
    finally:
        del N
This means the exception must be assigned to a different name to be able to refer to it after the except clause. Exceptions are cleared because with the traceback attached to them, they form a reference cycle with the stack frame, keeping all locals in that frame alive until the next garbage collection occurs.

这在这两个PEP中有所涉及。