python - "except Foo as bar"导致 "bar"从范围中删除

Question

给定以下代码:

msg = "test"
try:
    "a"[1]
except IndexError as msg:
    print("Error happened")
print(msg)

有人可以解释为什么这会导致 Python 3 中出现以下输出吗？

Error happened
Traceback (most recent call last):
  File "test.py", line 6, in <module>
    print(msg)
NameError: name 'msg' is not defined

Answer 1

except 子句中的

msg 与第一行的 msg 范围相同。

但是in Python 3 we have this new behavior too :

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.

所以，你在异常处理程序中“覆盖msg”，退出处理程序将删除变量以清除回溯引用循环。