记一次try finally中隐藏的坑

最近在工作的时候，遇到了一个问题，源码简化后如下：

import os

def test():
    b = None
    e = None
    try:
        for _ in b:
            break
    except Exception as e:
        raise e
    finally:
        if e is not None:
            os._exit(0)


if __name__ == '__main__':
    test()

简单来说，就是遇到异常后，抛出，最后判断异常如果存在，就退出进程
执行后出现错误UnboundLocalError: local variable 'e' referenced before assignment
详情如下：

Traceback (most recent call last):
  File "a.py", line 11, in test
    raise e
  File "a.py", line 8, in test
    for _ in b:
TypeError: 'NoneType' object is not iterable

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "a.py", line 18, in <module>
    test()
  File "a.py", line 13, in test
    if e is not None:
UnboundLocalError: local variable 'e' referenced before assignment

很奇怪，本来e已经在开始的时候定义了，为什么最后会说e未被定义就使用了呢？
于是打印出local()来看下实时的变量
ps:locals() 函数会以字典类型返回当前位置的全部局部变量。
代码如下:

import os

def test():
    b = None
    e = None
    try:
        for _ in b:
            break
    except Exception as e:
        print(locals())
        print("--------")
        raise e
    finally:
        print(locals())
        print("~~~~~~")
        if e is not None:
            os._exit(0)

if __name__ == '__main__':
    test()

结果如下：

{'e': TypeError("'NoneType' object is not iterable",), 'b': None}
--------
{'b': None}
~~~~~~
Traceback (most recent call last):
  File "a.py", line 13, in test
    raise e
  File "a.py", line 8, in test
    for _ in b:
TypeError: 'NoneType' object is not iterable

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "a.py", line 22, in <module>
    test()
  File "a.py", line 17, in test
    if e is not None:
UnboundLocalError: local variable 'e' referenced before assignment

由此可以发现，在进入finall模块前，e变量已经被回收了，因此会出现UnboundLocalError: local variable 'e' referenced before assignment的报错

总结：
except 的异常 会在进入finally前进行回收
如果想完成最初的逻辑可以做出如下修改：

import os


def test():
    b = None
    exc = None
    try:
        for _ in b:
            break
    except Exception as e:
        exc = e
        raise e
    finally:
        if exc is not None:
            os._exit(0)


if __name__ == '__main__':
    test()

不要将异常名与变量名使用同一个，而是定义一个新的变量名，将异常复制给该变量即可