在 finally 块中使用异常变量

Question

当运行这个例子func:

from typing import Tuple, Any, Optional

def func() -> Tuple[Any, Optional[Exception]]:
    exc = None
    ret = None
    try:
        # code here, if successful assign result to `ret`
        ret = "Result"
        # comment this line out and the code works
        raise Exception
    except Exception as exc:
        exc.__traceback__ = None
        # Error logging here
        pass
    finally:
        return ret, exc

print(func())  # expected: ("Result", <Exception instance>)

最后一行 (return ret, exc) 引发 UnboundLocalError: local variable 'exc' referenced before assignment 即使 exc 明确绑定在函数的第一行 (exc = None) 中。这可以通过像这样更改 except-子句来解决：

except Exception as exc1:
    exc = exc1
    exc.__traceback__ = None
    # Error logging here
    pass

问题:

是否可以避免使用另一个变量（在我的示例中 exc1），同时仍然避免使用 UnboundLocalError？
为什么 except <Exception> as <var> 语句“吞噬”了已经定义的局部变量？

Answer 1

这种情况在8.4. The try statement中有描述：

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
   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.

在 finally 块中使用异常变量

Use exception variable in finally block

python

scope

exception

try-catch

python-3.x