Use exception variable in finally block

Question

When running this example 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>)

the last line (return ret, exc) raises UnboundLocalError: local variable 'exc' referenced before assignment even tho exc is definitively bound in the first line of the function (exc = None). This can be fixed by changing the except-clause like so:

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

Questions:

Is it possible to avoid using another variable (in my example exc1) while still avoiding the UnboundLocalError?
Why does the except <Exception> as <var> statement "swallow" already defined local variables?

alex_noname · Accepted Answer · 2020-11-19 14:23:23Z

1

This case is described in 8.4. The try statement:

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.

answered Nov 19, 2020 at 14:23

alex_noname

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