Re: Access to return / exception context in finally block

On 30/08/2025 12:03, marius.spix--- via Python-list wrote:
> Dear mailing list,
>
> there is currently no direct way to observe the current interpreter state in a finally block without tracing.
>
> My idea is introducing an immutable __exit_context__ magic variable, which would have one of three possible values:
>
> * ReturnContext(value), if a return statement is about to exit the try, except or else block
> * ExceptionContext(exc, tb, caught: bool), if an exception has occured in the try, except or else block
> * None, if the try block completes normally without return or exception
>
> This variable would allow to inspect the cause, why a try block is left and make post-processing easier without using workarounds like storing the exception in a temporary variable.
>
> sys.exc_info() is not always useful, because it always returns (None, None, None) when the exception has been caught in the except block.
>
> This is an example, how __exit_context__ could be used:
>
> def f(x):
>      try:
>          return 10 / x
>      except ZeroDivisionError:
>          pass
>      finally:
>          if isinstance(__exit_context__, ExceptionContext):
>              log_error(__exit_context__.exc)
>              return 0
>          elif isinstance(__exit_context__, ReturnContext):
>              log_return(__exit_context__.value)
>              return __exit_context__.value + 1
>
> I wonder if it would be a candidate for a PEP to be implemented in the Python standard.
>
> Best regards
>
> Marius Spix
In your example when would isinstance(__exit_context__, ReturnContext) 
be True and when would it be False? What would __exit_context__.value 
be? I can't think of a sensible meaning for it. If no exception occurs, 
is the value returned by f supposed to be 10/x or __exit_context__.value 
+ 1 # whatever that is
Best wishes
Rob Cliffe
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