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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