[Python-ideas] Re: else without except

Aug. 1, 2023

      2023-06-30 19:54 UTC+02:00, MRAB <python@mrabarnett.plus.com>:
...
On 2023-06-30 14:55, Daniel Walker wrote:
...
As most of you probably know, you can use else with try blocks:
try:
     do_stuff()
except SomeExceptionClass:
    handle_error()
else:
    no_error_occurred()
Here, no_error_occurred will only be called if do_stuff() didn't raise
an exception.
However, the following is invalid syntax:
try:
    do_stuff()
else:
    no_error_occurred()
Now you might say that this isn't needed as you can achieve the same
result with
do_stuff()
no_error_occurred()
However, what if I want to use finally as well:
try:
    do_stuff()
else:
    no_error_occurred()
finally:
    cleanup()
and I need no_error_occurred to be called before cleanup?  For my actual
use case, I've done
try:
    do_stuff()
except Exception:
     raise
else:
    no_error_occurred()
finally:
    cleanup()
This seems very non-Pythonic.  Is there a reason why else without except
has to be invalid syntax?
What would be the difference between
try:
        do_stuff()
     else:
        no_error_occurred()
     finally:
        cleanup()
and
try:
        do_stuff()
        no_error_occurred()
     finally:
        cleanup()
?
One could argue that it's conceptually not the same although the
actual execution is the same.

In the first case you intend to only catch the exceptions generated by
do_stuff().
In the second case, you intend to catch those generated by do_stuff()
and those generated by no_error_occurred().

If later on an "except" is added, the developper doing the
modification should be reminded to move the call to
no_error_occurred() into an "else". With real-world non-trivial code,
it might not be so simple to see.

I'm not an expert, but I actually see no downside to having an "else"
without an "except".

Best regards,
Celelibi