try: finally: question

Peter Hansen peter at engcorp.com
Wed Jul 7 18:11:22 CEST 2004


Antoon Pardon wrote:

> Op 2004-07-06, Peter Hansen schreef <peter at engcorp.com>:
[about using finally and except on the same try block]
>>You used to be able to do that, according to a recent thread on the
>>subject.  It was removed because it was ambiguous what would happen.
>>For example, if an exception were raised in a finally clause which
>>came just before the except clause, would the except clause catch it?
>>Since the finally is technically outside of the try block, some
>>people thought it shouldn't be caught by the following except.
>>Others did.  Guido nuked it.
>>
>>Anyway, it would be purely syntactic sugar for what we have now,
>>saving you a few keystrokes and one extra level of indentation.
>>No big deal.
> 
> I don't agree. 

With what statement don't you agree?  The part about "you used to
be able to do that... [but] it was ambiguous ... Guido nuked it."?

Because that's a fact.  It's hard not to agree with it...

And if it's the part about "syntactic sugar", then you should
understand that the term refers to alternate syntax which is
seen as "cleaner" or "simpler" in some way, but which is not
otherwise different in any way that affects the parse tree or
resulting code.  And unless you think combined finally/except
clauses should be treated differently in the generated bytecode
than nested try/finally/except, it's *still* a fact that it
would just be "syntactic sugar", and arguing against facts is
pretty hard to do...

> It is not unusual to nest try blocks. If you then
> need to two instead of one because you can't combine a finally
> with an except clause, you almost double the level of indentations.

In fact, you would precisely double the level of indentations.
It seems that's the price we have to pay for being human, since
apparently our brains aren't all wired the same way and allowing
finally/except to be combined was likely to lead to subtle bugs.

> IMO it is almost as bad as would not having an elif and needing
> an extra indentation level for each else .. if

Almost, perhaps.  But can you see any way for similar ambiguity
to arise in the case of if/elif/else blocks?  I think everybody
understands how they work just fine.  The whole point is that
not everybody understood how the try/finally/except thing worked
(or they didn't agree with how it _should_ work).

I understand that you are arguing that it would be nicer to
allow try/finally/except.  I agree!  Unfortunately, (a) it was
removed and is probably unlikely to be added back in, (b) it
was ambiguous, and the ambiguity has not been resolved, so
until you or someone else can come up with a better approach
(and write a PEP) it will stay the way it is.  And that's just
another fact, so it's pointless to try to argue against it.

Hmm... Also, I think it really *is* unusual (as in, "not usual"
meaning not really common) to nest try blocks.  At least, I
recall lone try/finally or try/except blocks being much much
more common in most code than a mix.  Perhaps my memory is
poor on this.  Perhaps someone will actually go and research
it and prove it one way or the other.  If it really is
very common, it might be worth revisiting the subject...
If it's not very common, then this is once again just another
discussion about syntactic sugar for an uncommon use case,
and that kind of thing is so last year.  And the year before.
And....

-Peter



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