replacing `else` with `then` in `for` and `try`

[Jon Ribbens]

On 2017-11-03, Steve D'Aprano <steve+python at pearwood.info> wrote:
> On Fri, 3 Nov 2017 03:31 am, Jon Ribbens wrote:
>> No, it's an obvious bug. You have a 'for...else' with no 'break'.
>> Like I said, that should probably be a syntax error.
>
> It should absolutely not be a syntax error. There's no reason for it
> to be a syntax error, except to satisfy some arrogant and foolish
> idea of purity.

It'd be nice if you could be a little less rude. It's not an "arrogant
and foolish idea of purity", it's to help people catch bugs in their
code, and to aid their understanding of the language.

> There are uncountable ways of writing code which is seemingly
> "pointless", and we don't make it a syntax error.

And there's uncountable ways of writing code which we *do* make a
syntax error. I'm not sure what your point is there.

>> No, 'then' describes the opposite of what it does. The word 'then'
>> implies something that always happens next, 
>
> Right, which is what happens with the for...else block.

No. Ok, so look. It's obvious that you and I have different mental
models of the situation here. You're thinking of 'for...else' as two
arbitrary clauses that run consecutively unless the whole thing is
aborted by a 'break', whereas I'm thinking of the 'for' clause as
being a search for a situation that matches a condition and the
'else' clause being what happens if the condition is not matched
(i.e. exactly the same as 'if...else').

Now there's nothing inherently *wrong* with your choice of mental
model, except it's leading you into confusion because my model means
the meaning of the 'else' keyword is intuitive and obvious, and yours
means it's counter-intuitive and confusing. Your suggestion is that
the fix is to change the language, my suggestion is to fix your model.
I'd suggest that changing your mind is easier than changing the
language ;-)

Also, my model has the advantage that if what the 'for' clause is
doing doesn't match the concept of 'searching for a match' then it's
obvious that you shouldn't be using 'for...else' in the first place.
Again, sure, the language doesn't currently strictly enforce the
requirement for a 'break', but nevertheless if you don't have one
you almost certainly have a bug.

>> Yes, I saw that. It's possible you are the only person in the world
>> ever to have done that. It would not make the interactive interpreter
>> 'worse' in the slightest for that silly trick to be lost.
>
> Just because you personally haven't used this technique doesn't make it
> a "silly trick".

It's an incredibly obscure work-around for a different problem,
i.e. an infelicity in the way the interactive prompt parses input
blocks. If the parsing is a genuine issue then the answer is to
fix that, not to look for hacks that almost never help anyway.

> And while I thank you for the complement that I am the cleverest and most
> insightful Python coder in the world,

I didn't say anything even remotely resembling that.