[Python-ideas] Control Flow - Never Executed Loop Body

[Steven D'Aprano]

On Tue, Mar 22, 2016 at 06:21:24PM +0100, Sven R. Kunze wrote:
> On 22.03.2016 16:09, Stephen J. Turnbull wrote:
> >Chris Barker writes:
> >  > All that being said:
> >  >
> >  > how about  "elempty"?, to go with elif?
> >
> >-1 Not to my taste, to say the least.
> 
> Hmm, it seems there is no easy solution for this. 

Possibly with the exception of the three or four previously existing 
easy solutions :-)


> What do you think 
> about an alternative that can handle more than empty and else?
> 
> for item in collection:
>     # do for item
> except NeverExecuted:
>     # do if collection is empty
> 
> It basically merges "try" and "for" and make "for" emit EmptyCollection.

Does this mean that every single for-loop that doesn't catch 
NeverExecuted (or EmptyCollection) will raise an exception?

If not, then how will this work? Is this a special kind of 
exception-like process that *only* operates inside for loops?

What will an explicit "raise NeverExecuted" do?

 
> So, independent of the initial "never executed loop body" use-case, one 
> could also emulate the "else" clause by:
> 
> for item in collection:
>     # do for item
> except StopIteration:
>     # do after the loop

That doesn't work, for two reasons:

(1) Not all for-loops use iterators. The venerable old "sequence 
protocol" is still supported for sequences that don't support __iter__. 
So there may not be any StopIteration raised at all.

(2) Even if StopIteration is raised, the for-loop catches it (in a 
manner of speaking) and consumes it.

So to have this work, we would need to have the for-loop re-raise 
StopIteration... but what happens if you don't include an except 
StopIteration clause? Does every bare for-loop with no "except" now 
print a traceback and halt processing? If not, why not?


-- 
Steve