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

[Sven R. Kunze]

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. 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. 
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

Thinking this further, it might enable much more use-cases and stays 
extensible without the need to invent all kinds of special keywords.


So, this "for" might roughly be equivalent to (using the "old for"):

class NeverExecuted(StopIteration):
     pass

i = iter(collection)
try:
     e = next(i)
except StopIteration:
     if [NeverExecuted/StopIteration is catched]:
         raise NeverExecuted
else:
     # do for item e
     for e in i:
         # do for item e
     else:
         if [StopIteration is catched]:
             raise StopIteration


I hope that's not too convoluted.

Best,
Sven