Loop with else clause

DL Neil PythonList at DancesWithMice.info
Tue Feb 5 02:33:24 EST 2019


On 5/02/19 8:12 PM, Steve wrote:
> Would it be a hyphythonitical question?

Is that one of the new meta-classes in release 3.99, or a perhaps a 
project to remove the GIL and take advantage of multi-core architectures?

As well as embarrassing the poor coder, this question vexed quite a few 
minds this afternoon (even allowing for the gathering mood - tomorrow 
being a public holiday!).

Even though it seemed so straightforward to me, others felt it 
reasonable that there would be some way to tell that a loop never 
executed - there must be a simpler, more pythonic, construct...


> =================
> Footnote:
> Zamboni locks up after running into large patch of loose teeth.



> -----Original Message-----
> From: Python-list <python-list-bounces+gronicus=sga.ninja at python.org> On
> Behalf Of DL Neil
> Sent: Monday, February 4, 2019 11:29 PM
> To: 'Python' <python-list at python.org>
> Subject: Loop with else clause
> 
> What is the pythonic way to handle the situation where if a condition exists
> the loop should be executed, but if it does not something else should be
> done?
> 
> 
> Why am I asking?
> Today's code review included a for...else structure. I've rarely seen such a
> thing, and even knowing it exists, cannot recall ever using it!
> The coder intended to implement the scenario (above) but did not realise
> that the else-clause means 'execute if the loop ended without using break'.
> She thought it meant 'if there's nothing in the iterable, execute the else
> clause' (per if...then...else... ie the two clauses are
> mutually-exclusive*) - which one assumes is the reason why the BDfL is
> claimed to have said it should never have been implemented (this way).
> She neglected to test the exception properly, and was lulled into a false
> sense of security by the coverage reporting 100%. Oops!
> 
> *see also the more commonly-used try...except...else...[finally...]
> 
> 
> When/how does this occur?
> Our client is more than a little commercially-sensitive. So as a really
> simple scenario, imagine a report is required, naming people who have become
> eligible for something, eg students qualified to enter an advanced class,
> Oscar film award nominees, entrants who have fulfilled the requirements of a
> competition from which a winner will be randomly selected...
> 
> The names all appear in a list, so the most frequent use-case is trivial:
> 
> 	print( "And the winners are:" )
> 	for name in list:
> 		print( name )
> 
> but, if no-one actually qualifies, a warning message is required, eg
> 
> 	print( "Sorry, no-one is eligible" )
> 
> 
> Possible solution:
> To make anything more than the trivial case readable, I think I'd put the
> list processing into one function, and the exception into another (except
> that this case is so trivial), ie
> 
> 	if list:
> 		process_list() #the heading and for-loop, as above
> 	else:
> 		print( "Sorry...
> 
> 
> Others wanted to add a semaphore/flag inside the loop to indicate if it was
> executed at least once. Yes, could even use the else clause then!
> 
> The ideas went (rapidly) down-hill from there...
> 
> 
> Is there another, more pythonic, approach to conditional (for/while) loop
> processing?
> 
> --
> Regards,
> =dn
> --
> https://mail.python.org/mailman/listinfo/python-list
> 

-- 
Regards =dn


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