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Re: empty clause of for loops

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  Re: empty clause of for loops Steven D'Aprano <steve@pearwood.info> - 2016-03-17 11:27 +1100
    Re: empty clause of for loops "Sven R. Kunze" <srkunze@mail.de> - 2016-03-17 13:01 +0100

#105065 — Re: empty clause of for loops

On Thu, 17 Mar 2016 05:05 am, Sven R. Kunze wrote:

> What I don't understand is why Python features "if break, then no else
> clause", but "if empty, then empty clause".
> 
> I found this excellent post:
> https://shahriar.svbtle.com/pythons-else-clause-in-loops

That post describes the motivating use-case for the introduction
of "if...else", and why break skips the "else" clause:


for x in data:
    if meets_condition(x):
        break
else:
    # raise error or do additional processing 


It might help to realise that the "else" clause is misnamed. It should be
called "then":

for x in data:
    block
then:
    block


The "then" (actually "else") block is executed *after* the for-loop, unless
you jump out of that chunk of code by raising an exception, calling return,
or break.

As a beginner, it took me years of misunderstanding before I finally
understood for...else and while...else, because I kept coming back to the
thought that the else block was executed if the for/while block *didn't*
execute. I couldn't get code with for...else to work right and I didn't
understand why until finally the penny dropped and realised that "else"
should be called "then".



-- 
Steven

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#105087

On 17.03.2016 01:27, Steven D'Aprano wrote:
> That post describes the motivating use-case for the introduction
> of "if...else", and why break skips the "else" clause:
>
>
> for x in data:
>      if meets_condition(x):
>          break
> else:
>      # raise error or do additional processing
>
>
> It might help to realise that the "else" clause is misnamed. It should be
> called "then":
>
> for x in data:
>      block
> then:
>      block
>
>
> The "then" (actually "else") block is executed *after* the for-loop, unless
> you jump out of that chunk of code by raising an exception, calling return,
> or break.
>
> As a beginner, it took me years of misunderstanding before I finally
> understood for...else and while...else, because I kept coming back to the
> thought that the else block was executed if the for/while block *didn't*
> execute.

That's true. I needed to explain this to few people and I always need 
several attempts/starts to get it right in a simple statement:

'If you do a "break", then "else" is NOT executed.' I think the "NOT" 
results in heavy mental lifting.

> I couldn't get code with for...else to work right and I didn't
> understand why until finally the penny dropped and realised that "else"
> should be called "then".

That's actually a fine idea. One could even say: "finally".

Best,
Sven

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