Understanding while...else...

[Terry Reedy]

On 1/22/2013 3:09 PM, Ethan Furman wrote:
> On 01/22/2013 09:44 AM, Terry Reedy wrote:
>> Several people have trouble understanding Python's while-else and
>> for-else constructs. It is actually quite simple if one starts with
>> if-else, which few have any trouble with.
>>
>> Start with, for example
>>
>> if n > 0:
>>    n -= 1
>> else:
>>    n = None
>>
>> The else clause is executed if and when the condition is false. (That
>> the code is useless is not the point here.) Now use pseudo-Python label
>> and goto statements to repeatedly decrement n
>>
>> label: check
>> if n > 0:
>>    n -= 1
>>    goto: check
>> else:
>>    n = None
>>
>> The else clause is executed if and when the condition is false.
>> Now use a real Python while statement to do the *same
>> thing*.
>>
>> while n > 0:
>>    n -= 1
>> else:
>>    n = None
>
> I understand how it works (although it did take a while for it to sink
> in); my gripe, and probably why it is misunderstood so often, is that
> nine times out of ten when I /want/ to use a while-else or for-else I
> only want the true/false check /once/, at the beginning of the loop.

I do not understand what you are saying. There already is only one 
true/false check, at the beginning of the loop. If you only want the 
check *performed* once, you would use if-else. But I presume you know this.

-- 
Terry Jan Reedy