[Python-ideas] for/else syntax

[Ron Adam]

Nick Coghlan wrote:
> Ron Adam wrote:
>> How about this?
>>
>>    try:
>>       for x in xs:
>>           foo()
>>       esle:
>>           Print "foo didn't raise an exception."
>>    except:
>>       print "foo riased an exception"
>>
>> Using exceptions for flow control is very common in python.
> 
> The else is adding no value here. That example would be better written as:
> 
>    try:
>       for x in xs:
>           foo()
>       print "foo didn't raise an exception."
>    except:
>       print "foo riased an exception"
> 
>> Or this:
>>
>>    for x in xs:
>>        y = foo()
>>        if y is True:
>>           return
>>    else:
>>        Print "y was never True"
>>
>> This is perfectly acceptable in my opinion.
> 
> Again, using else here is redundant and misleading. The code is clearer
> if it is left out entirely:
> 
>    for x in xs:
>        y = foo()
>        if y is True:
>           return
>    print "y was never True"
> 
> The *only* time an else clause is a better alternative to just writing
> the code after the loop is when there is a break statement present that
> may skip over it.

All good points.  I need another cup of coffee. ;-)

There may be some obscure exception to the break-else pattern but I can't 
think of any at the moment.


Considering the following...

 >>> break
   File "<stdin>", line 1
SyntaxError: 'break' outside loop

Documenting for/break/else patterns seems very appropriate.
+1



I suppose that it would have been clearer if it was for/then and 
while/then. With a break stopping the loop, and skipping over the then block.

While it's much easer to explain that way, It adds a new keyword and isn't 
something we could or should change now I suppose.


Cheers,
    Ron