[Python-ideas] if condition: break idiom

[Arnaud Delobelle]

On 21 Sep 2008, at 20:24, Josiah Carlson wrote:
[...]
> Not all statements; all *control flow* statements.  Return, yield,
> continue, break, assert, ..., can all change program flow.  To say
> that break and continue should be special cased is silly, as "Special
> cases aren't special enough to break the rules".  As such, why
> continue and break but not return or yield?  Further, the syntax is
> *so very similar* to conditional expressions <X> if <C> vs. <X> if <C>
> else <Y>, the lack of an else clause could be confusing to those who
> have seen and used conditional expressions before, never mind the
> meaning of them.

Well my view was that break and continue are the only two statements  
that relate to loops.
[...]

>> I don't agree with that: the absence of do .. while liberates the  
>> loop
>> construct in python from its constraints and the first form above  
>> becomes
>> more common.
>
> But you were just arguing that the *lack* of do/while makes the
> embedded if necessary.  Now you are saying that it *liberates* us from
> the control-flow induced by do/while.  ;)  There's an argument that
> says rather than treat the symptom (breaks in the body), treat the
> disease (lack of a do/while).  But since the lack of a do/while isn't
> a disease, by your own words, then the symptom is not a bug, it's a
> feature ;)
>

There is a missing link in your interpretation of my argumentation.   
It is that I haved noticed that, as I do not have a do .. while  
construct at my disposal in Python, I do not try to shape my loops  
into this structure anymore.  I almost *never* write:

     while True:
         ...
         if condition: break

But most of the time it seems that the correct structure for a loop  
comes as

     while True:
         ...
         if condition: break
         ...

In fact, I would be happy with getting rid of the while loop and  
replacing it with a simple 'loop' constuct, where:

     while condition:
         ...

would be written as:

     loop:
         if condition: break
         ...

However I see this as too radical to propose :)

>>> Which *has* an idiom in Python.
>>>
>>> first = 1
>>> while first or condition:
>>>  first = 0
>>>  ...
>>>
>>
>> I would not use this, not because it is slower, but because it is  
>> uglier.
>
> That's funny, because whenever I use it in a production codebase,
> coworkers always comment how they like it because it pushes the
> condition for the loop at the loop header rather than embedding it in
> the (sometimes long) body.  In particular, I've seen the lack of a
> do-while in Python result in an if clause at the bottom of a 150 line
> while, which had been confusing as hell for anyone who got to touch
> that code.  Moving it up to the top made it clear and resulted in a
> removal of half a dozen lines of comments at the top explaining why
> the loop was constructed like this.  After the above translation,
> those comments became "this emulates a do-while clause".
>

This makes a lot of sense, if you use a do .. while concept in your  
loops.  Now I feel bad that I used the word "uglier" ...

>
>> Anyway, there hasn't been a flurry of positive responses so far so  
>> I don't
>> think this is going to go much further than this reply...
>
> Syntax changes are tough to get agreement on in Python.
>

Thank you for your comments, they all make a lot of sense.

-- 
Arnaud