[Python-ideas] Updating PEP 315: do-while loops

[Arnaud Delobelle]

On 27 Apr 2009, at 18:49, Terry Reedy wrote:

> A quick summary of my views:
>
> 1. When I programmed in C, I hardly if ever used do...while.  I have  
> read that this is true of other C coders also.  So I see no need for  
> a Python equivalent.
>

I haven't got much C that I wrote myself, but on my most recent C  
project, out of 100 loops:

* 65 are while loops
* 31 are for loops
* 4 are do-while loops

In the py3k C source, out of 100 loops I see:

* 60 for loops
* 32 while loops
* 8 do-while loops

> 2. On the other hand, loop and a half use is a regular occurrence.  
> Anything that does not arguably improve
>
> while True:
>  part1()
>  if cond: break
>  part2()
>
> such as
>
> loop
>  part1()
> whhile not cond:
>  part2()
>
> seems pretty useless.  But even the above does not work great for  
> multiple "if... break"s and not at all for "if...continue".
>

This is my opinion also.

> 3. I do not see 'if cond: break' as that much of a problem.  One can  
> emphasize with extra whitespace indent or comment.
>  if cond:                         break #EXIT#
>
> In any case, I see it as analogous to early return
>
> def f():
>  part1()
>  if cond: return
>  part2()
>
> and while there are purists who object to *that*, I have not seen  
> any proposals to officially support better emphasis (other than the  
> existing whitespace or comment mechanisms).
>

My idea with 'break/continue if' was, in my mind, such a proposal.  I  
thought that it would enshrine what I perceived as a very common  
construct ('if ...: break/continue') into the language and make it  
easier to spot when scanning code.

> In fact, it is not uncomment to use 'return expr' instead of  
> 'ret=expr; break' when a loop (for or while) is the last part of a  
> def.
>
> 4. "do ... while cond:" strikes me as ugly.  Aside from that, I do  
> not like and would not use anything that puts the condition out of  
> place, other than where it is executed.  I would hate to read such  
> code and expect it would confuse many others.
>

I definitely agree with this.

-- 
Arnaud