PEP 315: Enhanced While Loop

[Alex Martelli]

Ben Allfree wrote:

> Can you provide an example snippit?
> 
> I'm interested, but I can't come up with a scenario where the loop
> couldn't be rewritten, except for do..until loops that must execute at
> least once, which require the first loop iteration to be placed before a
> while loop.

Those "can be rewritten" too, of course.

In general, the proposed syntax:

  do:
     setup
  while condition:
     remainder

has exactly the same semantics as:

  while True:
    setup
    if not condition: break
    remainder

so the rewrite is always trivially easy.  The PEP claims the current
form is unreadable -- and that syntax-sugar issue is all that needs
to be discussed (as well as choice of new keyword -- if the new PEP
was accepted I'd rather have the more readable 'loop' instead of
the totally obscure 'do').


> And for that scenario, I would propose a simple "until" loop construct
> that is guaranteed to execute once before evaluation:
> 
> # Old way
> MakeMoney()
> while Poor():
>    MakeMoney()
> 
> # New way
> until not Poor():
>    MakeMoney()

Rewritable as:

  while True:
      MakeMoney()
      if not Poor(): break

> The above example assumes one must MakeMoney() before running the Poor()
> test ;)
> 
> Python programmers would just have to understand that the "until"
> construct executes once before evaluation. Or maybe it could be named
> something more obvious. "AfterWhile" - heh - no pun intended.

I find myself quite grateful that the designer of Python will never
allow anything as absurd as your proposal -- an unreadable "backwards"
ordering (condition appears before but is tested after) AND a new
keyword, all to "solve" *HALF* of an (arguable) problem, when it's
just as easy, as the PEP shows, to solve ALL of it WITHOUT imposing
"out of order" sequencing of source code vs execution order!


Alex