[Python-ideas] PEP 315: do-while

[Greg Ewing]

Łukasz Langa wrote:
Alternatively, the only way I think we can improve on
> the syntax above is something like this:
> 
>   do:
>     <code>
>     if condition:
>       break

One of the problems with a 'while' clause as suggested is that the
comprehension version of it would complicate the equivalence rules
between comprehensions and statements. Currently, a comprehension
such as

    [... for a in x if c]

corresponds to

    for a in x:
       if c:
          ...

But if we take a comprehension with a 'while' clause,

    [... for a in x while c]

and try to expand it the same way, we get

    for a in x:
       while c:
          ...

which is not the same thing at all! To make it work, we need to
expand it as

    for a in x:
       if not c:
          break

But this only works if there is only one for-loop.
If we have two nested loops,

    [... for a in x for b in y while c]

the following expansion doesn't work,

    for a in x:
       for b in y:
          if not c:
             break

because the break only exits the innermost loop.

The proposed 'while' clause has a similar problem. In

    for a in x:
       for b in y while c:
          ...

the 'while' would presumably break only the innermost
loop. If we try to write it like this instead,

    for a in x while c:
       for b in y:
          ...

we have a problem if the condition c involves both a
and b.

I'm not sure what the lesson is from all this. I think
it's that any form of early-exit construct, including
'break' and the proposed 'while' clause, is a hack of
limited usefulness without some way of scoping the
amount of stuff to be broken out of. Comprehensions
provide a natural way of specifying that scope, whereas
the looping statements don't.

So, I would probably support adding a 'while' clause
to comprehensions, but not to the for-loop statement.

-- 
Greg