[Python-Dev] Wishlist: dowhile

Guido van Rossum gvanrossum at gmail.com
Mon Jun 13 06:01:42 CEST 2005

On 6/12/05, Nick Coghlan <ncoghlan at gmail.com> wrote:
> Raymond Hettinger wrote:
> > More than case-statement semantics or PEP343, I wish for a dowhile
> > statement.
> >
> > The most straight-forward way is to put the conditional expression at
> > the beginning of the block with the understanding that a dowhile keyword
> > will evaluate the condition only after the block runs
> Out-of-order code execution rarely counts as 'straightforward' ;)

Right. Millions of years of programming language design have shown us
how to write a loop with the condition tested at the end -- the
condition is written after the loop body.

> With PEP 315, a do-while loop would look like:
>    do:
>        <body>
>    while <cond>:
>        pass
> But then, I'm reasonably happy with the 'break out of an infinite
> loop' approach, so *shrug*.


If we have to do this, PEP 315 has my +0.

It is Pythonically minimal and the motivation rings true: I've often
written code like this in the past:

    line = f.readline()
    while line:
        <do something>
        line = f.readline()

But these days we do that using "for line in f".

I wonder if other similar use cases can't be rewritten using better iterators?

Maybe Raymond can show us some motivating use cases.

--Guido van Rossum (home page: http://www.python.org/~guido/)

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