Status of PEP's?

[David Eppstein]

In article <mailman.1015019284.27486.python-list at python.org>,
 James_Althoff at i2.com wrote:

> In other words, if I want to iterate up (smaller up to larger) I need to
> write the for-loop as
>     for smaller <= i <= larger:  # can *only* iterate up
> but if I want to iterate down (larger down to smaller) I need to write the
> for-loop with the reverse relational operators
>     for larger >= i >= smaller:  # can *only* iterate down
> 
> So if you want/need "from/to" functionality for an "interval" using the
> suggested for+relational idiom, you need to have two for-loops at hand,
> test the bounds, *and* pick the corresponding, correct for-loop (whose
> relational operators are in the correct direction).
> 
> Note that the existing range and xrange functions (when used in a
> for-loop), although requiring a test on the bounds in order to determine
> the correct order of the args, do *not* require the use of two for-loops in
> this context.
>     for i in range(n,m,step):  # can iterate up or down depending on
> n,m,step

Ok, I missed your point that the iteration order and not just the endpoints 
are important in your example.

But I think the current range system does too require two loops.
Again, let's consider closed intervals.

If from <= to:
     range(from,to+1)

If from > to:
     range(from,to-1,-1)

You could sort of avoid this by using a signum function:
     range(from,to+signum(from-to),signum(from-to))
but (besides being confusing) this breaks when from=to.

Anyway, while you may need two ranges, you don't have to have two separate 
for-loops over those ranges:
    if from <= to: range = [x for from <= x <= to]
    else: range = [x for from >= x >= to]
    for x in range:
        ...
-- 
David Eppstein       UC Irvine Dept. of Information & Computer Science
eppstein at ics.uci.edu http://www.ics.uci.edu/~eppstein/