PEP 276 Simple Iterator for ints (fwd)

[David Eppstein]

In article <3C150188.C31AC4B8 at ccvcorp.com>,
 Jeff Shannon <jeff at ccvcorp.com> wrote:

> (I still strongly dislike the proposed notation, though....  Maybe
> it's a style thing--I'd usually much rather have a clear function call
> than clever operator tricks.)

The two objections I have to the current "clear function call" (range and 
friends) are first, you can't understand it unless you crack that manual 
and have it tell you what the arguments mean and what the difference is 
between range and xrange, which is no good for writing code that 
non-Python-literate programmers can quickly and easily understand, and 
second, even for Pythonists, it doesn't lead to clear code unless what you 
need is half-open-on-the-right intervals with integer bounds, which only 
cover some of the uses of range.

For example, suppose you want to loop over the set of integers i satisfying 
x <= i < y (the usual half-open interval), but you need them in reverse 
order, and x and y may be non-integer.  As far as I can tell, the "one 
right way" of doing this is
    import math
    for i in range(int(math.ceil(y-1)),int(math.ceil(x-1)),-1): ...
Despite having some idea how range works I checked this in the interpreter 
and had to correct multiple mistakes: forgot the "math.", misspelled "ceil" 
as "ceiling", and incorrectly used floor(x+1) instead of ceil(x-1) in the 
second argument. And there's still a possible bug when integer unification 
comes around and makes long an acceptable argument for a range. This is a 
"clear function call" compared to
    for y > i >= x: ...
?
-- 
David Eppstein       UC Irvine Dept. of Information & Computer Science
eppstein at ics.uci.edu http://www.ics.uci.edu/~eppstein/