[Python-3000] Math in Python 3.0

[Fredrik Johansson]

On 5/12/06, Tim Peters <tim.peters at gmail.com> wrote:
> What did he state twice?  I see him checking the parity just once there.

The information "n is odd" appears once as "n&1" and once in the name
of a function. The point is that any if statement has the following
form:

    if conditionX:
        consequence of conditionX

So the if statement *inevitably* references conditionX twice, although
it may do so in form of a function or variable explicitly named
"conditionX" or implicitly in terms of code that the reader interprets
as "conditionX"/"consequence of conditionX".

Raymond's example code says "consequence of conditionX" explicitly
("handle_odd"). That's what I think is rare. Your code illustrates my
point:

    if a & 1:
        a = b-a

    while a & 1 == 0:
        a >>= 1

Here the heads read "conditionX" ("a is odd/even"), but "a = b-a" and
"a >>= 1" certainly aren't as trivial as "handle_odd", so in this case
the repetitiveness Raymond speaks about doesn't exist.

The version

    if isodd(a):
        a = b-a

    assert iseven(a)

    while iseven(a):
        a >>= 1

seems more readable to me. The advantage is perhaps questionable in
the context of any code that does bit manipulation (in which my mental
model of integers automatically turns them into bit strings), but not
all code that is concerned with whether a number is odd does that.

Anyway, the particular example of isodd/iseven wasn't the intended
topic of this thread. Better go ahead with that library so people can
try it to decide if there's anything in it they like instead of
arguing over trivialities on a mailing list :-)

Fredrik Johansson