how to convert code that uses cmp to python3

[Antoon Pardon]

Op 08-04-16 om 16:25 schreef Chris Angelico:
> On Sat, Apr 9, 2016 at 12:20 AM, Antoon Pardon
> <antoon.pardon at rece.vub.ac.be> wrote:
>>> You only need ONE comparison, and the other is presumed to be its
>>> opposite. When, in the Python 3 version, would you need to compare
>>> twice?
>>
>> About 50% of the time. When I traverse the tree I go left when the
>> argument key is smaller than the node key, I go right when it is
>> greater than the node key and I have found the node I want when
>> they are equal.
> 
> How about this:
> 
> You have found the node if they are equal.
> Otherwise, go left if your argument is smaller than the node.
> Otherwise, go right.
> 
> You don't have to do three comparisons, only two - and one of them is
> an equality, rather than an inequality, which is often cheaper.

You don't seem to understand. I only do two comparisons and no the
equality is not necesarrily cheaper.

I am talking about the difference between the following two:

    if arg.key < node.key:   # possible expensive computation
        go_left()
    elif arg.key == node.key # possible expensive computation
        found_node()
    else:
        go_right()

and

    delta = cmp(arg.key, node.key) # possible expensive computation
    if delta < 0:                  # cheap computation
        go_left()
    elif delta == 0:               # cheap computation
	found_node()
    else:
        go_right()

> But
> hey. If you really can't handle the double comparison, *write your
> own* special-purpose comparison function - nobody's stopping you! It's
> just not something that exists *in the language*. If your specific
> objects need this, write it!

Sure I could do that, but it may not be worth the trouble without
language support. Just writing a cmp function is not sufficient, because
the trivial way to write it, will just hide the expense in that function.

For the moment I will probably go with Ben Finney's suggestion and go
with somekind of lru cache. So when I do one of the comparisons with
my own objects, it will do the expensive cmp-like computation behind
the scene and then return the result of the cheap computation. I just
check the cache before to see whether the result of the expensive
computation is already available.

-- 
Antoon Pardon