how to convert code that uses cmp to python3

[Steven D'Aprano]

On Fri, 8 Apr 2016 06:34 pm, Marko Rauhamaa wrote:

> Antoon Pardon <antoon.pardon at rece.vub.ac.be>:
> 
>> In python2 descending the tree would only involve at most one
>> expensive comparison, because using cmp would codify that comparison
>> into an integer which would then be cheap to compare with 0. Now in
>> python3, I may need to do two expensive comparisons, because there is
>> no __cmp__ method, to make such a codefication.
> 
> I think you should base your tree implementation on key.__lt__() only.
> Only compare keys using <, nothing else, ever.

I believe that's how list.sort() and sorted() work:

py> class Spam(object):
...     def __init__(self, n):
...             self.n = n
...     def __lt__(self, other):
...             return self.n < other.n
...     def __repr__(self):
...             return repr(self.n)
...
py> L = [Spam(5), Spam(3), Spam(9), Spam(1), Spam(2)]
py> L
[5, 3, 9, 1, 2]
py> sorted(L)
[1, 2, 3, 5, 9]


as well as max() and min().



-- 
Steven