Sorting: too different times. Why?

[Steven D'Aprano]

In the subject line, you write "too different times". You actually want 
"two", the number, not "too" as in "too many", "too much". Lots of native 
English speakers get this wrong too :)

On Sun, 22 Nov 2009 01:21:42 -0800, n00m wrote:

> Any comment:
> 
> class Vector:
>     def __init__(self, x, y):
>         self.x = x
>         self.y = y
>     def __cmp__(self, v):
>         if self.x < v.x and self.y > v.y:
>             return -1
>         return 0


Modern versions of Python (since 2.2 I think?) use __lt__ or __gt__ for 
sorting. If the class doesn't have a __lt__ method, Python falls back on 
__cmp__.

> b.sort(cmp=v_cmp)

This is relatively fast, because you pass a comparison function directly, 
so Python doesn't need to look for a __lt__ method then fall back to 
__cmp__. It just uses v_cmp, every time.


> c.sort()

This is slower, because every comparison looks up the __lt__ and fails, 
then tries the __cmp__.

If you change the definition of Vector to include rich comparison methods 
as detailed here:

http://docs.python.org/reference/datamodel.html#object.__lt__

sorting will probably be significantly faster still.



-- 
Steven