custom sorting and __cmp__

[Lee Harr]

Let's say I have a class

class A:
def __init__(self, level):
self.level = level

and I want to put some of these in a list and sort the list
by level

a1 = A(1)
a2 = A(2)
l = [a1, a2]
l.sort()

Am I better off creating a __cmp__ method for my class or
making a cmp function to pass to sort?


My thought was that __cmp__ would be perfect, but then I
started thinking about this ...
[color=blue][color=green][color=darkred]
>>> class A:[/color][/color][/color]
.... def __init__(self, level):
.... self.level = level
.... def __cmp__(self, other):
.... if self.level > other.level:
.... return 1
.... elif self.level < other.level:
.... return -1
.... else:
.... return 0
....[color=blue][color=green][color=darkred]
>>> a1 = A(1)
>>> a2 = A(2)
>>> class C:[/color][/color][/color]
.... pass
....[color=blue][color=green][color=darkred]
>>> c = C()
>>> a1 == a2[/color][/color][/color]
False[color=blue][color=green][color=darkred]
>>> a1 < a2[/color][/color][/color]
True[color=blue][color=green][color=darkred]
>>> a1 == c[/color][/color][/color]
Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "<stdin>", line 5, in __cmp__
AttributeError: C instance has no attribute 'level'


Should I be catching comparisons to objects that do not have
my 'level' attribute and falling back to id comparison?
Or am I worried about nothing (YAGNI :o) ?


Is this related in any way to interfaces?

[Emile van Sebille]

Re: custom sorting and __cmp__

Lee Harr[color=blue]
> My thought was that __cmp__ would be perfect, but then I
> started thinking about this ...
>[color=green][color=darkred]
> >>> class A:[/color][/color]
> ... def __init__(self, level):
> ... self.level = level
> ... def __cmp__(self, other):
> ... if self.level > other.level:
> ... return 1
> ... elif self.level < other.level:
> ... return -1
> ... else:
> ... return 0[/color]

Instead, how about:
def __cmp__(self,ot her):
try: return cmp(self.level, other.level)
except return 1 # or -1

--

Emile van Sebille
emile@fenx.com

[Kristian Ovaska]

Re: custom sorting and __cmp__

Lee Harr <missive@fronti ernet.net>:[color=blue]
>Should I be catching comparisons to objects that do not have
>my 'level' attribute and falling back to id comparison?[/color]

I usually solve it like this:

class A:
def __init__(self, level):
self.level = level

def __cmp__(self, other):
if isinstance(othe r, A):
return cmp(self.level, other.level)
else:
return -1

--
Kristian Ovaska - http://www.cs.helsinki.fi/u/hkovaska/

[Aahz]

Re: custom sorting and __cmp__

In article <slrnbsk5ms.1pe .missive@homer. easthighschool. net>,
Lee Harr <missive@hotmai l.com> wrote:[color=blue]
>[color=green][color=darkred]
>>>> a1 == c[/color][/color]
>Traceback (most recent call last):
> File "<stdin>", line 1, in ?
> File "<stdin>", line 5, in __cmp__
>AttributeError : C instance has no attribute 'level'
>
>Should I be catching comparisons to objects that do not have
>my 'level' attribute and falling back to id comparison?
>Or am I worried about nothing (YAGNI :o) ?[/color]

Python is these days moving more and more toward preventing
heterogeneous comparisons:
[color=blue][color=green][color=darkred]
>>> import datetime, time
>>> d=datetime.date .fromtimestamp( time.time())
>>> d>1[/color][/color][/color]
Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: can't compare datetime.date to int

However, you should set things up so that you can use ``==``:
[color=blue][color=green][color=darkred]
>>> d==1[/color][/color][/color]
False

The easy way to handle this is to use the new special methods for rich
comparisons instead of __cmp__.
--
Aahz (aahz@pythoncra ft.com) <*> http://www.pythoncraft.com/

Weinberg's Second Law: If builders built buildings the way programmers wrote
programs, then the first woodpecker that came along would destroy civilization.