__cmp__ method

[JH]

Hi

Can anyone explain to me why the following codes do not work? I want to
try out using __cmp__ method to change the sorting order. I subclass
the str and override the __cmp__ method so the strings can be sorted by
the lengh. I expect the shortest string should be in the front. Thanks
[color=blue][color=green][color=darkred]
>>> class myStr(str):[/color][/color][/color]
def __init__(self, s):
str.__init__(se lf, s) # Ensure super class is initialized
def __cmp__(self, other):
return cmp(len(self), len(other))
[color=blue][color=green][color=darkred]
>>> a = myStr('abc')
>>> b = myStr('Personal ')
>>> c = myStr('Personal firewall')
>>> sorted([c, b, a])[/color][/color][/color]
['Personal', 'Personal firewall', 'abc'][color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

[marek.rocki@wp.pl]

Re: __cmp__ method

Python documentation says:[color=blue]
>__cmp__( self, other)
> Called by comparison operations if rich comparison (see above) is not defined.[/color]
So it seems you have to redefine rich comparisons __lt__, __gt__,
__eq__ etc as well.

If all you need is change sorting order, why not use appropriate
parameters of sorted() function (cmp=... or key=...)?

[Jon Clements]

Re: __cmp__ method

This probably isn't exactly what you want, but, unless you wanted to do
something especially with your own string class, I would just pass a
function to the sorted algorithm.

eg:

sorted( [a,b,c], cmp=lambda a,b: cmp(len(a),len( b)) )

gives you the below in the right order...

Never tried doing what you're doing, but something about builtin types,
and there's a UserString module...

Hope that helps a bit anyway,

Jon.

JH wrote:
[color=blue]
> Hi
>
> Can anyone explain to me why the following codes do not work? I want to
> try out using __cmp__ method to change the sorting order. I subclass
> the str and override the __cmp__ method so the strings can be sorted by
> the lengh. I expect the shortest string should be in the front. Thanks
>[color=green][color=darkred]
> >>> class myStr(str):[/color][/color]
> def __init__(self, s):
> str.__init__(se lf, s) # Ensure super class is initialized
> def __cmp__(self, other):
> return cmp(len(self), len(other))
>[color=green][color=darkred]
> >>> a = myStr('abc')
> >>> b = myStr('Personal ')
> >>> c = myStr('Personal firewall')
> >>> sorted([c, b, a])[/color][/color]
> ['Personal', 'Personal firewall', 'abc'][color=green][color=darkred]
> >>>[/color][/color][/color]

[George Sakkis]

Re: __cmp__ method

Jon Clements wrote:
[color=blue]
> This probably isn't exactly what you want, but, unless you wanted to do
> something especially with your own string class, I would just pass a
> function to the sorted algorithm.
>
> eg:
>
> sorted( [a,b,c], cmp=lambda a,b: cmp(len(a),len( b)) )
>
> gives you the below in the right order...[/color]

Or even better in 2.4 or later:
sorted([a,b,c], key=len)

George

[bruno at modulix]

Re: __cmp__ method

JH wrote:[color=blue]
> Hi
>
> Can anyone explain to me why the following codes do not work? I want to
> try out using __cmp__ method to change the sorting order. I subclass
> the str and override the __cmp__ method so the strings can be sorted by
> the lengh. I expect the shortest string should be in the front. Thanks[/color]

AFAIK (please a Guru correct me if I'm wrong), __cmp__ is used as a
fallback when other comparison operators ( __lt__ etc) are not implemented :
[color=blue][color=green][color=darkred]
>>> class MyStr(str):[/color][/color][/color]
.... def __lt__(self, other):
.... return len(self) < len(other)
.... def __le__(self, other):
.... return len(self) <= len(other)
.... def __gt__(self, other):
.... return len(self) > len(other)
.... def __ge__(self, other):
.... return len(self) >= len(other)
....[color=blue][color=green][color=darkred]
>>> items = [MyStr("lolo lala"), MyStr("lolo"), MyStr("alal"), MyStr("a")]
>>> sorted(items)[/color][/color][/color]
['a', 'lolo', 'alal', 'lolo lala'][color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

But anyway, all this is a WTF. Just using the correct params for sorted
is enough:[color=blue][color=green][color=darkred]
>>> items2 = ['lolo lala', 'lolo', 'alal', 'a']
>>> sorted(items2, key=len)[/color][/color][/color]
['a', 'lolo', 'alal', 'lolo lala']

[color=blue]
>[color=green][color=darkred]
>>>>class myStr(str):[/color][/color]
>
> def __init__(self, s):
> str.__init__(se lf, s) # Ensure super class is initialized[/color]

This is useless. If you don't override it, parent's init will be called
anyway.

(snip)

HTH
--
bruno desthuilliers
python -c "print '@'.join(['.'.join([w[::-1] for w in p.split('.')]) for
p in 'onurb@xiludom. gro'.split('@')])"