On 28 Jan 2007 12:46:07 -0800, Thomas Nelson <[EMAIL PROTECTED]> wrote:
>My code:
>
>class Policy(list):
> def __cmp__(self,other):
> return cmp(self.fitness,other.fitness)
>
>j = Policy()
>j.fitness = 3
>k = Policy()
>k.fitness = 1
>l = Policy()
>l.fitness = 5
>print max([j,k,l]).fitness
>
>prints 3, when I was expecting it to print 5. What have I done wrong?
>
max() uses >, apparently. Which means the builtin list.__gt__ is consulted,
not your __cmp__ implementation:
>>> class x(list):
... def __cmp__(self, other):
... print 'yes'
... return 1
...
>>> max([x(), x(), x()])
[]
>>> class y(list):
... def __lt__(self, other):
... print 'yes'
... return cmp(id(self), id(other))
...
>>> max([y(), y(), y()])
[]
>>> class z(list):
... def __le__(self, other):
... print 'yes'
... return cmp(id(self), id(other))
...
>>> max([z(), z(), z()])
[]
>>> class w(list):
... def __ge__(self, other):
... print 'yes'
... return cmp(id(self), id(other))
...
>>> max([w(), w(), w()])
[]
>>> class v(list):
... def __gt__(self, other):
... print 'yes'
... return cmp(id(self), id(other))
...
>>> max([v(), v(), v()])
yes
yes
[]
Note that this is different from the operation used by list.sort():
>>> [v(), v(), v()].sort()
>>> [w(), w(), w()].sort()
>>> [x(), x(), x()].sort()
>>> [z(), z(), z()].sort()
>>> [y(), y(), y()].sort()
yes
yes
>>>
So, if you implement __gt__, max will work right. If you also implement
__le__, sorting will work right. Your best bet is probably to implement
all of the rich comparison methods.
Jean-Paul
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