Inheritance question

[Eric Brunel]

On Tue, 25 Mar 2008 16:37:00 +0100, Brian Lane <bcl at brianlane.com> wrote:

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>
> Gerard Flanagan wrote:
>
>> Use the child class when calling super:
>>
>> --------------------------------------
>> class Foo(object):
>>     def __init__(self):
>>         self.id = 1
>>
>>     def getid(self):
>>         return self.id
>>
>> class FooSon(Foo):
>>     def __init__(self):
>>         Foo.__init__(self)
>>         self.id = 2
>>
>>     def getid(self):
>>         a = super(FooSon, self).getid()
>>         b = self.id
>>         return '%d.%d' % (a,b)
>>
>> print FooSon().getid()
>> --------------------------------------
>
> That still doesn't do what he's trying to do.
>
> The problem here is that you only have one instance. self refers to it,
> so when you set self.id in the super you have set the instance's id to
> 1. When you set it to 2 after calling the super's __init__ you are now
> setting the *same* variable to a 2.
>
> Basically, you can't do what you are trying to do without using a
> different variable, or keeping track of a separate instance for the
> super instead of sub-classing it.

If for any reason it's better to have the same attribute name, it might be  
a case where a "pseudo-private" attribute - i.e. prefixed with __ - can be  
handy:

--------------------------------------
class Foo(object):
     def __init__(self):
         self.__id = 1
     def getid(self):
         return self.__id

class FooSon(Foo):
     def __init__(self):
         Foo.__init__(self)
         self.__id = 2
     def getid(self):
         a = super(FooSon, self).getid()
         b = self.__id
         return '%d.%d' % (a,b)

print FooSon().getid()
--------------------------------------

For an explanation, see here:  
http://docs.python.org/ref/atom-identifiers.html

HTH
-- 
python -c "print ''.join([chr(154 - ord(c)) for c in  
'U(17zX(%,5.zmz5(17l8(%,5.Z*(93-965$l7+-'])"