Frank Millman wrote:
> Hi all
>
> I recently posted a question about subclassing. I did not explain my
> full requirement very clearly, and my proposed solution was not pretty.
> I will attempt to explain what I am trying to do more fully, and
> describe a possible solution. It is still not pretty, so I would
> appreciate any comments.
>
> I have a base class (ClassA), which is an abstract class. Most of the
> methods and attributes are common to all subclasses, so there is not
> much they have to override.
>
> I have a subclass (ClassB) of my base class, which is also abstract. It
> represents a subset of ClassA, and overrides some of its methods. When
> I create a concrete class (is that the correct term?) I subclass either
> from ClassA or from ClassB.
>
> Now I want to represent a different subset of ClassA, which overrides
> some of its methods. This subset can apply to ClassB as well as to
> ClassA.
>
> In pseudo terms, I want ClassA1, ClassA2, ClassB1, and ClassB2 where A1
> is the base class, B overides some methods, and 2 overrides other
> methods, and I want to subclass from any of them.
>
> My original solution involved passing 1 or 2 as an argument, and
> putting some code into __init__ which redefined certain methods if it
> received a 2. This worked, but it meant that I could not then easily
> redefine the method again in a concrete class.
>
> My new idea is to use multiple inheritance. This is how it would work.
>
> class ClassA(object):
> def __init__(self):
> pass
> def test1(self):
> print 'Base method 1'
> def test2(self):
> print 'Base method 2'
>
> class ClassB(ClassA):
> def __init__(self):
> ClassA.__init__(self)
> def test1(self):
> print 'Overriding method 1'
>
> class Class2(object):
> def test2(self):
> print 'Overriding method 2'
To be pedantic, Class2.test2 is not overridding anything, since there's
no "test2" method in it's parent class.
> Now I can set up the following concrete classes -
>
> class ClassA1(ClassA):
> def __init__(self):
> ClassA.__init__(self)
If that's the only thing you do in the __init__, then don't bother write
an init method at all.
> class ClassA2(Class2,ClassA):
> def __init__(self):
> ClassA.__init__(self)
May I suggest having a look at super() ?
> class ClassB1(ClassB):
> def __init__(self):
> ClassB.__init__(self)
>
> class ClassB2(Class2,ClassB):
> def __init__(self):
> ClassB.__init__(self)
>
> Now if I do the following, I get the results shown, which is what I
> want -
>
> ClassA1().test1() - 'Base method 1'
> ClassA1().test2() - 'Base method 2'
> ClassB1().test1() - 'Overriding method 1'
> ClassB1().test2() - 'Base method 2'
> ClassA2().test1() - 'Base method 1'
> ClassA2().test2() - 'Overriding method 2'
> ClassB2().test1() - 'Overriding method 1'
> ClassB2().test2() - 'Overriding method 2'
>
> Now for the real test -
>
> class ClassC3(Class2,ClassB):
> def __init__(self):
> ClassB.__init__(self)
> def test1(self):
> print 'Overriding method 1 from ClassC3'
> def test2(self):
> print 'Overriding method 2 from ClassC3'
>
> ClassC3().test1() - 'Overriding method 1 from ClassC3'
> ClassC3().test2() - 'Overriding method 2 from ClassC3'
>
> So it works. However, using multiple inheritance is not ideal,
Why so ? Multiple inheritence is a pretty useful tool - but it can
become tricky very soon. IMHO, it's best use is for mixin classes...
> and I
> believe it is not even supported in some languages.
A lot of things aren't even supported in some languages !-)
> Can anyone suggest
> a better way of tackling this problem?
Not out of my hat. Just a few considerations on Python and OO: Python
being dynamically typed, inheritence is only about sharing
implementation. There's another way to do share implementation -
composition/delegation. It's more flexible, and can avoid "cartesian
product" multiplication of classes. It's also less advertised than
inheritance - probably because of "some languages" that fail to offer
any support for it. The good news here is that Python makes it a breeze,
thanks to the __getattr__/__setattr__ hooks. Now I don't know if it
makes any sense WRT/ your current problem...
--
bruno desthuilliers
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p in '[EMAIL PROTECTED]'.split('@')])"
--
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