FAQ

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

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