How can I call a subclass method from parent class ?

[bonono@gmail.com]

Hi,

Suppose my class definition is like this :

class A:
name = "A"

@classmethod
def foo(cls):
cls.__super.foo ()
cls.bar()

@classmethod
def bar(cls):
print cls.name

class B(A):
name = "B"

class C(B):
name = "C"

What I want is

C.foo() prints 'ABC'
B.foo() prints 'BC'
A.foo() prints 'A'

But when I call C.foo(), it said

AttributeError: class C has no attribute '_A__super'

How should this be coded ?

[Jason Lai]

Re: How can I call a subclass method from parent class ?

If you use a newstyle class, e.g. class A(object), then you can get the
superclass with cls.__base__. You could also use super(cls,cls),
although note that it returns a <super> object that isn't exactly the
same thing as a class -- but good enough for just accessing attributes.

Make sure to check that your superclass isn't <object>, otherwise it'll
complain about <object> not having a foo attribute. __base__ is
probably easier for this purpose. Also be careful with multiple
inheritance.

No such thing as __super though.

[bonono@gmail.com]

Re: How can I call a subclass method from parent class ?

thanks, it works. Though I don't quite understand what super(cls,cls)
returns, and it doesn't work if I do a super(cls,cls). foo(). But
cls.__base__.fo o() do the trick.

thankfully, I don't have multiple inheritance.

Jason Lai wrote:[color=blue]
> If you use a newstyle class, e.g. class A(object), then you can get the
> superclass with cls.__base__. You could also use super(cls,cls),
> although note that it returns a <super> object that isn't exactly the
> same thing as a class -- but good enough for just accessing attributes.
>
> Make sure to check that your superclass isn't <object>, otherwise it'll
> complain about <object> not having a foo attribute. __base__ is
> probably easier for this purpose. Also be careful with multiple
> inheritance.
>
> No such thing as __super though.[/color]

[Steve Holden]

Re: How can I call a subclass method from parent class ?

bonono@gmail.co m wrote:[color=blue]
> Jason Lai wrote:
>[color=green]
>>If you use a newstyle class, e.g. class A(object), then you can get the
>>superclass with cls.__base__. You could also use super(cls,cls),
>>although note that it returns a <super> object that isn't exactly the
>>same thing as a class -- but good enough for just accessing attributes.
>>
>>Make sure to check that your superclass isn't <object>, otherwise it'll
>>complain about <object> not having a foo attribute. __base__ is
>>probably easier for this purpose. Also be careful with multiple
>>inheritance .
>>
>>No such thing as __super though.[/color]
>
> thanks, it works. Though I don't quite understand what super(cls,cls)
> returns, and it doesn't work if I do a super(cls,cls). foo(). But
> cls.__base__.fo o() do the trick.
>
> thankfully, I don't have multiple inheritance.
>
>[/color]
In point of fact super() is most useful when you *do* have multiple
inheritance. The classic example is when classes B and C have a common
supertype A, and then class D is a subclass of both B and C.

Under these circumstances (the so-called "diamond-shaped inheritance
graph") it can be problematic to ensure that each superclass's methods
are called exactly once when methods are being extended rather than
overridden: should a D call B's method, which then calls A's, and if so
how does a D ensure that C's method gets called without it also calling
A's method.

Calls to super() are used to effectively place a linearised oredering on
the superclasses to ansure that the diamond-shaped inheritance pattern
is correctly handled to give the correct method resolution order.

regards
Steve
--
Steve Holden +44 150 684 7255 +1 800 494 3119
Holden Web LLC www.holdenweb.com
PyCon TX 2006 www.python.org/pycon/

[bonono@gmail.com]

Re: How can I call a subclass method from parent class ?

Thanks for the explanation but some how my code fail and since I don't
need multiple inheritance for the moment, I would settle for the not so
clean version.

The documentation of super is not very clear to me too. As seen in my
code, I am using classmethod which may cause some problem.

Steve Holden wrote:
[color=blue]
> In point of fact super() is most useful when you *do* have multiple
> inheritance. The classic example is when classes B and C have a common
> supertype A, and then class D is a subclass of both B and C.
>
> Under these circumstances (the so-called "diamond-shaped inheritance
> graph") it can be problematic to ensure that each superclass's methods
> are called exactly once when methods are being extended rather than
> overridden: should a D call B's method, which then calls A's, and if so
> how does a D ensure that C's method gets called without it also calling
> A's method.
>
> Calls to super() are used to effectively place a linearised oredering on
> the superclasses to ansure that the diamond-shaped inheritance pattern
> is correctly handled to give the correct method resolution order.
>
> regards
> Steve
> --
> Steve Holden +44 150 684 7255 +1 800 494 3119
> Holden Web LLC www.holdenweb.com
> PyCon TX 2006 www.python.org/pycon/[/color]