Class type coercion

Robert Cragie rcc at jennic.com
Wed Apr 5 14:17:19 CEST 2000


(Apologies if this has been covered before, I'm a bit of a Python newbie)

I posted a question to wxWindows list, and got an answer from Robin Dunn,
but it raised a more general question...

In C++, it is easy to use a pointer to a object declared as a super class of
that object's class, and then coerce it to be the same class type as the
object, e.g.

// Declare instance of class 'MyClass', which is derived from class
'MySuperClass'
MyClass MyClassObj;

// Declare pointer to class 'MySuperClass'
MySuperClass* pMySuperClass;

pMySuperClass = &MyClassObj; // OK, class 'MyClass' is derived from class
'MySuperClass'

// Coerce pointer to get to method which is only in class 'MyClass'
((MyClass*)pMySuperClass)->MyMethodOnlyInMyClass();

Can you do the equivalent in Python? I feel there must be some
straightforward way to do this, I just haven't figured it out yet. Or is
there...? As there is no type declaration in Python maybe this just isn't
possible.

There is a real-world need for this e.g. parent Window object contains
various child control objects. All the control classes are subclassed from
the Window class. All control objects have an ID. There often exists a
Window method to get a child window by ID. However, it can only return a
type of Window class. Therefore, to get to the actual control, this return
type needs to be subclassed.







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