I want to implement dynamic polymorphism in python like in java. Is it possible? if yes please guide me how.I want to implement something like this:
class A {
public void show() {
System.out.println("Base class");
}
}
class B extends A{
public void show() {
System.out.println("Derived class");
}
}
class C {
public static void main(String args[]) {
A a = new B();
a.show(); // so it will call B's show method.
}
}
Of course it is possible. Actually Python is even more powerful in this sense because it has only dynamic types. In the example below you do not even need to derive B from A.
class A:
def show(self):
print "Base class"
class B(A): # this would also work fine without deriving: class B:
def show(self):
print "Derived class"
a = A()
a.show()
a = B()
a.show()
When interpretter meets the expression a.show(), it only looks up the object 'a' and checks if it has method 'show' implemented. If yes, then it runs this method regardless of inheritance or whatever. If 'show' is not found, then exception is raised.
Your issue is here:
A a = new B();
Because Python is dynamic, there is no way to declare that a variable is of type A, hence after a = B() the name a will always be bound to a B object until you delete or rebind the name.
To me, the question seems irrelevant. Since Python uses "duck typing" for classes, the show method for the class you actually create an instance of would be the one called.
Duck typing:
In Python (2.x) you could do something like:
class A(object): # inherit from base object
def show(self): # define function to inherit
print "Base class"
class B(A): # inherit from A
def show(self): # override A.show()
print "Derived class"
a = B() # instance of derived class B
a.show() # would print "Derived class"
Note that, due to the inheritance from A in B:
isinstance(a, A) == True
Alternatively, you could do:
a = A() # instance of base class A
a.show = B().show # use B's method for this instance of A
B's show for an A instance; is that what you had in mind?
Bclass to be derived fromA? Because the code says it's not.