Convert superclass instance to subclass instance

Question

• I have an external library which I cannot touch. This library has a function, genA(), that returns the instance of class A.
• In my side I define class B as a subclass of class A.
• I want to use the instance of class B in my project, but the instance should be generated by genA().

Is there any standard and easy way to do this?

---

# I cannnot tweak these code

def genA():
    a = A
    return(a)

class A:
    def __init__():
        self.a = 1

# ---

# code in my side

class B(A):
    def __init__():
        self.b = 2


a = genA()
# like a copy-constructor, doesn't work
# b = B(a)

# I want to get this
b.a # => 1
b.b # => 2

---

Here is an equivalent c++ code:

#include <iostream>

// library side code
class A {
public:
  int a;
  // ... many members

  A() { a = 1; }
};

void fa(A a) {
  std::cout << a.a << std::endl;
}

A genA() { A a; return a; }

// ///
// my code

class B : public A {
public:
  int b;
  B() : A() { init(); }
  B(A& a) : A(a) { init(); }
  void init() { b = 2; }
};

void fb(B b) {
  std::cout << b.b << std::endl;
}


int main(void) {
  A a = genA();
  B b(a);

  fa(b); // => 1
  fb(b); // => 2
}

Answer 1

You shouldn't do it with __new__, but it works only with new-style classes:

class A(object):
    def __init__(self):
        self.a = 10

class B(A):
    def __new__(cls, a):
        a.__class__ = cls
        return a

    def __init__(self, a):
        self.b = 20

a = A()
b = B(a)

print type(b), b.a, b.b   # <class '__main__.B'> 10 20

But as I said, don't do that, you should probably use aggregation, not a subclassing in such cases. If you wish to make B so it will have same interface as A, you may write transparent proxy with __getattr__:

class B(object):
    def __init__(self, a):
        self.__a = a
        self.b = 20

    def __getattr__(self, attr):
        return getattr(self.__a, attr)

    def __setattr__(self, attr, val):
        if attr == '_B__a':
            object.__setattr__(self, attr, val)

        return setattr(self.__a, attr, val)