GATE CS 2011 Question 64

This is a question asked in the GATE (Graduate Aptitude Test in Engineering) for Computer Science in the year 2011.

Question

Consider the following code:

class A:
    def __init__(self, i = 0):
        self.i = i

class B(A):
    def __init__(self, j = 0):
        self.j = j

def main():
    b=B()

How can method main() of class B be corrected to properly initialize an object of class B?

(A) Add A.__init__(self) at the beginning of B.__init__(self, j = 0).

(B) Add A.__init__(self) at the end of B.__init__(self, j = 0).

(C) Add A.__init__(self) at the beginning of method main().

(D) Add A.__init__(i, self) at the beginning of B.__init__(self, j = 0).

Correct Answer

(B)

Explanation

While creating an object of class B, the constructor of class A is not called automatically. Hence, the instance variable i of class A is not initialized.

To initialize the instance variable i of class A, we need to explicitly call the constructor of class A from the constructor of class B. We can achieve this by calling A.__init__(self) in the constructor of class B.

Therefore, the correct way of initializing an object of class B is to add A.__init__(self) at the end of B.__init__(self, j = 0).

class A:
    def __init__(self, i = 0):
        self.i = i

class B(A):
    def __init__(self, j = 0):
        self.j = j
        A.__init__(self)

def main():
    b=B()

Hence, option (B) is the correct answer.