Simple interview question Python

gistfile1.py

# What's wrong in the code?
# A: No call to parent __init__. To call, uncomment the code

class A:
    def __init__(self):
        self._greeting = 'hello'
    
    def greet(self):
        print self._greeting
 
class B(A):
    def __init__(self):
       # A.__init__(self)
       self._greeting_add = ', world'
    
    def greet(self):
        print self._greeting + self._greeting_add  # No _greeting because of 
 
b = B()
b.greet()