python sub class constructor

Question

In python OOP, lets say, Person is a parent class with its own constructor; then Student is a sub class of Person, before I use Student, must Person.__init__(self) be called first in the constructor of Student? Plus, can I define a new constructor in Student class?

class Person():      
    def __init__(self):  

Above is class Person with its constructor

class Student(Person):    
    def __init__(self):  
        Person.__init__(self)   
    def __init__(self, age)

What I mean is, could Student have its own constructor? If so, must Person.__init__(self) be called in the Student constructor in this case?

Answer 1

It doesn't have to be, no, but it really should. If the base class has any methods or properties which aren't overridden in the derived class then the base class should be initialized properly, otherwise those methods and properties may not work.

EDIT: let me clarify. You can define a constructor for Student, but that constructor really should call Person.__init__ to make sure the base class is initialized properly.

Answer 2

Student can have a own constructor. In python you may or may not call the base class constructor, but it is good practice to do so.

Answer 3

No it would look like this:

class Person(object):
    def __init__(self, name, age, height, weight):
        self.name = name
        self.age = age
        self.height = height
        self.weigth = weight


class Student(Person):
    def __init__(self, name, age, height, weight, school):
        Person.__init__(self, name, age, heigth, weight)
        self.school = school

If you didn't have Person.__init__ in the Student class, you would have to do all the things you do in the Person class in the Student class if you wanted to be able to use it like a person.

Answer 4

Adapted from Zimm3r's answer, but using super for maximum correctness:

class Person(object):
    def __init__(self, name, age, height, weight):
        self.name = name
        self.age = age
        self.height = height
        self.weigth = weight


class Student(Person):
    def __init__(self, name, age, height, weight, school):
        super(Student, self).__init__(name, age, heigth, weight)
        self.school = school

Answer 5

Of course, Student can have its own constructor. However, a class can only have one constructor in Python, there's nothing like constructor overload.

So when we say a sub class has its own constructor, we really mean something like this:

class Worker(People):
    def __init__(self, company):
        self.company = company

As @IanH pointed out, you don't have to call the super class constructor. And when you think you should call it (probably for some common initialization), you can do it like this:

class People:
    def __init__(self, name):
        self.name = name

class Student(People):
    def __init__(self, name, school):
        super(Student, self).__init__(name)
        self.school = school