python sub class initialiser

Question

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

class Person():      
    def __init__(self):  

Above is class Person with its initialiser

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

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

Solution 1

Of course, Student can have its own initialiser. However, a class can only have one initialiser in Python, because that is a special method called within the constructor (the class __new__ method).

So when we say a sub class has its own initialiser, 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 initialiser. 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

Solution 2

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.

Solution 3

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 initialiser from the base class should be called, otherwise those methods and properties may not work.

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

Solution 4

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

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Comments

[ladyfafa]

@sth, btw, may I know how to edit the code into a nice format when I tried to post any piece of code here? Your code looks nice

[IanH]

ladyfafa: Just indent the code lines by 4 spaces (+ the spaces for the python code) and Stackoverflow will colorize it automagically.

[sth]

@ladyfafa: For code blocks indent all the lines with for spaces, for example by selecting the code and clicking the 101010-button in the editor (Or using the Ctrl-K keyboard shortcut). For inline code use backticks, or that same button/shortcut. For more see stackoverflow.com/editing-help or the explanations shown to the right of the edit window.

[habnabit]

Also, you should be using super.

[ladyfafa]

that could be brilliant, my concern is the so called "constructor overload", now i am on my way

[Zimm3r]

@ Aaron Gallapher you can but I find it causes over confusion for something that is already confusing, Object Oriented Programing.

[habnabit]

@Zimm3r, super makes things less confusing. What's so much more confusing about the method call in my answer?

[Zimm3r]

@ Aaron I guess it is a matter of what parts of OOP you understand at the time.

[clacke]

Regarding super() there are some caveats, read this: fuhm.net/super-harmful

[clacke]

This is the right way to do it, but learn more about what super() really does here: fuhm.net/super-harmful . It's a good read in a more general sense also, as the point of using **kwargs and keyword parameters is useful in many situations.

[satoru]

Thanks for sharing the link. But if I understood this essay correctly, it is evil only when you try to use multi-inheritance, right? So I think it is multi-inheritance that's harmful :)