June 14, 2016

Python classes in detail

simple class defination - its just a logical grouping of objects and methods

suppose we want a define a person object and with some attributes and methods on the person objects. we do it like this.

class User:
    def __init__(self, age, gender, location, contact):
        self.age = age
        self.gender = gender
        self.location = location
        self.contact = contact

    def setContact(new_contact):
        self.contact = new_contact

    def setLocation(new_location):
        self.location = new_location

notice def __init__(self, age, gender, location, contact): This is called the constructor of the class. as name suggests it is used to construct a class instance from the class defination of User.

A class can be considered as a blueprint or instruction manula of how the class should be constructed. until now memory is not allocated for the class above its just a blueprint/instruction manual.

What if we want to use above class? for that we need to create a class instance or an object of class User

This is how we do it:

John = User(20,'male','tokyo','98924676488')

The above is like asking python “ if you happen to know how to create a User ,then create it with the values that i provide and remember to follow the instruction manual for creating a User John

and then to use its attributes


here John is just one instance of User class yo ucan create any number of instances of a class you want.

Now whats going on with that self in function definations inside class?

self refers to instance itself that we created. calling John..setLocation('UK') puts those instruction to John instance. simply put self is the instance of the John that setLocation is being called upon.you could have written User.setLocation(John,'UK')

# Instance Level v/s Class Lvel.

until now all we saw was creating attributes and methods at instance level i.e each class has its own copy of its attributes.What if we wanted some attribute to be present at class level?

its actually quite simple. This is how its done

 class User:
    defaultAge = 25

    def __ini__(age,location,gender):
        self.location = location
        self.gneder = gender
        if gender:
            self.age = age
            self.age = defaultAge

now every object of User will have a common value defaultAge