3.4 methods

This lesson discusses the concept of method in python.

Forget for the time being what the word methods mean in English. The term method here means functions associated with classes Let’s write a simple function which takes an object as input and prints the name attribute of that object”

def say_salam(obj):
    print("Salam, I am " + obj.name + "!")
    return

Let’s also define a simple class named Insan as we did in previous examples.

class Insan:
    pass

We can use the function salam by using the instance of class Insan which is x.

x = Insan()
x.name = "Ali"
say_salam(x)  # >> Salam, I am Ali!

Salam, I am Ali!

say_salam is a function at this time, and we can verify this by checking its type

type(say_salam)

but we can bind/link it to class Insan as following

def say_salam(obj):
    print("Salam, I am " + obj.name)


class Insan:
    taruf = say_salam

Now we can make use of the function say_salam which is linked to class Insan as following

x = Insan()

x.name = "Ali"

Insan.taruf(x)

Salam, I am Ali

attributes in the Insan class are:

print(Insan.__dict__)

{'__module__': '__main__', 'taruf': <function say_salam at 0x7ffad8136d30>, '__dict__': <attribute '__dict__' of 'Insan' objects>, '__weakref__': <attribute '__weakref__' of 'Insan' objects>, '__doc__': None}

taruf is a method and can be called as

x.taruf()

Salam, I am Ali

so Insan.taruf and x.taruf are equivalent. Although the method say_salam takes one argument obj as input but we did not provide any input argument while calling it through x.taruf() and no error was thrown? This is because when the function say_salam was linked to the class Insan, and when we call this method, the first argument is by default the instance i.e. x in this case. In this case we defined the method outside the body of class Insan and then linked it to class, but this is not the usual way to define methods. The proper way is to define it inside the class(indented) We connect this function which is method now to the class by its first argument which is usually self. self corresponds to the Insan object x

class Insan:
    def say_salam(self):
        print("Salam, I am " + self.name)
        return


ali = Insan()

uncomment following line ali.say_salam() # AttributeError

The problem with above code is that the class Insan does not have an attribute name.So the class must have an attribute name before using the method say_salam.

print(ali.__dict__)

{}

Lets define the attribute before using the method say_salam

class Insan:
    def say_salam(self):
        print("Salam, I am " + self.name)


ali = Insan()
ali.name = 'Ali'  # define an attribute of instance ali
ali.say_salam()

Salam, I am Ali

print(ali.__dict__)

{'name': 'Ali'}

so whats the difference btw method and function? self is just a convention, we can use this, apna or any other keyword but it is better to just follow the convention so that others can follow your work

class Insan:

    def say_salam(apna):
        print("Salam, I am " + apna.name)


ali = Insan()
ali.name = 'Ali'  # define an attribute of instance ali
ali.say_salam()

Salam, I am Ali

As said earlier, defining class attributes outside class is not proper way. We need the class Insan to have the attribute name before using the method say_salam. So we need a systematic way that the class, upon its creation (initialization) must have the attribute name.This is done with init method, which we will learn in next lesson.