Tukukkk

Let’s take a simple vector class:

class Vector:

    def __init__(self, x, y):

        self.x = x

        self.y = y

We want to have a method which calculates the length. What would it look like if we wanted to define it inside the class?

    def length(self):

        return math.sqrt(self.x ** 2 + self.y ** 2)

What should it look like when we were to define it as a global method/function?

def length_global(vector):

    return math.sqrt(vector.x ** 2 + vector.y ** 2)

So the whole structure stays the same. How can me make use of this? If we assume for a moment that we hadn’t written a length method for our Vector class, we could do this:

Vector.length_new = length_global

v = Vector(3, 4)

print(v.length_new()) # 5.0

This works because the first parameter of length_global, can be re-used as the self parameter in length_new. This would not be possible without an explicit self.

Another way of understanding the need for the explicit self is to see where Python adds some syntactical sugar. When you keep in mind, that basically, a call like

v_instance.length()

is internally transformed to

Vector.length(v_instance)

it is easy to see where the self fits in. You don't actually write instance methods in Python; what you write is class methods which must take an instance as a first parameter. And therefore, you’ll have to place the instance parameter somewhere explicitly.

Let's say you have a class ClassA which contains a method methodA defined as:

def methodA(self, arg1, arg2):

    # do something

and ObjectA is an instance of this class.

Now when ObjectA.methodA(arg1, arg2) is called, python internally converts it for you as:

ClassA.methodA(ObjectA, arg1, arg2)

The self variable refers to the object itself.

When objects are instantiated, the object itself is passed into the self parameter.

Because of this, the object’s data is bound to the object. Below is an example of how you might like to visualize what each object’s data might look. Notice how ‘self’ is replaced with the objects name. I'm not saying this example diagram below is wholly accurate but it hopefully with serve a purpose in visualizing the use of self.

The Object is passed into the self parameter so that the object can keep hold of its own data.

Although this may not be wholly accurate, think of the process of instantiating an object like this: When an object is made it uses the class as a template for its own data and methods. Without passing it's own name into the self parameter, the attributes and methods in the class would remain as a general template and would not be referenced to (belong to) the object. So by passing the object's name into the self parameter it means that if 100 objects are instantiated from the one class, they can all keep track of their own data and methods.

See the illustration below:

I like this example:

class A: 

    foo = 

a, b = A(), A()

a.foo.append(5)

b.foo

ans: [5]



class A: 

    def __init__(self): 

        self.foo = 

a, b = A(), A()

a.foo.append(5)

b.foo

ans: 

I will demonstrate with code that does not use classes:

def state_init(state):

    state['field'] = 'init'



def state_add(state, x):

    state['field'] += x



def state_mult(state, x):

    state['field'] *= x



def state_getField(state):

    return state['field']



myself = {}

state_init(myself)

state_add(myself, 'added')

state_mult(myself, 2)



print( state_getField(myself) )

#--> 'initaddedinitadded'

Classes are just a way to avoid passing in this "state" thing all the time (and other nice things like initializing, class composition, the rarely-needed metaclasses, and supporting custom methods to override operators).

Now let's demonstrate the above code using the built-in python class machinery, to show how it's basically the same thing.

class State(object):

    def __init__(self):

        self.field = 'init'

    def add(self, x):

        self.field += x

    def mult(self, x):

        self.field *= x



s = State()

s.add('added')    # self is implicitly passed in

s.mult(2)         # self is implicitly passed in

print( s.field )

[migrated my answer from duplicate closed question]

As well as all the other reasons already stated, it allows for easier access to overridden methods; you can call Class.some_method(inst).

An example of where it’s useful:

class C1(object):

    def __init__(self):

         print "C1 init"



class C2(C1):

    def __init__(self): #overrides C1.__init__

        print "C2 init"

        C1.__init__(self) #but we still want C1 to init the class too

>>> C2()

"C2 init"

"C1 init"

The following excerpts are from the Python documentation about self:

As in Modula-3, there are no shorthands [in Python] for referencing the object’s members from its methods: the method function is declared with an explicit first argument representing the object, which is provided implicitly by the call.

Often, the first argument of a method is called self. This is nothing more than a convention: the name self has absolutely no special meaning to Python. Note, however, that by not following the convention your code may be less readable to other Python programmers, and it is also conceivable that a class browser program might be written that relies upon such a convention.

For more information, see the Python documentation tutorial on classes.

Python is not a language built for Object Oriented Programming unlike Java or C++.

When calling a static method in Python, one simply writes a method with regular arguments inside it.

class Animal():

    def staticMethod():

        print "This is a static method"

However, an object method, which requires you to make a variable, which is an Animal, in this case, needs the self argument

class Animal():

    def objectMethod(self):

        print "This is an object method which needs an instance of a class"

The self method is also used to refer to a variable field within the class.

class Animal():

    #animalName made in constructor

    def Animal(self):

        self.animalName = "";





    def getAnimalName(self):

        return self.animalName

In this case, self is referring to the animalName variable of the entire class. REMEMBER: If you have a variable within a method, self will not work. That variable is simply existent only while that method is running. For defining fields (the variables of the entire class), you have to define them OUTSIDE the class methods.

If you don't understand a single word of what I am saying, then Google "Object Oriented Programming." Once you understand this, you won't even need to ask that question :).

Its use is similar to the use of this keyword in Java, i.e. to give a reference to the current object.

It’s there to follow the Python zen “explicit is better than implicit”. It’s indeed a reference to your class object. In Java and PHP, for example, it's called this.

If user_type_name is a field on your model you access it by self.user_type_name.

self is an object reference to the object itself, therefore, they are same.
Python methods are not called in the context of the object itself.
self in Python may be used to deal with custom object models or something.

I'm surprised nobody has brought up Lua. Lua also uses the 'self' variable however it can be omitted but still used. C++ does the same with 'this'. I don't see any reason to have to declare 'self' in each function but you should still be able to use it just like you can with lua and C++. For a language that prides itself on being brief it's odd that it requires you to declare the self variable.

First of all, self is a conventional name, you could put anything else (being coherent) in its stead.

It refers to the object itself, so when you are using it, you are declaring that .name and .age are properties of the Student objects (note, not of the Student class) you are going to create.

class Student:

    #called each time you create a new Student instance

    def __init__(self,name,age): #special method to initialize

        self.name=name

        self.age=age



    def __str__(self): #special method called for example when you use print

        return "Student %s is %s years old" %(self.name,self.age)



    def call(self, msg): #silly example for custom method

        return ("Hey, %s! "+msg) %self.name



#initializing two instances of the student class

bob=Student("Bob",20)

alice=Student("Alice",19)



#using them

print bob.name

print bob.age

print alice #this one only works if you define the __str__ method

print alice.call("Come here!") #notice you don't put a value for self



#you can modify attributes, like when alice ages

alice.age=20

print alice

Code is here

Take a look at the following example, which clearly explains the purpose of self

class Restaurant(object):  

    bankrupt = False



    def open_branch(self):

        if not self.bankrupt:

           print("branch opened")



#create instance1

>>> x = Restaurant()

>>> x.bankrupt

False



#create instance2

>>> y = Restaurant()

>>> y.bankrupt = True   

>>> y.bankrupt

True



>>> x.bankrupt

False  

self is used/needed to distinguish between instances.

Is because by the way python is designed the alternatives would hardly work. Python is designed to allow methods or functions to be defined in a context where both implicit this (a-la Java/C++) or explicit @ (a-la ruby) wouldn't work. Let's have an example with the explicit approach with python conventions:

def fubar(x):

    self.x = x



class C:

    frob = fubar

Now the fubar function wouldn't work since it would assume that self is a global variable (and in frob as well). The alternative would be to execute method's with a replaced global scope (where self is the object).

The implicit approach would be

def fubar(x)

    myX = x



class C:

    frob = fubar

This would mean that myX would be interpreted as a local variable in fubar (and in frob as well). The alternative here would be to execute methods with a replaced local scope which is retained between calls, but that would remove the posibility of method local variables.

However the current situation works out well:

 def fubar(self, x)

     self.x = x



 class C:

     frob = fubar

here when called as a method frob will receive the object on which it's called via the self parameter, and fubar can still be called with an object as parameter and work the same (it is the same as C.frob I think).

The use of the argument, conventionally called self isn't as hard to understand, as is why is it necessary? Or as to why explicitly mention it? That, I suppose, is a bigger question for most users who look up this question, or if it is not, they will certainly have the same question as they move forward learning python. I recommend them to read these couple of blogs:

1: Use of self explained

Note that it is not a keyword.

The first argument of every class method, including init, is always a reference to the current instance of the class. By convention, this argument is always named self. In the init method, self refers to the newly created object; in other class methods, it refers to the instance whose method was called. For example the below code is the same as the above code.

2: Why do we have it this way and why can we not eliminate it as an argument, like Java, and have a keyword instead

Another thing I would like to add is, an optional self argument allows me to declare static methods inside a class, by not writing self.

Code examples:

class MyClass():

    def staticMethod():

        print "This is a static method"



    def objectMethod(self):

        print "This is an object method which needs an instance of a class, and that is what self refers to"

PS:This works only in Python 3.x.

In previous versions, you have to explicitly add @staticmethod decorator, otherwise self argument is obligatory.

In the __init__ method, self refers to the newly created object; in other class methods, it refers to the instance whose method was called.

self, as a name, is just a convention, call it as you want ! but when using it, for example to delete the object, you have to use the same name: __del__(var), where var was used in the __init__(var,[...])

You should take a look at cls too, to have the bigger picture. This post could be helpful.