What init and self do in Python

Question

I m learning the Python programming language and I ve came across something I don t fully understand    In a method like   def method self  blah       def   init                               What does self do   What is it meant to be   Is it mandatory   What does the   init   method do   Why is it necessary   etc    I think they might be OOP constructs  but I don t know very much

User · Answer

A brief illustrative example

In the hope it might help a little, here's a simple example I used to understand the difference between a variable declared inside a class, and a variable declared inside an __init__ function:

class MyClass(object):
    i = 123
    def __init__(self):
        self.i = 345
     
a = MyClass()
print(a.i)
print(MyClass.i)

Output:

345
123

User · Answer

Just a demo for the question   class MyClass       def   init   self           print    init   is the constructor for a class        def   del   self           print    del   is the destructor for a class        def   enter   self           print    enter   is for context manager           return self      def   exit   self  exc type  exc value  traceback           print    exit   is for context manager        def greeting self           print  hello python     if   name         main         with MyClass   as mycls          mycls greeting         python3 class objects instantiation py   init   is the constructor for a class   enter   is for context manager hello python   exit   is for context manager   del   is the destructor for a class

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note that self could actually be any valid python identifier   For example  we could just as easily write  from Chris B s example   class A object       def   init   foo           foo x    Hello       def method a bar  foo           print bar x         foo   and it would work exactly the same   It is however recommended to use self because other pythoners will recognize it more easily

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Had trouble undestanding this myself  Even after reading the answers here    To properly understand the   init   method you need to understand self    The self Parameter  The arguments accepted by the   init   method are    def   init   self  arg1  arg2     But we only actually pass it two arguments    instance   OurClass  arg1    arg2     Where has the extra argument come from    When we access attributes of an object we do it by name  or by reference   Here instance is a reference to our new object  We access the printargs method of the instance object using instance printargs   In order to access object attributes from within the   init   method we need a reference to the object   Whenever a method is called  a reference to the main object is passed as the first argument  By convention you always call this first argument to your methods self   This means in the   init   method we can do    self arg1   arg1 self arg2   arg2   Here we are setting attributes on the object  You can verify this by doing the following    instance   OurClass  arg1    arg2   print instance arg1 arg1   values like this are known as object attributes  Here the   init   method sets the arg1 and arg2 attributes of the instance   source  http   www voidspace org uk python articles OOP shtml the-init-method

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The  self  is a reference to the class instance  class foo      def bar self               print  hi    Now we can create an instance of foo and call the method on it  the self parameter is added by Python in this case   f   foo   f bar     But it can be passed in as well if the method call isn t in the context of an instance of the class  the code below does the same thing  f   foo   foo bar f    Interestingly the variable name  self  is just a convention  The below definition will work exactly the same   Having said that it is very strong convention which should be followed always  but it does say something about flexible nature of the language  class foo      def bar s               print  hi

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init   is basically a function which will  initialize   activate  the properties of the class for a specific object  once created and matched  to the corresponding class   self represents that object which will inherit those properties

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Here  the guy has written pretty well and simple  https   www jeffknupp com blog 2014 06 18 improve-your-python-python-classes-and-object-oriented-programming   Read above link  as a reference to this      self  So what s with that self parameter to all of the Customer   methods  What is it  Why  it s the instance  of course  Put another   way  a method like withdraw defines the instructions for withdrawing   money from some abstract customer s account  Calling   jeff withdraw 100 0  puts those instructions to use on the jeff   instance       So when we say def withdraw self  amount    we re saying   here s how   you withdraw money from a Customer object  which we ll call self  and   a dollar figure  which we ll call amount   self is the instance of the   Customer that withdraw is being called on  That s not me making   analogies  either  jeff withdraw 100 0  is just shorthand for   Customer withdraw jeff  100 0   which is perfectly valid  if not often   seen  code       init self may make sense for other methods  but what about init  When we call init  we re in the process of creating an object  so how can there already be a self  Python allows us to extend   the self pattern to when objects are constructed as well  even though   it doesn t exactly fit  Just imagine that jeff   Customer  Jeff   Knupp   1000 0  is the same as calling jeff   Customer jeff   Jeff   Knupp   1000 0   the jeff that s passed in is also made the result       This is why when we call init  we initialize objects by saying   things like self name   name  Remember  since self is the instance    this is equivalent to saying jeff name   name  which is the same as   jeff name    Jeff Knupp  Similarly  self balance   balance is the same   as jeff balance   1000 0  After these two lines  we consider the   Customer object  initialized  and ready for use       Be careful what you   init        After init has finished  the caller can rightly assume that the   object is ready to use  That is  after jeff   Customer  Jeff Knupp     1000 0   we can start making deposit and withdraw calls on jeff  jeff is a fully-initialized object

User · Answer

Class objects support two kinds of operations  attribute references and instantiation  Attribute references use the standard syntax used for all attribute references in Python  obj name  Valid attribute names are all the names that were in the class   s namespace when the class object was created  So  if the class definition looked like this   class MyClass         A simple example class        i   12345      def f self           return  hello world    then MyClass i and MyClass f are valid attribute references  returning an integer and a function object  respectively  Class attributes can also be assigned to  so you can change the value of MyClass i by assignment    doc   is also a valid attribute  returning the docstring belonging to the class   A simple example class    Class instantiation uses function notation  Just pretend that the class object is a parameterless function that returns a new instance of the class  For example   x   MyClass     The instantiation operation     calling    a class object  creates an empty object  Many classes like to create objects with instances customized to a specific initial state  Therefore a class may define a special method named   init      like this   def   init   self       self data        When a class defines an   init     method  class instantiation automatically invokes   init     for the newly-created class instance  So in this example  a new  initialized instance can be obtained by   x   MyClass     Of course  the   init     method may have arguments for greater flexibility  In that case  arguments given to the class instantiation operator are passed on to   init      For example   class Complex      def   init   self  realpart  imagpart           self r   realpart         self i   imagpart  x   Complex 3 0  -4 5  x r  x i   Taken from official documentation which helped me the most in the end     Here is my example  class Bill        def   init   self apples figs dates           self apples   apples         self figs   figs         self dates   dates         self bill   apples   figs   dates         print   Buy  self apples  apples   self figs  figs                  and  self dates  dates                   Total fruitty bill is  self bill   pieces of fruit        When you create instance of class Bill   purchase   Bill 5 6 7    You get        gt  Buy 5 apples 6 figs and 7 dates  Total fruitty bill is 18  pieces of  gt  fruit

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Basically  you need to use the  self  keyword when using a variable in multiple functions within the same class  As for init  it s used to setup default values incase no other functions from within that class are called

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Python   init   and self what do they do       What does self do  What is it meant to be  Is it mandatory       What does the   init   method do  Why is it necessary   etc     The example given is not correct  so let me create a correct example based on it   class SomeObject object        def   init   self  blah           self blah   blah      def method self           return self blah    When we create an instance of the object  the   init   is called to customize the object after it has been created  That is  when we call SomeObject with  blah  below  which could be anything   it gets passed to the   init   function as the argument  blah   an object   SomeObject  blah     The self argument is the instance of SomeObject that will be assigned to an object    Later  we might want to call a method on this object   an object method     Doing the dotted lookup  that is  an object method  binds the instance to an instance of the function  and the method  as called above  is now a  bound  method - which means we do not need to explicitly pass the instance to the method call   The method call gets the instance because it was bound on the dotted lookup  and when called  then executes whatever code it was programmed to perform   The implicitly passed self argument is called self by convention  We could use any other legal Python name  but you will likely get tarred and feathered by other Python programmers if you change it to something else     init   is a special method  documented in the Python datamodel documentation  It is called immediately after the instance is created  usually via   new   - although   new   is not required unless you are subclassing an immutable datatype

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In this code   class A object       def   init   self           self x    Hello       def method a self  foo           print self x         foo       the self variable represents the instance of the object itself   Most object-oriented languages pass this as a hidden parameter to the methods defined on an object  Python does not   You have to declare it explicitly   When you create an instance of the A class and call its methods  it will be passed automatically  as in      a   A                   We do not pass any argument to the   init   method a method a  Sailor      We only pass a single argument   The   init   method is roughly what represents a constructor in Python   When you call A   Python creates an object for you  and passes it as the first parameter to the   init   method   Any additional parameters  e g   A 24   Hello    will also get passed as arguments--in this case causing an exception to be raised  since the constructor isn t expecting them

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What does self do  What is it meant to be  Is it mandatory    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   Python doesn t force you on using  self   You can give it any name you want  But remember the first argument in a method definition is a reference to the object  Python adds the self argument to the list for you  you do not need to include it when you call the methods  if you didn t provide self in init method then you will get an error  TypeError    init      takes no arguments  1 given       What does the init method do  Why is it necessary   etc     init is short for initialization  It is a constructor which gets called when you make an instance of the class and it is not necessary  But usually it our practice to write init method for setting default state of the object  If you are not willing to set any state of the object initially then you don t need to write this method

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Try out this code  Hope it helps many C programmers like me to Learn Py       usr bin python2  class Person          Doc - Inside Class          def   init   self  name              Doc -   init   Constructor            self n name   name              def show self  n1  n2              Doc - Inside Show            print self n name         print  Sum       n1   n2       def   del   self           print  Destructor Deleting object -    self n name  p Person  Jay   p show 2  3  print p   doc   print p   init     doc   print p show   doc     Output   Jay  Sum    5  Doc - Inside Class  Doc -   init   Constructor  Doc - Inside Show  Destructor Deleting object -  Jay

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Source  Class and Instance Variables   https   docs python org 2 tutorial classes html class-and-instance-variables  class MyClass object         class variable     my CLS var   10        sets  init ial  state to objects instances  use self argument     def   init   self             self usage   gt  instance variable  per object          self my OBJ var   15            also possible  class name is used   gt  init class variable         MyClass my CLS var   20   def run example func          PRINTS    10     class variable      print MyClass my CLS var        executes   init   for obj1 instance       NOTE    init   changes class variable above     obj1   MyClass          PRINTS    15     instance variable      print obj1 my OBJ var        PRINTS    20     class variable  changed value      print MyClass my CLS var   run example func

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In this code   class Cat      def   init   self  name           self name   name     def info self           print  I am a cat and I am called   self name   Here   init   acts as a constructor for the class and when an object is instantiated  this function is called  self represents the instantiating object   c   Cat  Kitty   c info     The result of the above statements will be as follows   I am a cat and I am called Kitty

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Yep  you are right  these are oop constructs     init   is the constructor for a class   The self parameter refers to the instance of the object  like this in C      class Point      def   init   self  x  y           self  x   x         self  y   y   The   init   method gets called when memory for the object is allocated   x   Point 1 2    It is important to use the self parameter inside an object s method if you want to persist the value with the object   If  for instance  you implement the   init   method like this   class Point      def   init   self  x  y            x   x          y   y   Your x and y parameters would be stored in variables on the stack and would be discarded when the init method goes out of scope   Setting those variables as self  x and self  y sets those variables as members of the Point object  accessible for the lifetime of the object

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In short    self as it suggests  refers to itself- the object which has called the method  That is  if you have N objects calling the method  then self a will refer to a separate instance of the variable for each of the N objects  Imagine N copies of the variable a for each object   init   is what is called as a constructor in other OOP languages such as C   Java  The basic idea is that it is a special method which is automatically called when an object of that Class is created

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init   does act like a constructor  You ll need to pass  self  to any class functions as the first argument if you want them to behave as non-static methods   self  are instance variables for your class

[python] What init and self do in Python?

A brief illustrative example

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