What is the difference between init and call

Question

I want to know the difference between   init   and   call   methods     For example   class test     def   init   self       self a   10    def   call   self        b   20

User · Answer

Short and sweet answers are already provided above. I wanna provide some practical implementation as compared with Java.

 class test(object):
        def __init__(self, a, b, c):
            self.a = a
            self.b = b
            self.c = c
        def __call__(self, a, b, c):
            self.a = a
            self.b = b
            self.c = c


    instance1 = test(1, 2, 3)
    print(instance1.a) #prints 1

    #scenario 1
    #creating new instance instance1
    #instance1 = test(13, 3, 4)
    #print(instance1.a) #prints 13


    #scenario 2
    #modifying the already created instance **instance1**
    instance1(13,3,4)
    print(instance1.a)#prints 13

Note: scenario 1 and scenario 2 seems same in terms of result output. But in scenario1, we again create another new instance instance1. In scenario2, we simply modify already created instance1. __call__ is beneficial here as the system doesn't need to create new instance.

Equivalent in Java

public class Test {

    public static void main(String[] args) {
        Test.TestInnerClass testInnerClass = new Test(). new TestInnerClass(1, 2, 3);
        System.out.println(testInnerClass.a);

        //creating new instance **testInnerClass**
        testInnerClass = new Test().new TestInnerClass(13, 3, 4);
        System.out.println(testInnerClass.a);

        //modifying already created instance **testInnerClass**
        testInnerClass.a = 5;
        testInnerClass.b = 14;
        testInnerClass.c = 23;

        //in python, above three lines is done by testInnerClass(5, 14, 23). For this, we must define __call__ method

    }

    class TestInnerClass /* non-static inner class */{

        private int a, b,c;

        TestInnerClass(int a, int b, int c) {
            this.a = a;
            this.b = b;
            this.c = c;
        }
    }
}

User · Answer

gt  gt  gt  class A          def   init   self               print  From init             gt  gt  gt  a   A   From init       gt  gt  gt  a   Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  AttributeError  A instance has no   call   method  gt  gt  gt    gt  gt  gt  class B          def   init   self               print  From init               def   call   self               print  From call             gt  gt  gt  b   B   From init       gt  gt  gt  b   From call       gt  gt  gt

User · Answer

call   allows to return arbitrary values  while   init   being an constructor returns the instance of class implicitly  As other answers properly pointed out    init   is called just once  while it s possible to call   call   multiple times  in case the initialized instance is assigned to intermediate variable    gt  gt  gt  class Test          def   init   self               return  Hello        gt  gt  gt  Test   Traceback  most recent call last     File   lt console gt    line 1  in  lt module gt  TypeError    init     should return None  not  str   gt  gt  gt  class Test2          def   call   self               return  Hello        gt  gt  gt  Test2      Hello   gt  gt  gt    gt  gt  gt  Test2      Hello   gt  gt  gt

User · Answer

init   would be treated as Constructor where as   call   methods can be called with objects any number of times  Both   init   and   call   functions do take default arguments

User · Answer

In Python  functions are first-class objects  this means  function references can be passed in inputs to other functions and or methods  and executed from inside them   Instances of Classes  aka Objects   can be treated as if they were functions  pass them to other methods functions and call them  In order to achieve this  the   call   class function has to be specialized    def   call   self   args           It takes as an input a variable number of arguments  Assuming x being an instance of the Class X  x   call   1  2  is analogous to calling x 1 2  or the instance itself as a function   In Python    init     is properly defined as Class Constructor  as well as   del     is the Class Destructor   Therefore  there is a net distinction between   init     and   call      the first builds an instance of Class up  the second makes such instance callable as a function would be without impacting the lifecycle of the object itself  i e    call   does not impact the construction destruction lifecycle  but it can modify its internal state  as shown below    Example   class Stuff object        def   init   self  x  y  range           super Stuff  self    init             self x   x         self y   y         self range   range      def   call   self  x  y           self x   x         self y   y         print    call   with   d  d      self x  self y       def   del   self           del self x         del self y         del self range   gt  gt  gt  s   Stuff 1  2  3   gt  gt  gt  s x 1  gt  gt  gt  s 7  8    call   with  7 8   gt  gt  gt  s x 7

User · Answer

Defining a custom   call     method in the meta-class allows the class s instance to be called as a function  not always modifying the instance itself   In  1   class A              def   init   self                   print  init                               def   call   self                   print  call                                     In  2   a   A   init  In  3   a   call

User · Answer

You can also use   call   method in favor of implementing decorators   This example taken from Python 3 Patterns  Recipes and Idioms  class decorator without arguments object       def   init   self  f                       If there are no decorator arguments  the function         to be decorated is passed to the constructor                      print  Inside   init               self f   f      def   call   self   args                       The   call   method is not called until the         decorated function is called                      print  Inside   call               self f  args          print  After self f    args       decorator without arguments def sayHello a1  a2  a3  a4       print  sayHello arguments    a1  a2  a3  a4    print  After decoration   print  Preparing to call sayHello     sayHello  say    hello    argument    list   print  After first sayHello   call   sayHello  a    different    set of    arguments   print  After second sayHello   call     Output

User · Answer

So    init   is called when you are creating an instance of any class and initializing the instance variable also   Example    class User       def   init   self first n last n age           self first n   first n         self last n   last n         self age   age  user1   User  Jhone   Wrick   40     And   call   is called when you call the object like any other function    Example   class USER      def   call   self arg            todo here           print f I am in   call   with arg    arg       user1 USER   user1  One    calling the object user1 and that s gonna call   call   dunder functions

User · Answer

call   makes the instance of a class callable   Why would it be required     Technically   init   is called once by   new   when object is created  so that it can be initialized    But there are many scenarios where you might want to redefine your object  say you are done with your object  and may find a need for a new object  With   call   you can redefine the same object as if it were new    This is just one case  there can be many more

User · Answer

The first is used to initialise newly created object  and receives arguments used to do that  class Foo      def   init   self  a  b  c                  x   Foo 1  2  3      init    The second implements function call operator  class Foo      def   call   self  a  b  c                  x   Foo   x 1  2  3      call

User · Answer

We can use call method to use other class methods as static methods   class  Callable      def   init   self  anycallable           self   call     anycallable  class Model       def get instance conn  table name                do something         get instance    Callable get instance   provs fac   Model get instance connection   users

User · Answer

init   is a special method in Python classes  it is the constructor method for a class  It is called whenever an object of the class is constructed or we can say it initialises a new object  Example       In  4   class A              def   init   self  a                   print a                  a   A 10    An argument is necessary 10   If we use A    it will give an error  TypeError    init     missing 1 required positional argument   a  as it requires 1 argument a because of   init                   call   when implemented in the Class helps us invoke the Class instance as a function call   Example   In  6   class B              def   call   self b                   print b                  b   B     Note we didn t pass any arguments here         b 20      Argument passed when the object is called         20   Here if we use B    it runs just fine because it doesn t have an   init   function here

User · Answer

I will try to explain this using an example  suppose you wanted to print a fixed number of terms from fibonacci series  Remember that the first 2 terms of fibonacci series are 1s  Eg  1  1  2  3  5  8  13      You want the list containing the fibonacci numbers to be initialized only once and after that it should update  Now we can use the   call   functionality  Read  mudit verma s answer  It s like you want the object to be callable as a function but not re-initialized every time you call it   Eg   class Recorder      def   init   self           self  weights              for i in range 0  2               self  weights append 1          print self  weights -1          print self  weights -2          print  no  above is from   init         def   call   self  t           self  weights    self  weights -1   self  weights -1    self  weights -2           print self  weights -1          print  no  above is from   call     weight recorder   Recorder   for i in range 0  10       weight recorder i    The output is   1 1 no  above is from   init   2 no  above is from   call   3 no  above is from   call   5 no  above is from   call   8 no  above is from   call   13 no  above is from   call   21 no  above is from   call   34 no  above is from   call   55 no  above is from   call   89 no  above is from   call   144 no  above is from   call     If you observe the output   init   was called only one time that s when the class was instantiated for the first time  later on the object was being called without re-initializing

[python] What is the difference between init and call?

Examples related to python

Examples related to class

Examples related to oop

Examples related to object

Examples related to callable-object

[python] What is the difference between __init__ and __call__?