Why do we use init in Python classes

Question

I am having trouble understanding the Initialization of classes   What s the point of them and how do we know what to include in them  Does writing in classes require a different type of thinking versus creating functions  I figured I could just create functions and then just wrap them in a class so I can re-use them  Will that work    Here s an example   class crawler      Initialize the crawler with the name of database   def   init   self dbname       self con sqlite connect dbname     def   del   self       self con close      def dbcommit self       self con commit     Or another code sample   class bicluster    def   init   self vec left None right None distance 0 0 id None       self left left     self right right     self vec vec     self id id     self distance distance   There are so many classes with   init   I come across when trying to read other people s code  but I don t understand the logic in creating them

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The   init   function is setting up all the member variables in the class  So once your bicluster is created you can access the member and get a value back   mycluster   bicluster    actual values go here     mycluster left   returns the value passed in as  left    Check out the Python Docs for some info  You ll want to pick up an book on OO concepts to continue learning

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It seems like you need to use   init   in Python if you want to correctly initialize mutable attributes of your instances   See the following example    gt  gt  gt  class EvilTest object           attr            gt  gt  gt  evil test1   EvilTest    gt  gt  gt  evil test2   EvilTest    gt  gt  gt  evil test1 attr append  strange    gt  gt  gt    gt  gt  gt  print  This is evil    evil test1 attr  evil test2 attr This is evil    strange     strange    gt  gt  gt    gt  gt  gt    gt  gt  gt  class GoodTest object           def   init   self               self attr            gt  gt  gt  good test1   GoodTest    gt  gt  gt  good test2   GoodTest    gt  gt  gt  good test1 attr append  strange    gt  gt  gt    gt  gt  gt  print  This is good    good test1 attr  good test2 attr This is good    strange        This is quite different in Java where each attribute is automatically initialized with a new value   import java util ArrayList  import java lang String   class SimpleTest       public ArrayList lt String gt  attr   new ArrayList lt String gt        class Main       public static void main String    args                SimpleTest t1   new SimpleTest            SimpleTest t2   new SimpleTest             t1 attr add  strange             System out println t1 attr         t2 attr             produces an output we intuitively expect    strange       But if you declare attr as static  it will act like Python    strange   strange

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Classes are objects with attributes  state  characteristic  and methods  functions  capacities  that are specific for that object  like the white color and fly powers  respectively  for a duck    When you create an instance of a class  you can give it some initial personality  state or character like the name and the color of her dress for a newborn   You do this with   init     Basically   init   sets the instance characteristics automatically when you call instance   MyClass some individual traits

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By what you wrote  you are missing a critical piece of understanding  the difference between a class and an object    init   doesn t initialize a class  it initializes an instance of a class or an object  Each dog has colour  but dogs as a class don t  Each dog has four or fewer feet  but the class of dogs doesn t  The class is a concept of an object  When you see Fido and Spot  you recognise their similarity  their doghood  That s the class   When you say  class Dog      def   init   self  legs  colour           self legs   legs         self colour   colour  fido   Dog 4   brown   spot   Dog 3   mostly yellow     You re saying  Fido is a brown dog with 4 legs while Spot is a bit of a cripple and is mostly yellow  The   init   function is called a constructor  or initializer  and is automatically called when you create a new instance of a class  Within that function  the newly created object is assigned to the parameter self  The notation self legs is an attribute called legs of the object in the variable self  Attributes are kind of like variables  but they describe the state of an object  or particular actions  functions  available to the object   However  notice that you don t set colour for the doghood itself - it s an abstract concept  There are attributes that make sense on classes  For instance  population size is one such - it doesn t make sense to count the Fido because Fido is always one  It does make sense to count dogs  Let us say there re 200 million dogs in the world  It s the property of the Dog class  Fido has nothing to do with the number 200 million  nor does Spot  It s called a  class attribute   as opposed to  instance attributes  that are colour or legs above   Now  to something less canine and more programming-related  As I write below  class to add things is not sensible - what is it a class of  Classes in Python make up of collections of different data  that behave similarly  Class of dogs consists of Fido and Spot and 199999999998 other animals similar to them  all of them peeing on lampposts  What does the class for adding things consist of  By what data inherent to them do they differ  And what actions do they share   However  numbers    those are more interesting subjects  Say  Integers  There s a lot of them  a lot more than dogs  I know that Python already has integers  but let s play dumb and  implement  them again  by cheating and using Python s integers    So  Integers are a class  They have some data  value   and some behaviours   add me to this other number    Let s show this   class MyInteger      def   init   self  newvalue            imagine self as an index card            under the heading of  value   we will write           the contents of the variable newvalue          self value   newvalue     def add self  other             when an integer wants to add itself to another integer            we ll take their values and add them together            then make a new integer with the result value          return MyInteger self value   other value   three   MyInteger 3    three now contains an object of class MyInteger   three value is now 3 five   MyInteger 5    five now contains an object of class MyInteger   five value is now 5 eight   three add five    here  we invoked the three s behaviour of adding another integer   now  eight value is three value   five value   3   5   8 print eight value      gt  8   This is a bit fragile  we re assuming other will be a MyInteger   but we ll ignore now  In real code  we wouldn t  we d test it to make sure  and maybe even coerce it   you re not an integer  by golly  you have 10 nanoseconds to become one  9    8        We could even define fractions  Fractions also know how to add themselves   class MyFraction      def   init   self  newnumerator  newdenominator          self numerator   newnumerator         self denominator   newdenominator           because every fraction is described by these two things     def add self  other           newdenominator   self denominator   other denominator         newnumerator   self numerator   other denominator   self denominator   other numerator         return MyFraction newnumerator  newdenominator    There s even more fractions than integers  not really  but computers don t know that   Let s make two   half   MyFraction 1  2  third   MyFraction 1  3  five sixths   half add third  print five sixths numerator      gt  5 print five sixths denominator      gt  6   You re not actually declaring anything here  Attributes are like a new kind of variable  Normal variables only have one value  Let us say you write colour    grey   You can t have another variable named colour that is  fuchsia  - not in the same place in the code   Arrays solve that to a degree  If you say colour     grey    fuchsia    you have stacked two colours into the variable  but you distinguish them by their position  0  or 1  in this case    Attributes are variables that are bound to an object  Like with arrays  we can have plenty colour variables  on different dogs  So  fido colour is one variable  but spot colour is another  The first one is bound to the object within the variable fido  the second  spot  Now  when you call Dog 4   brown    or three add five   there will always be an invisible parameter  which will be assigned to the dangling extra one at the front of the parameter list  It is conventionally called self  and will get the value of the object in front of the dot  Thus  within the Dog s   init    constructor   self will be whatever the new Dog will turn out to be  within MyInteger s add  self will be bound to the object in the variable three  Thus  three value will be the same variable outside the add  as self value within the add   If I say the mangy one   fido  I will start referring to the object known as fido with yet another name  From now on  fido colour is exactly the same variable as the mangy one colour   So  the things inside the   init    You can think of them as noting things into the Dog s birth certificate  colour by itself is a random variable  could contain anything  fido colour or self colour is like a form field on the Dog s identity sheet  and   init   is the clerk filling it out for the first time   Any clearer   EDIT  Expanding on the comment below   You mean a list of objects  don t you   First of all  fido is actually not an object  It is a variable  which is currently containing an object  just like when you say x   5  x is a variable currently containing the number five  If you later change your mind  you can do fido   Cat 4   pleasing    as long as you ve created a class Cat   and fido would from then on  contain  a cat object  If you do fido   x  it will then contain the number five  and not an animal object at all   A class by itself doesn t know its instances unless you specifically write code to keep track of them  For instance   class Cat      census       define census array      def   init   self  legs  colour           self colour   colour         self legs   legs         Cat census append self    Here  census is a class-level attribute of Cat class   fluffy   Cat 4   white   spark   Cat 4   fiery   Cat census      gt    lt   main   Cat instance at 0x108982cb0 gt    lt   main   Cat instance at 0x108982e18 gt     or something like that   Note that you won t get  fluffy  sparky   Those are just variable names  If you want cats themselves to have names  you have to make a separate attribute for the name  and then override the   str   method to return this name  This method s  i e  class-bound function  just like add or   init    purpose is to describe how to convert the object to a string  like when you print it out

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class Dog object          Class Object Attribute     species    mammal       def   init   self breed name           self breed   breed         self name   name   In above example we use species as a global since it will be always same Kind of constant you can say   when you call   init   method then all the variable inside   init   will be initiated eg breed name    class Dog object       a    12       def   init   self breed name a           self breed   breed         self name   name         self a  a   if you print the above example by calling below like this  Dog a 12  Dog  Lab   Sam   10   Dog a 10   That means it will be only initialized during object creation  so anything which you want to declare as constant make it as global and anything which changes use    init

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To contribute my 5 cents to the thorough explanation from Amadan   Where classes are a description  of a type  in an abstract way  Objects are their realizations  the living breathing thing  In the object-orientated world there are principal ideas you can almost call the essence of everything  They are    encapsulation  won t elaborate on this  inheritance polymorphism   Objects have one  or more characteristics    Attributes  and behaviors    Methods   The behavior mostly depends on the characteristics  Classes define what the behavior should accomplish in a general way  but as long as the class is not realized  instantiated  as an object it remains an abstract concept of a possibility   Let me illustrate with the help of  inheritance  and  polymorphism        class Human          gender         nationality         favorite drink         core characteristic         favorite beverage         name         age          def love             def drink         def laugh         def do your special thing                      class Americans Humans          def drink beverage               if beverage    favorite drink  print  You call that a drink               else  print  Great         class French Humans          def drink beverage  cheese               if beverage    favourite drink and cheese    None  print  No cheese                elif beverage    favourite drink and cheese    None  print  R  volution        class Brazilian Humans          def do your special thing             win every football world cup        class Germans Humans          def drink beverage               if favorite drink    beverage  print  I need more beer              else  print  Lecker         class HighSchoolStudent Americans           def   init   self  name  age                self name   name              self age   age  jeff   HighSchoolStudent name  age   hans   Germans   ronaldo   Brazilian   amelie   French    for friends in  jeff  hans  ronaldo       friends laugh       friends drink  cola       friends do your special thing    print amelie love jeff   gt  gt  gt  True print ronaldo love hans   gt  gt  gt  False   Some characteristics define human beings  But every nationality differs somewhat  So  national-types  are kinda Humans with extras   Americans  are a type of  Humans   and inherit some abstract characteristics and behavior from the human type  base-class    that s inheritance  So all Humans can laugh and drink  therefore all child-classes can also  Inheritance  2    But because they are all of the same kind  Type base-class   Humans  you can exchange them sometimes  see the for-loop at the end  But they will expose an individual characteristic  and thats Polymorphism  3    So each human has a favorite drink  but every nationality tend towards a special kind of drink  If you subclass a nationality from the type of Humans you can overwrite the inherited behavior as I have demonstrated above with the drink   Method  But that s still at the class-level and because of this it s still a generalization   hans   German favorite drink    Cola     instantiates the class German and I  changed  a default characteristic at the beginning   But if you call hans drink  Milk   he would still print  I need more beer  - an obvious bug     or maybe that s what i would call a feature if i would be a Employee of a bigger Company   -      The characteristic of a type e g  Germans  hans  are usually defined through the constructor  in python     init    at the moment of the instantiation  This is the point where you define a class to become an object  You could say breath life into an abstract concept  class  by filling it with individual characteristics and becoming an object   But because every object is an instance of a class they share all some basic characteristic-types and some behavior  This is a major advantage of the object-orientated concept    To protect the characteristics of each object you encapsulate them - means you try to couple behavior and characteristic and make it hard to manipulate it from outside the object  That s Encapsulation  1

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It is just to initialize the instance s variables     E g  create a crawler instance with a specific database name  from your example above

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Following with your car example  when you get a car  you just don t get a random car  I mean  you choose the color  the brand  number of seats  etc  And some stuff is also  initialize  without you choosing for it  like number of wheels or registration number   class Car      def   init   self  color  brand  number of seats           self color   color         self brand   brand         self number of seats   number of seats         self number of wheels   4         self registration number   GenerateRegistrationNumber     So  in the   init   method you defining the attributes of the instance you re creating  So  if we want a blue Renault car  for 2 people  we would initialize or instance of Car like   my car   Car  blue    Renault   2    This way  we are creating an instance of the Car class  The   init   is the one that is handling our specific attributes  like color or brand  and its generating the other attributes  like registration number    More about classes in Python More about the   init   method

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