Python constructors and init

Question

Why are constructors indeed called  quot Constructors quot   What is their purpose and how are they different from methods in a class  Also  can there be more that one   init   in a class  I tried the following  can someone please explain the result   gt  gt  gt  class test      def   init   self           print  quot init 1 quot      def   init   self           print  quot init 2 quot    gt  gt  gt  s test   init 2  Finally  is   init   an operator overloader

User · Answer

There is no notion of method overloading in Python  But you can achieve a similar effect by specifying optional and keyword arguments

User · Answer

coonstructors are called automatically when you create a new object  thereby  constructing  the object   The reason you can have more than one init is because names are just references in python  and you are allowed to change what each variable references whenever you want  hence dynamic typing   def func     now func refers to an empty funcion     pass     func 5       now func refers to the number 5 def func        print  something      now func refers to a different function   in your class definition  it just keeps the later one

User · Answer

Classes are simply blueprints to create objects from  The constructor is some code that are run every time you create an object  Therefor it does nt make sense to have two constructors  What happens is that the second over write the first   What you typically use them for is create variables for that object like this    gt  gt  gt  class testing          def   init   self  init value               self some value   init value   So what you could do then is to create an object from this class like this    gt  gt  gt  testobject   testing 5    The testobject will then have an object called some value that in this sample will be 5    gt  gt  gt  testobject some value 5   But you don t need to set a value for each object like i did in my sample  You can also do like this    gt  gt  gt  class testing          def   init   self               self some value   5   then the value of some value will be 5 and you don t have to set it when you create the object    gt  gt  gt  testobject   testing    gt  gt  gt  testobject some value 5   the     and     in my sample is not what you write  It s how it would look in pyshell

User · Answer

There is no function overloading in Python  meaning that you can t have multiple functions with the same name but different arguments   In your code example  you re not overloading   init      What happens is that the second definition rebinds the name   init   to the new method  rendering the first method inaccessible   As to your general question about constructors  Wikipedia is a good starting point  For Python-specific stuff  I highly recommend the Python docs

User · Answer

Why are constructors indeed called  Constructors     The constructor  named   new    creates and returns a new instance of the class  So the C   new   class method is the constructor for the class C   The C   init   instance method is called on a specific instance  after it is created  to initialise it before being passed back to the caller  So that method is the initialiser for new instances of C   How are they different from methods in a class   As stated in the official documentation   init   is called after the instance is created  Other methods do not receive this treatment   What is their purpose   The purpose of the constructor C   new   is to define custom behaviour during construction of a new C instance   The purpose of the initialiser C   init   is to define custom initialisation of each instance of C after it is created   For example Python allows you to do   class Test object       pass  t   Test    t x   10     here you re building your object t print t x   But if you want every instance of Test to have an attribute x equal to 10  you can put that code inside   init     class Test object       def   init   self           self x   10  t   Test   print t x   Every instance method  a method called on a specific instance of a class  receives the instance as its first argument  That argument is conventionally named self   Class methods  such as the constructor   new    instead receive the class as their first argument   Now  if you want custom values for the x attribute all you have to do is pass that value as argument to   init     class Test object       def   init   self  x           self x   x  t   Test 10  print t x z   Test 20  print t x   I hope this will help you clear some doubts  and since you ve already received good answers to the other questions I will stop here

[python] Python constructors and init

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[python] Python constructors and init