init and arguments in Python

Question

I want to understand arguments of the constructor   init   in Python   class Num      def   init   self num           self n   num     def getn self           return self n     def getone            return 1 myObj   Num 3   print myObj getn     RESULT  3  I call the getone   method   print myObj getone     RESULT  Error  getone    takes no arguments  1given    So I replace   def getone        return 1   with  def getone self       return 1   RESULT 1 This is OK   But getone   method needs no arguments   Do I have to use meaningless argument

User · Answer

The fact that your method does not use the self argument  which is a reference to the instance that the method is attached to  doesn t mean you can leave it out  It always has to be there  because Python is always going to try to pass it in

User · Answer

In python you must always pass in at least one argument to class methods  the argument is self and it is not meaningless its a reference to the instance itself

User · Answer

If you print type Num getone   you will get  lt class  function  gt   It is just a plain function  and be called as usual  with no arguments   Num getone     returns 1  as expected  but if you print print type myObj getone    you will get  lt class  method  gt   So when you call getone   from an instance of the class  Python automatically  quot transforms quot  the function defined in a class into a method  An instance method requires the first argument to be the instance object  You can think myObj getone    as syntactic sugar for Num getone myObj    this explains the Error  getone    takes no arguments  1 given    For  example  class Num      def   init   self num           self n   num     def getid self           return id self   myObj Num 3   Now if you print id myObj     myObj getid          returns True  As you can see self and myObj are the same object

User · Answer

In Python     Instance methods  require the self argument  Class methods  take the class as a first argument    Static methods  do not require either the instance  self  or the class  cls  argument      init   is a special function and without overriding   new   it will always be given the instance of the class as its first argument     An example using the builtin classmethod and staticmethod decorators   import sys  class Num      max   sys maxint      def   init   self num           self n   num      def getn self           return self n       staticmethod     def getone            return 1       classmethod     def getmax cls           return cls max  myObj   Num 3    with the appropriate decorator these should work fine myObj getone   myObj getmax   myObj getn     That said  I would try to use  classmethod  staticmethod sparingly   If you find yourself creating objects that consist of nothing but staticmethods the more pythonic thing to do would be to create a new module of related functions

User · Answer

Every method needs to accept one argument  The instance itself  or the class if it is a static method    Read more about classes in Python

User · Answer

The current object is explicitly passed to the method as the first parameter   self is the conventional name   You can call it anything you want but it is strongly advised that you stick with this convention to avoid confusion

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