Python __init__ and classmethod, do they have to have the same number of args?

Question

So classmethods can be used as an alternative "constructor" in python, they are bound to the class and not to an instance, pretty clear so far. But my question is, if it is mandatory to have the same number of arguments in the returned instance of the class method as in the __init__ . More exactly :

    class MyClass(object):
      def __init__(self,name):
         self.name=name

      @classmethod
      def alternative_init(cls,name,surname):
          return cls(name,surname)

And if I try to create an instance Myclass("alex") works fine, but if I try to create an instance Myclass.alternative_init("alex","james") I have a TypeError , because I pass to many arguments, and init take only 2 . Am I missing something?

Answer 1

__init__ only takes one parameter, the name. Thus, you can pass either name or surname to cls , but not both. However, you can create a class instance in classmethod , and add an additional paramter:

class MyClass(object):
  def __init__(self,name):
    self.name=name
  def __setattr__(self, name, val):
     self.__dict__[name] = val
  @classmethod
  def alternative_init(cls,name,surname):
    v = cls(name)
    v.surname = surname
    return v

Answer 2

You could do what you want like this:

class MyClass(object):
    def __init__(self,name):
        self.name=name

    @classmethod
    def alternative_init(cls,name,surname):
        new_instance = cls(name)
        new_instance.surname = surname
        return new_instance

a = MyClass.alternative_init('Bob', 'Spongy')
print(a.name, a.surname)
# Bob Spongy

Answer 3

Because Myclass.alternative_init("alex","james") calls the cls(name, surname) which same as MyClass(name,surname) which also same as __init__(self,name,surname) but your __init__ function don't have surname parameter. You can make surname optional by __init__(self,name,surname=None)

class MyClass(object):
  def __init__(self,name,surname=None):
     self.name=name
     self.surname=surname

  @classmethod
  def alternative_init(cls,name,surname):
      return cls(name,surname)

Answer 4

In Python, the first argument Passed to a method is always the object itself. If you call now your method with a name, you will get self as first parameter and the name as second.

When you call now the init method from inside of your classmethod, python has no idea what it should do with the surname.

Answer 5

class MyClass(object):
    def __init__(self,name):
        self.name=name

    @classmethod
    def alternative_init(cls,name,surname):
        return cls(name)


a = MyClass("alex")
MyClass.alternative_init("alex","james")