simplified custom type in python

custom_type.py

#!/usr/bin/env python

class MyType(object):
  def __init__(self, value):
    self.value = value

  def __add__(self, other):                           #overload '+'
    if not isinstance(other, MyType):                 #only works on MyType types
      return NotImplemented
    return self.__class__(self.value + other.value)

  def __str__(self):                                  #overload print() 
    return str(self.value)                            #well, not exactly.

if __name__ == "__main__":
  obj  = MyType(5)
  obj2 = MyType(6)

  obj3 = obj + obj2
  print(obj3)                                         #this should work just fine

  obj4 = obj3 + 7
  print(obj4)                                         #this should err out