Python OOP - Unable to change the value of class variable using self

Question

I'm following this Python Design pattern, and there is something I don't understand about the initialization() function:

class ObjectFactory:

   """ Manages prototypes.
   Static factory, that encapsulates prototype
   initialization and then allows instatiation
   of the classes from these prototypes.
   """

   __type1Value1 = None
   __type1Value2 = None
   __type2Value1 = None
   __type2Value2 = None

   @staticmethod
   def initialize():
      ObjectFactory.__type1Value1 = Type1(1)
      ObjectFactory.__type1Value2 = Type1(2)
      ObjectFactory.__type2Value1 = Type2(1)
      ObjectFactory.__type2Value2 = Type2(2)

Why the init for the variables are using the name of the class (i.e. ObjectFactory.__type1Value1) and not using self (i.e. self.__type1Value1)? When I changed to self:

   def initialize(self):
      self.__type1Value1 = Type1(1)
      self.__type1Value2 = Type1(2)
      self.__type2Value1 = Type2(1)
      self.__type2Value2 = Type2(2)

I got error TypeError: initialize() missing 1 required positional argument: 'self'.

But in another example, using "self" worked:

class Geek: 

    # Variable defined inside the class. 
    inVar = 'inside_class'
    print("Inside_class2", inVar) 

    def access_method(self): 
        self.inVar="a"
        print("Inside_class3", self.inVar) 

uac = Geek() 
uac.access_method()

Output:

Inside_class2 inside_class
Inside_class3 a

What am I missing?

Please update your question with your call to initialize() — quamrana
– quamrana, Commented Jul 8, 2019 at 15:04
Side note: in the linked design pattern, it would have probably been better to decorate the methods with @classmethod instead of @staticmethod to avoid typing the class name. You can read about there difference here: stackoverflow.com/questions/136097/… — Mike Scotty
– Mike Scotty, Commented Jul 8, 2019 at 15:08
Frankly speaking, that tutorial does not seem to teach typical Python patterns and conventions. — Klaus D.
– Klaus D., Commented Jul 8, 2019 at 15:10
This is one more useless tutorial written by someone that has obviously not much experience with the language. Actually the whole serie is just as bad - the part on "the iterator pattern" just shows one simple generator function without even any explanation :facepalm: — bruno desthuilliers
– bruno desthuilliers, Commented Jul 8, 2019 at 15:20
@aronot design patterns are mostly language independant. If you understand the pattern itself - it's principle and the problem it's supposed to solve - and know the language, the effective implementation is just a detail. The best text about design patterns (and possibly one of the best texts about OO design) is still the original: "Design Patterns - Elements of Reusable Object-Oriented Software" by Gamma, Helm, Johnson and Vlissides (aka "the GOF"). — bruno desthuilliers
– bruno desthuilliers, Commented Jul 9, 2019 at 7:29

chepner · Accepted Answer · 2019-07-08 15:06:08Z

2

When looking up the value of an attribute, self.foo will fall back to type(self).foo (roughly speaking) if there is no instance attribute named foo.

When setting a value, though, self.foo will always update (or create, if necessary) an instance attribute. You have to explicitly refer to the class to modify a class attribute.

In your other example, self "worked" in the sense that you verified the value of the new instance attribute inVar of uac. The class attribute Geek.inVar remained unchanged.

answered Jul 8, 2019 at 15:06

chepner

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