correct way to define class variables in Python

Question

I noticed that in Python  people initialize their class attributes in two different ways  The first way is like this  class MyClass      element1   123     element2    quot this is Africa quot     def   init   self        pass or something else  The other style looks like  class MyClass    def   init   self       self   element1   123     self   element2    quot this is Africa quot   Which is the correct way to initialize class attributes

User · Accepted Answer

Neither way is necessarily correct or incorrect, they are just two different kinds of class elements:

Elements outside the __init__ method are static elements; they belong to the class.
Elements inside the __init__ method are elements of the object (self); they don't belong to the class.

You'll see it more clearly with some code:

class MyClass:
    static_elem = 123

    def __init__(self):
        self.object_elem = 456

c1 = MyClass()
c2 = MyClass()

# Initial values of both elements
>>> print c1.static_elem, c1.object_elem 
123 456
>>> print c2.static_elem, c2.object_elem
123 456

# Nothing new so far ...

# Let's try changing the static element
MyClass.static_elem = 999

>>> print c1.static_elem, c1.object_elem
999 456
>>> print c2.static_elem, c2.object_elem
999 456

# Now, let's try changing the object element
c1.object_elem = 888

>>> print c1.static_elem, c1.object_elem
999 888
>>> print c2.static_elem, c2.object_elem
999 456

As you can see, when we changed the class element, it changed for both objects. But, when we changed the object element, the other object remained unchanged.

User · Answer

I think this sample explains the difference between the styles   james bodacious-wired   cat test py     usr bin env python  class MyClass      element1    Hello       def   init   self           self element2    World   obj   MyClass    print dir MyClass  print  --  print dir obj  print  --  print obj element1  print obj element2 print MyClass element1         MyClass element2 james bodacious-wired     test py      doc        init        module      element1   --     doc        init        module      element1    element2   -- Hello World Hello Traceback  most recent call last     File    test py   line 17  in  lt module gt      print MyClass element2 AttributeError  class MyClass has no attribute  element2    element1 is bound to the class  element2 is bound to an instance of the class

[python] correct way to define class variables in Python

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