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 · 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 <module>
    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.

User · 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  gt  gt  gt  print c1 static elem  c1 object elem  123 456  gt  gt  gt  print c2 static elem  c2 object elem 123 456    Nothing new so far        Let s try changing the static element MyClass static elem   999   gt  gt  gt  print c1 static elem  c1 object elem 999 456  gt  gt  gt  print c2 static elem  c2 object elem 999 456    Now  let s try changing the object element c1 object elem   888   gt  gt  gt  print c1 static elem  c1 object elem 999 888  gt  gt  gt  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

[python] correct way to define class variables in Python

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