When you want to set or change a value of the class variable, you should access it via the class, but not the class instance, as you can do for reading.
When you try to set a value for the class variable using the object (a variable referring to the object or self keyword) but not the class, you are creating an instance variable that holds the same name as the class variable. The following snippet shows such a case – remember this in order to avoid wasting time hunting for bugs!
The snippet contains additional comments for direct explanations.
shared variable
shared variable
....................
contents of d1: {}
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contents of d1: {'class_var': "I'm messing with the class variable"}
I'm messing with the class variable
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contents of d2: {}
contents of class variable accessed via d2: shared variable
output
Code
class Demo:class_var = 'shared variable'
d1 = Demo()
d2 = Demo()
# both instances allow access to the class variable
print(d1.class_var)
print(d2.class_var)
print('.' * 20)
# d1 object has no instance variable
print('contents of d1:', d1.__dict__)
print('.' * 20)
# d1 object receives an instance variable named 'class_var'
d1.class_var = "I'm messing with the class variable"
# d1 object owns the variable named 'class_var' which holds a different value than the class variable named in the same way
print('contents of d1:', d1.__dict__)
print(d1.class_var)
print('.' * 20)
# d2 object variables were not influenced
print('contents of d2:', d2.__dict__)
# d2 object variables were not influenced
print('contents of class variable accessed via d2:', d2.class_var)
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