There are class attributes, and instance attributes.
When you say
class base :
derived_val = 1
You are defining a class attribute. derived_val becomes a key in
base.__dict__.
t2=base()
print(base.__dict__)
# {'derived_val': 1, '__module__': '__main__', '__doc__': None}
print(t2.__dict__)
# {}
When you say t2.derived_val Python tries to find ‘derived_val’ in t2.__dict__. Since it is not there, it looks if there is a 'derived_val' key in any of t2‘s base classes.
print(t2.derived_val)
print(t2.__dict__)
# 1
# {}
But when you assign a value to t2.derived_val, you are now adding an instance attribute to t2. A derived_val key is added to t2.__dict__.
t2.derived_val = t2.derived_val+1
print(t2.derived_val)
print(t2.__dict__)
# 2
# {'derived_val': 2}
Note that at this point, there are two derived_val attributes, but only
the instance attribute is easily accessible. The class attribute becomes accessible only through referencing base.derived_val or direct access to the class dict base.__dict__.