class GetAttr:
    attr1 = 1
    def __init__(self):
        self.attr2 = 2
    def __getattr__(self, attr):            # On undefined attrs only
        print('get:', attr)                 # Not on attr1: inherited from class
        if attr == 'attr3':                 # Not on attr2: stored on instance
            return 3
        else:
            raise AttributeError(attr)

X = GetAttr()
print(X.attr1)
print(X.attr2)
print(X.attr3)
print('-'*20)

class GetAttribute:
    attr1 = 1
    def __init__(self):
        self.attr2 = 2
    def __getattribute__(self, attr):       # On all attr fetches
        print('get:',  attr)                # Use superclass to avoid looping here
        if attr == 'attr3':
            return 3
        else:
            return object.__getattribute__(self, attr)

X = GetAttribute()
print(X.attr1)
print(X.attr2)
print(X.attr3)