Example code that illustrates the (possibly surprising) behavior of Python instance and class attributes

test_class_behavior.py

class MyNewClass(object):
    a = 1 # class attribute
    def __init__(self):
        self.b = 1 # instance attribute
    def get_a_via_instance(self):
        return self.a
    def get_a_via_class(self):
        return self.__class__.a # = MyNewClass.a

class MyOldClass():
    # This is an old style class (Python 2 only)
    a = 1 # class attribute
    def __init__(self):
        self.b = 1 # instance attribute
    def get_a_via_instance(self):
        return self.a
    def get_a_via_class(self):
        return self.__class__.a # = MyOldClass.a

class MySlotClass(object):
    __slots__ = ['b', ]
    a = 1 # class attribute
    def __init__(self):
        self.b = 1 # instance attribute
    def get_a_via_instance(self):
        return self.a
    def get_a_via_class(self):
        return self.__class__.a # = MySlotClass.a

def assert_dict(inst, attrs):
    try:
        assert sorted(list(inst.__dict__)) == sorted(attrs)
    except AttributeError:
        print(str(inst.__class__)+" has no __dict__")


def test_class_behavior():
    for cls in [MyNewClass, MyOldClass, MySlotClass]:

        inst1 = cls()
        inst2 = cls()

        # all instances share the same class attribute
        assert inst1.a == inst2.a == 1
        assert inst1.get_a_via_instance() == inst1.get_a_via_class()
        cls.a = 2
        assert inst1.a == inst2.a == 2
        # the class attribute is *not* part of the instance __dict__, only the
        # instance attribute are!
        assert_dict(inst1, ['b', ])
        assert_dict(inst2, ['b', ])

        # we can create arbitrary new instance attributes, which are added to
        # the instance __dict__
        try:
            inst1.new_attr = 1
            assert_dict(inst1, ['b', 'new_attr'])
        except AttributeError as exc:
            # ... except if the class has __slots__
            print("%s: %s" % (str(cls), str(exc)))

        # Assigning to the class attribute name of an instance does *not* set
        # the value of the class attribute, but creates a new instance
        # attribute that shadows the class attribute
        try:
            inst1.a = 1
            assert inst1.a == inst1.get_a_via_instance() == 1
            assert inst2.a == 2
            assert inst1.get_a_via_class() == inst2.a
            assert_dict(inst1, ['a', 'b', 'new_attr'])
            assert_dict(inst2, ['b', ])
        except AttributeError as exc:
            # ... except if the class has __slots__
            print("%s: %s" % (str(cls), str(exc)))

        # One could actually remove the shadowing instance attribute, again
        # allowing access to the class attribute
        try:
            del inst1.a
            assert_dict(inst1, ['b', 'new_attr'])
        except AttributeError as exc:
            print("%s: %s" % (str(cls), str(exc)))
        assert inst1.a == 2


if __name__ == "__main__":
    test_class_behavior()