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cls_atr_vs_ins_atr.py

class A():
  attr = "Cls attr"

instance1 = A()
instance2 = A()
print("I1 attr", instance1.attr)
instance2.attr = "New Attr"
print("I2 attr", instance2.attr)
print("I1 attr", instance1.attr)
A.attr = "New Cls"
print("I1 attr", instance1.attr)

prevent_adding.py

class A():
    attr = "Cls Attr"

    def __init__(self):
        self.item = "instance item"
        self._initiate = True

    def __setattr__(self, key, value):
        print('setattr ', key, ' : ', value)
        if hasattr(self, key) or \
                not hasattr(self, '_initiate') or \
                self._initiate is False:
            object.__setattr__(self, key, value)
        else:
            print("Attirbute Error {}".format(key))

instance1 = A()
instance1.item = "this item"
instance1.sth = "what?"
instance1.attr = "wooow"

prevent_adding_only_instance.py

class A():
    attr = "Cls Attr"

    def __init__(self):
        self.item = "instance item"
        self._initiate = True

    def __setattr__(self, key, value):
        print('setattr ', key, ' : ', value)
        if key in self.__dict__ or \
                '_initiate' not in self.__dict__ or \
                self._initiate is False:
            object.__setattr__(self, key, value)
        else:
            print("Attirbute Error {}".format(key))

instance1 = A()
instance1.item = "this item"
instance1.sth = "what?"
instance1.attr = "wooow"

prevent_freeze.py

class Freeze:
    def __setattr__(self, key, value):
        if key in self.__dict__ or \
                '_frozen' not in self.__dict__ or \
                self._frozen is False:
            object.__setattr__(self, key, value)
        else:
            print("Attribute Error {}".format(key))

    def _freeze(self):
        self._frozen = True


class A(Freeze):
    attr = "Cls Attr"

    def __init__(self):
        self.item = "instance item"
        self._freeze()


instance1 = A()
instance1.item = "this item"
instance1.sth = "what?"
instance1.attr = "wooow"

prevent_freeze_collab_class_attr.py

from functools import reduce


class Freeze:
    def __setattr__(self, key, value):
        if key in self.__dict__ or \
                '_frozen' not in self.__dict__ or \
                self._frozen is False:
            object.__setattr__(self, key, value)
            return

        def rec(c, k):
            return c if k in c.__dict__ else \
                (reduce(lambda x, y: x or y, list(map(lambda x: rec(x, k), c.__bases__)), None))

        cls = rec(self.__class__, key)
        if cls is not None:
            setattr(cls, key, value)
            return

        print("Attribute Error {}".format(key))

    def _freeze(self):
        self._frozen = True


class A(Freeze):
    attr = "Cls Attr"

    def __init__(self):
        self.item = "instance item"
        self._freeze()


class B(A):
    bttr = "weeew"

    def __init__(self):
        self.btem = "b item"
        super().__init__()


instance1 = B()
instance1.item = "this item"
instance1.sth = "what?"
instance1.attr = "wooow"