classes and object-oriented programming can be problem for examplesuppose you had an object that was implementing variant of the "observer pattern class account(object)def _init_ (self,name,balance)self name name self balance balance self observers set(def _del_ (self)for ob in self observersob close(del self observers def register(self,observer)self observers add(observerdef unregister(self,observer)self observers remove(observerdef notify(self)for ob in self observersob update(def withdraw(self,amt)self balance -amt self notify(class accountobserver(object)def _init_ (selftheaccount)self theaccount theaccount theaccount register(selfdef _del_ (self)self theaccount unregister(selfdel self theaccount def update(self)print("balance is % fself theaccount balancedef close(self)print("account no longer in use"example setup account('dave', a_ob accountobserver(ain this codethe account class allows set of accountobserver objects to monitor an account instance by receiving an update whenever the balance changes to do thiseach account keeps set of the observers and each accountobserver keeps reference back to the account each class has defined _del_ (in an attempt to provide some sort of cleanup (such as unregistering and so onhoweverit just doesn' work insteadthe classes have created reference cycle in which the reference count never drops to and there is no cleanup not only thatthe garbage collector (the gc modulewon' even clean it upresulting in permanent memory leak one way to fix the problem shown in this example is for one of the classes to create weak reference to the other using the weakref module weak reference is way of creating reference to an object without increasing its reference count to work with weak referenceyou have to add an extra bit of functionality to check whether the object being referred to still exists here is an example of modified observer classimport weakref class accountobserver(object)def _init_ (selftheaccount)self accountref weakref ref(theaccounttheaccount register(selfcreate weakref lib fl ff