python - How to inherit __del__ function -

i reading python essential reference 4th ed. , cannot figure out how fix problem in following code

class account(object):     num_accounts = 0      def __init__(self, name, balance):         self.name = name         self.balance = balance         account.num_accounts += 1      def __del__(self):         account.num_accounts -= 1      def deposit(self, amt):         self.balance += amt      def withdraw(self, amt):         self.balance -= amt      def inquire(self):         return self.balance  class evilaccount(account):         def inquire(self):         if random.randint(0,4) == 1:             return self.balance * 1.1         else:             return self.balance  ea = evilaccount('joe',400) 

if understand correctly, ea object goes out of scope when program ends , inherited __del__ function should called, correct? receive 'nonetype' object has no attribute num_accounts in __del__. why doesn't complain earlier in __init__ function?

from the docs:

warning: due precarious circumstances under __del__() methods invoked, exceptions occur during execution ignored, , warning printed sys.stderr instead. also, when __del__() invoked in response module being deleted (e.g., when execution of program done), other globals referenced __del__() method may have been deleted or in process of being torn down (e.g. import machinery shutting down). reason, __del__() methods should absolute minimum needed maintain external invariants. starting version 1.5, python guarantees globals name begins single underscore deleted module before other globals deleted; if no other references such globals exist, may in assuring imported modules still available @ time when __del__() method called.