Can someone explain what I've done wrong...

[Jason]

Hi,

I'm following a tutorial about classes, and have created the following 
(well, copied it from the manual buy added my own and wifes names)...

class Person:
     population=0

     def __init__(self,name):
         self.name=name
         print '(Initialising %s)' % self.name
         Person.population += 1

     def __del__(self):
         print "%s says bye." % self.name

         Person.population -= 1

         if Person.population == 0:
             print "I am the last one"
         else:
             print "There are still %d people left." % Person.population

     def sayHi(self):
         '''Greeting by the person.

         That's all it does.'''
         print "Hi, my name is %s" % self.name

     def howMany(self):
         if Person.population==1:
             print "I am on the only person here."
         else:
             print "We have %d persons here." % Person.population

Jason=Person("Jason")
Jason.sayHi()
Jason.howMany()

Sophie=Person("Sophie")
Sophie.sayHi()
Sophie.howMany()

Jason.sayHi()

The code, when run, should produce the following...

Hi, my name is Jason.
I am the only person here.
(Initializing Sophie)
Hi, my name is Sophie.
We have 2 persons here.
Hi, my name is Jason.
We have 2 persons here.
Jason says bye.
There are still 1 people left.
Sophie says bye.
I am the last one.

But what I actually get is...

(Initialising Jason)
Hi, my name is Jason
I am on the only person here.
(Initialising Sophie)
Hi, my name is Sophie
We have 2 persons here.
Hi, my name is Jason
We have 2 persons here.
Jason says bye.
There are still 1 people left.
Sophie says bye.
Exception exceptions.AttributeError: "'NoneType' object has no attribute 
'popula
tion'" in <bound method Person.__del__ of <__main__.Person instance at 
0x0097B53
0>> ignored

I've looked through the code but can't find anything obvious.

I also want to apologise if this isn't the write newsgroup to post on, 
but it's the only one I know of.  IF anyone knows a good newsgroup, I'd 
appreciate it.

TIA