Please explain what I've done wro

it seems to me that you should not write Person.population
etc, but self.population

When all person. is replaced by self.  the code works as promised!
:

class Person:
    population=0

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

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

        self.population -= 1

        if self.population == 0:
            print "I am the last one"
        else:
            print "There are still %d people left." % self.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 self.population==1:
            print "I am on the only person here."
        else:
            print "We have %d persons here." % self.population

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

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

Jason.sayHi()