Exception AttributeError: "'NoneType' object has no attribute 'population'"

[344276105]

Hi all,
I am a python learner. I encountered a problem when i was testing the following code. What is strange is that if I replace the object name with zhang, the program would be ok. And if I replace the Person.population with self.__class__.population, it will also be ok. So what is the matter?
Here is the code,

#!/usr/bin/python
 
# Filename: objvar.py
 


 
class Person:        
 
        population = 0
 


 
        def __init__(self, name):
 
                self.name = name
 
                print '(Initializing %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):
 
                print 'Hi, my name is %s.' % self.name
 


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


 
zhangsan = Person('Swaroop')
 
zhangsan.sayHi()
 
zhangsan.howMany()



 
lisi = Person('LiSi')
 
lisi.sayHi()
 
lisi.howMany()
 



zhangsan.sayHi()
 
zhangsan.howMany()
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