python异常属性错误: "' NoneType' object has no attribute 'var' "

我在 Python 中得到这个异常，

Exception AttributeError: "'NoneType' object has no attribute 'population'" in del of <main.Robot instance at 0x104eb7098>> ignored 

这是我的代码，

class Robot: 
 
    population = 0 #class variable, number of robots 
 
    def __init__(self, name): 
 
        self.name = name 
        print ('(initializing {0})'.format(self.name)) 
 
        Robot.population += 1 
 
    def __del__(self): 
        print('{0} is being destroyed!'.format(self.name)) 
        Robot.population -= 1 
 
        if Robot.population == 0: 
            print ('{0} was the last one.'.format(self.name)) 
        else: 
            print('there are still {0:d} robots working.'.format(Robot.population)) 
 
    def sayHi(self): 
        print('hole mi mestre me llama {0}'.format(self.name)) 
 
    def howMany(): 
        print('hay {0:d} robots'.format(Robot.population)) 
    howMany = staticmethod (howMany) 
 
 
#instantiate 2 robots 
mingos = Robot('alvergas') 
mingos.sayHi() 
Robot.howMany() 
 
pingos = Robot('chupacabra') 
pingos.sayHi() 
Robot.howMany() 
 
#destroy one 
del mingos 
 
Robot.howMany() 

谢谢!

请您参考如下方法:

我更改了代码以在 Python 2.7 上运行并添加了一些打印，结果如下:

# So I changed the code as follows: 
 
    class Robot: 
 
        population = 0 #class variable, number of robots 
 
        def __init__(self, name): 
 
            self.name = name 
            print '(initializing %s)' % self.name 
 
            Robot.population += 1 
 
        def __del__(self): 
            print'%s is being destroyed!' % self.name 
            print 'pre1 %s type %s' % (Robot, type(Robot)) 
            Robot.population -= 1 
 
            print 'pre2' 
            if Robot.population == 0: 
                print '%s was the last one.' % self.name 
            else: 
                print 'there are still %d robots working.' % Robot.population 
 
        def sayHi(self): 
            print '%s says hi' % self.name 
 
        def howMany(): 
            print 'there are %d robots' % Robot.population 
        howMany = staticmethod(howMany) 
 
 
    #instantiate 2 robots 
    mingos = Robot('alvergas') 
    mingos.sayHi() 
    Robot.howMany() 
    print 'end program' 
 
# and the output is: 
 
    (initializing alvergas) 
    alvergas says hi 
    there are 1 robots 
    end prog 
    alvergas is being destroyed! 
    pre1 None type <type 'NoneType'> 
    Exception AttributeError: "'NoneType' object has no attribute 'population'" in 
    <bound method Robot.__del__ of <__main__.Robot instance at 0x0223C918>> ignored 

所以异常发生在程序结束之后。 作为__del__ description说:“当 __del__() 被调用以响应模块被删除时(例如，当程序执行完成时)， __del__() 引用的其他全局变量方法可能已经被删除或正在被拆除(例如进口机械关闭)。”

我认为在您的情况下，当 Robot 类已被拆除并变为 None 时，将调用 Robot.population -= 1 行。尝试访问 None 的属性会导致异常。