python 类中 self 和 类本身变量的区别

杨奇龙

当使用 self 来调用变量population的时候，此变量是属于实例的。当 Jack 构造完成，结束howmany（）函数结束时Jack 实例的population 就注销了，而Tom 的population 变量还是存在的，

1. #!/etc/bin/python
   
2. # Filename: objvar.py
   
3. 
   
4. class Person:
   
5.     '''Represents a person.'''
   
6.     population = 0
   
7. 
   
8.     def __init__(self, name):
   
9.         '''Initializes the person's data.'''
   
10.         self.name = name
    
11. #!/etc/bin/python
    
12. # Filename: objvar.py
    
13. 
    
14. class Person:
    
15.     '''Represents a person.'''
    
16.     population = 0
    
17. 
    
18.     def __init__(self, name):
    
19.         '''Initializes the person's data.'''
    
20.         self.name = name
    
21.         print '(Initializing %s)' % self.name
    
22. 
    
23.         # When this person is created, he/she
    
24.         # adds to the population
    
25.       [b]  self.population += 1[/b]
    
26. 
    
27.     def __del__(self):
    
28.         '''I am dying.'''
    
29.         print '%s is dying ' %self.name
    
30.         print '%s says bye.' % self.name
    
31. 
    
32.         [b]self.population -= 1[/b]
    
33. 
    
34. 
    
35.         if self.population == 0:
    
36.             print 'I am the last one.'
    
37.         else:
    
38.             print 'There are still %d people left.' % Person.population
    
39. 
    
40.     def sayHi(self):
    
41.         '''Greeting by the person.
    
42. 
    
43.         Really, that's all it does.'''
    
44.         print 'Hi, my name is %s.' % self.name
    
45. 
    
46.     def howMany(self):
    
47.         '''Prints the current population.'''
    
48.         if self.population == 1:
    
49.             print 'I am the only person here.'
    
50.         else:
    
51.             print 'We have %d persons here.' % Person.population
    
52. 
    
53. Jack = Person('Jack')
    
54. Jack.sayHi()
    
55. Jack.howMany()
    
56. 
    
57. 
    
58. Tom = Person('Tom')
    
59. Tom.sayHi()
    
60. Tom.howMany()
    
61. print '\n Jack.population ',Jack.population
    
62. print '\n Tom.population =',Tom.population
    
63. 
    
64. 
    
65. #Jcak.sayHi()
    
66. #Jack.howMany()
    
67. "objvar.py" 56L, 1287C written
    
68. [yang@rac1 python]$ python objvar.py
    
69. (Initializing Jack)
    
70. Hi, my name is Jack.
    
71. I am the only person here.
    
72. (Initializing Tom)
    
73. Hi, my name is Tom.
    
74. I am the only person here.
    
75. 
    
76. Jack.population  1
    
77. 
    
78. Tom.population = 1
    
79. Jack is dying
    
80. Jack says bye.
    
81. I am the last one.
    
82. Tom is dying
    
83. Tom says bye.
    
84. I am the last one.

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然后看看下面的帖子：

杨奇龙

当使用 类来调用 population时，实例jack 调用完成之后，population变量随之也被注销了。Tom，Jack 不能再调用population变量了。

1. #!/etc/bin/python
   
2. # Filename: objvar.py
   
3. 
   
4. class Person:
   
5.     '''Represents a person.'''
   
6.     population = 0
   
7. 
   
8.     def __init__(self, name):
   
9.         '''Initializes the person's data.'''
   
10.         self.name = name
    
11.         print '(Initializing %s)' % self.name
    
12. 
    
13.         # When this person is created, he/she
    
14.         # adds to the population
    
15.        [b] Person.population += 1[/b]
    
16. 
    
17.     def __del__(self):
    
18.         '''I am dying.'''
    
19.         print '%s is dying ' %self.name
    
20.         print '%s says bye.' % self.name
    
21. 
    
22.        [b] Person.population -= 1[/b]
    
23. 
    
24. 
    
25.         if Person.population == 0:
    
26.             print 'I am the last one.'
    
27.         else:
    
28.             print 'There are still %d people left.' % Person.population
    
29. 
    
30.     def sayHi(self):
    
31.         '''Greeting by the person.
    
32. 
    
33.         Really, that's all it does.'''
    
34.         print 'Hi, my name is %s.' % self.name
    
35. 
    
36.     def howMany(self):
    
37.         '''Prints the current population.'''
    
38.         if Person.population == 1:
    
39.             print 'I am the only person here.'
    
40.         else:
    
41.             print 'We have %d persons here.' % Person.population
    
42. 
    
43. Jack = Person('Jack')
    
44. Jack.sayHi()
    
45. Jack.howMany()
    
46. print Jack.population
    
47. Tom = Person('Tom')
    
48. Tom.sayHi()
    
49. Tom.howMany()
    
50. #Jcak.sayHi()
    
51. #Jack.howMany()
    
52. "objvar.py" 51L, 1226C written                                                                                                                       
    
53. [yang@rac1 python]$ python objvar.py
    
54. (Initializing Jack)
    
55. Hi, my name is Jack.
    
56. I am the only person here.
    
57. 1
    
58. (Initializing Tom)
    
59. Hi, my name is Tom.
    
60. We have 2 persons here.
    
61. Jack is dying
    
62. Jack says bye.        
    
63. Exception exceptions.AttributeError: "'NoneType' object has no attribute 'population'" in <bound method Person.__del__ of <__main__.Person instance at 0x2acbd3e31e60>> ignored
    
64. Tom is dying
    
65. Tom says bye.
    
66. Exception exceptions.AttributeError: "'NoneType' object has no attribute 'population'" in <bound method Person.__del__ of <__main__.Person instance at 0x2acbd3e31f38>> ignored

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杨奇龙

我想要的输出是：

1. 
   
2. (Initializing Jack)
   
3. Hi, my name is Jack.
   
4. I am the only person here.
   
5. (Initializing Tom)
   
6. Hi, my name is Tom.
   
7. We have 2 persons here.
   
8. Jack.population =2
   
9. Tom.population =2
   
10. Jack is dying
    
11. Jack says bye.
    
12. There are still 1 people left.
    
13. Tom is dying
    
14. Tom says bye.
    
15. I am the last one.
    

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杨奇龙

怎么编写，让jack 实例注销的时候 population的值为1 而不是 0或者 被注销

杨奇龙

问题解决了。。很无力的原因：和源程序换了一个名称，就不行了，使用源程序是可以的
源程序地址：http://sebug.net/paper/python/ch11s06.html

106033177

回复 1# 杨奇龙
这是由于python不保证对象销毁顺序造成的。
when __del__() is invoked in response to a module being deleted (e.g., when execution of the program is done), other globals referenced by the __del__() method may already have been deleted or in the process of being torn down (e.g. the import machinery shutting down). For this reason, __del__() methods should do the absolute minimum needed to maintain external invariants.
从1.5开始能够保证的仅仅是以单下划线开始的早于其他的。所以针对你的例子可以改成这样

1. _Jack = Person('Jack')
   
2. _Jack.sayHi()
   
3. _Jack.howMany()
   
4. 
   
5. _Tom = Person('Tom')
   
6. _Tom.sayHi()
   
7. _Tom.howMany()
   
8. _Jack.sayHi()
   
9. _Jack.howMany()
   

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106033177

回复 1# 杨奇龙

另外一个办法就是用self.__class__.population 代替Person.population。这样即使Person这个名字早于Jack  Tom从当前模块删除也没问题。

1.   def __del__(self):
   
2.         '''I am dying.'''
   
3.         print '%s is dying ' %self.name
   
4.         print '%s says bye.' % self.name
   
5.         self.__class__.population -= 1
   
6.         if self.__class__.population == 0
   
7.             print 'I am the last one.'
   
8.         else:
   
9.             print 'There are still %d people left.' % self.__class__.population
   
10. 
    
11. ......
    
12. 
    
13. Jack = Person('Jack')
    
14. Jack.sayHi()
    
15. Jack.howMany()
    
16. 
    
17. Tom = Person('Tom')
    
18. Tom.sayHi()
    
19. Tom.howMany()
    
20. Jack.sayHi()
    
21. Jack.howMany()
    

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lovekaiyuan

回复 7# 106033177


    你好，这几天也遇到了这种情况，在网络上搜索了很多也没有解决一些主要的问题，有几个疑惑 1、为什么实例还没有结束，Person类就把自己的population给注销了，他是怎么判断出来用户不再使用了呢？ 2、为什么 使用 self.__class__.population 就可以改变结果不在出现错误？和Person.population有什么区别呢？