Python單例模式的4種實現方法

Python單例模式的4種實現方法：

1. #-*- encoding=utf-8 -*-
2. print '----------------------方法1--------------------------'
3. #方法1,實現__new__方法
4. #並在將一個類的實例綁定到類變量_instance上,
5. #如果cls._instance為None說明該類還沒有實例化過,實例化該類,並返回
6. #如果cls._instance不為None,直接返回cls._instance
7. class Singleton(object):
8. def __new__(cls, *args, **kw):
9. if not hasattr(cls, '_instance'):
10. orig = super(Singleton, cls)
11. cls._instance = orig.__new__(cls, *args, **kw)
12. return cls._instance
14. class MyClass(Singleton):
15. a = 1
17. one = MyClass()
18. two = MyClass()
20. two.a = 3
21. print one.a
22. #3
23. #one和two完全相同,可以用id(), ==, is檢測
24. print id(one)
25. #29097904
26. print id(two)
27. #29097904
28. print one == two
29. #True
30. print one is two
31. #True
33. print '----------------------方法2--------------------------'
34. #方法2,共享屬性;所謂單例就是所有引用(實例、對象)擁有相同的狀態(屬性)和行為(方法)
35. #同一個類的所有實例天然擁有相同的行為(方法),
36. #只需要保證同一個類的所有實例具有相同的狀態(屬性)即可
37. #所有實例共享屬性的最簡單最直接的方法就是__dict__屬性指向(引用)同一個字典(dict)
38. #可參看:http://code.activestate.com/recipes/66531/
39. class Borg(object):
40. _state = {}
41. def __new__(cls, *args, **kw):
42. ob = super(Borg, cls).__new__(cls, *args, **kw)
43. ob.__dict__ = cls._state
44. return ob
46. class MyClass2(Borg):
47. a = 1
49. one = MyClass2()
50. two = MyClass2()
52. #one和two是兩個不同的對象,id, ==, is對比結果可看出
53. two.a = 3
54. print one.a
55. #3
56. print id(one)
57. #28873680
58. print id(two)
59. #28873712
60. print one == two
61. #False
62. print one is two
63. #False
64. #但是one和two具有相同的（同一個__dict__屬性）,見:
65. print id(one.__dict__)
66. #30104000
67. print id(two.__dict__)
68. #30104000
70. print '----------------------方法3--------------------------'
71. #方法3:本質上是方法1的升級（或者說高級）版
72. #使用__metaclass__（元類）的高級python用法
73. class Singleton2(type):
74. def __init__(cls, name, bases, dict):
75. super(Singleton2, cls).__init__(name, bases, dict)
76. cls._instance = None
77. def __call__(cls, *args, **kw):
78. if cls._instance is None:
79. cls._instance = super(Singleton2, cls).__call__(*args, **kw)
80. return cls._instance
82. class MyClass3(object):
83. __metaclass__ = Singleton2
85. one = MyClass3()
86. two = MyClass3()
88. two.a = 3
89. print one.a
90. #3
91. print id(one)
92. #31495472
93. print id(two)
94. #31495472
95. print one == two
96. #True
97. print one is two
98. #True
100. print '----------------------方法4--------------------------'
101. #方法4:也是方法1的升級（高級）版本,
102. #使用裝飾器(decorator),
103. #這是一種更pythonic,更elegant的方法,
104. #單例類本身根本不知道自己是單例的,因為他本身(自己的代碼)並不是單例的
105. def singleton(cls, *args, **kw):
106. instances = {}
107. def _singleton():
108. if cls not in instances:
109. instances[cls] = cls(*args, **kw)
110. return instances[cls]
111. return _singleton
113. @singleton
114. class MyClass4(object):
115. a = 1
116. def __init__(self, x=0):
117. self.x = x
119. one = MyClass4()
120. two = MyClass4()
122. two.a = 3
123. print one.a
124. #3
125. print id(one)
126. #29660784
127. print id(two)
128. #29660784
129. print one == two
130. #True
131. print one is two
132. #True
133. one.x = 1
134. print one.x
135. #1
136. print two.x
137. #1