Python Singleton ;__ init _ call __;__ metaclass __; Python Singleton

1. There are many implementation methods on the Internet, and the stackflow contains various implementation details. Google by yourself, so you don't need to post a URL.

I will briefly describe the principle here and give a simple demo to help you understand it.

  1 class Singleton(type):                                                                                                 2     def __init__(self, name, bases, dic):  3         print ".... Singleton.init ...."  4         super(Singleton, self).__init__(name, bases, dic)  5         self.instance = None  6   7     def __call__(self, *args, **kwargs):  8         print ".... Singleton.call ...."  9         if self.instance is None: 10             self.instance = super(Singleton, self).__call__(*args, **kwargs) 11         return self.instance 12  13 print "====================== 1 =======================" 14 class test: 15     __metaclass__ = Singleton #call Singleton.__init__ 16     def __init__(self): 17         print "test.init ..." 18     def __call__(self): 19         print "test.call ..." 20  21 class test2: 22     __metaclass__ = Singleton #call Singleton.__init__ 23     def __init__(self): 24         print "test2.init ..." 25     def __call__(self): 26         print "test2.call ..." 27  28 print "====================== 2 =======================" 29 a = test() #call Singleton.__call__ 30 b = test2() #call sSngleton.__call__ 31  32 print "====================== 3 =======================" 33 a1 = test() 34 b1 = test2() 35 print id(a),id(a1),id(b),id(b1) 36 print "====================== 4 =======================" 37 a1() 38 b1() 39  40 print "====================== 5 =======================" 41 a.c = 100 42 b.c = 111 43 print a1.c 44 print b1.c

Output result:

1  bash-3.2$ python test.py 2  ====================== 1 =======================3  .... Singleton.init ....4  .... Singleton.init ....5  ====================== 2 =======================6  .... Singleton.call ....7  test.init ...8  .... Singleton.call ....9  test2.init ...10 ====================== 3 =======================11 .... Singleton.call ....12 .... Singleton.call ....13 4476624848 4476624848 4476670096 447667009614 ====================== 4 =======================15 test.call ...16 test2.call ...17 ====================== 5 =======================18 10019 111

Two Singleton classes, test and test2, are created in the demo. Both use _ metaclass _ = Singleton.

Test results:

The test output shows that id (a) = id (a1), id (B) = id (b1), and create a c attribute to, a1 will also get an identical c, including the c value. The Singleton is OK.

Principle:

Here, the Singleton is implemented in the _ metaclass _ method._ Metaclass _ indicates the class of the class.

Therefore, once the _ metaclass _ = Singleton of test and test2 is set,InstanceAs the class of test and test2. (A bit closed)

Note that line 14 and line 22 correspond to line 2 and line3 printing in the output results. Note:Once we set test, test2's _ metaclass _ = Singleton, Singleton immediately _ init _, and for each Singleton class (test or test2 here) this is done only once during definition.

In the following example, test, test2: a = test (), B = test2 (); then the system will call the _ call __of Singleton; instantiate several times test, test2, call Singleton's _ call _ several times __. Therefore, we only need to implement a Singleton in Singleton's _ call.

The specific implementation method is very simple, just look at the source code.

Add:

The _ init _ function of the test class corresponds to a = test () call.

_ Call _ function corresponds to ()