""" A Python Singleton mixin class that makes use of some of the ideas found at http://c2.com/cgi/wiki?PythonSingleton. Just inherit from it and you have a singleton. No code is required in subclasses to create singleton behavior -- inheritance from Singleton is all that is needed. Singleton creation is threadsafe. USAGE: Just inherit from Singleton. If you need a constructor, include an __init__() method in your class as you usually would. However, if your class is S, you instantiate the singleton using S.getInstance() instead of S(). Repeated calls to S.getInstance() return the originally-created instance. For example: class S(Singleton): def __init__(self, a, b=1): pass S1 = S.getInstance(1, b=3) Most of the time, that's all you need to know. However, there are some other useful behaviors. Read on for a full description: 1) Getting the singleton: S.getInstance() returns the instance of S. If none exists, it is created. 2) The usual idiom to construct an instance by calling the class, i.e. S() is disabled for the sake of clarity. For one thing, the S() syntax means instantiation, but getInstance() usually does not cause instantiation. So the S() syntax would be misleading. Because of that, if S() were allowed, a programmer who didn't happen to notice the inheritance from Singleton (or who wasn't fully aware of what a Singleton pattern does) might think he was creating a new instance, which could lead to very unexpected behavior. So, overall, it is felt that it is better to make things clearer by requiring the call of a class method that is defined in Singleton. An attempt to instantiate via S() will result in a SingletonException being raised. 3) Use __S.__init__() for instantiation processing, since S.getInstance() runs S.__init__(), passing it the args it has received. If no data needs to be passed in at instantiation time, you don't need S.__init__(). 4) If S.__init__(.) requires parameters, include them ONLY in the first call to S.getInstance(). If subsequent calls have arguments, a SingletonException is raised by default. If you find it more convenient for subsequent calls to be allowed to have arguments, but for those argumentsto be ignored, just include 'ignoreSubsequent = True' in your class definition, i.e.: class S(Singleton): ignoreSubsequent = True def __init__(self, a, b=1): pass 5) For testing, it is sometimes convenient for all existing singleton instances to be forgotten, so that new instantiations can occur. For that reason, a forgetAllSingletons() function is included. Just call forgetAllSingletons() and it is as if no earlier instantiations have occurred. 6) As an implementation detail, classes that inherit from Singleton may not have their own __new__ methods. To make sure this requirement is followed, an exception is raised if a Singleton subclass includ es __new__. This happens at subclass instantiation time (by means of the MetaSingleton metaclass. By Gary Robinson, grobinson@flyfi.com. No rights reserved -- placed in the public domain -- which is only reasonable considering how much it owes to other people's code and ideas which are in the public domain. The idea of using a metaclass came from a comment on Gary's blog (see http://www.garyrobinson.net/2004/03/python_singleto.html#comments). Other improvements came from comments and email from other people who saw it online. (See the blog post and comments for further credits.) Not guaranteed to be fit for any particular purpose. Use at your own risk. """ import threading class SingletonException(Exception): pass _stSingletons = set() _lockForSingletons = threading.RLock() _lockForSingletonCreation = threading.RLock() # Ensure only one instance of each Singleton # class is created. This is not bound to the # individual Singleton class since we need to # ensure that there is only one mutex for each # Singleton class, which would require having # a lock when setting up the Singleton class, # which is what this is anyway. So, when any # Singleton is created, we lock this lock and # then we don't need to lock it again for that # class. def _createSingletonInstance(cls, lstArgs, dctKwArgs): _lockForSingletonCreation.acquire() try: if cls._isInstantiated(): # some other thread got here first return instance = cls.__new__(cls) try: instance.__init__(*lstArgs, **dctKwArgs) except TypeError, e: if e.message.find('__init__() takes') != -1: raise SingletonException, 'If the singleton requires __init__ args, supply them on first call to getInstance().' else: raise cls.cInstance = instance _addSingleton(cls) finally: _lockForSingletonCreation.release() def _addSingleton(cls): _lockForSingletons.acquire() try: assert cls not in _stSingletons _stSingletons.add(cls) finally: _lockForSingletons.release() def _removeSingleton(cls): _lockForSingletons.acquire() try: if cls in _stSingletons: _stSingletons.remove(cls) finally: _lockForSingletons.release() def forgetAllSingletons(): '''This is useful in tests, since it is hard to know which singletons need to be cleared to make a test work.''' _lockForSingletons.acquire() try: for cls in _stSingletons.copy(): cls._forgetClassInstanceReferenceForTesting() # Might have created some Singletons in the process of tearing down. # Try one more time - there should be a limit to this. iNumSingletons = len(_stSingletons) if len(_stSingletons) > 0: for cls in _stSingletons.copy(): cls._forgetClassInstanceReferenceForTesting() iNumSingletons -= 1 assert iNumSingletons == len(_stSingletons), 'Added a singleton while destroying ' + str(cls) assert len(_stSingletons) == 0, _stSingletons finally: _lockForSingletons.release() class MetaSingleton(type): def __new__(metaclass, strName, tupBases, dct): if dct.has_key('__new__'): raise SingletonException, 'Can not override __new__ in a Singleton' return super(MetaSingleton, metaclass).__new__(metaclass, strName, tupBases, dct) def __call__(cls, *lstArgs, **dictArgs): raise SingletonException, 'Singletons may only be instantiated through getInstance()' class Singleton(object): __metaclass__ = MetaSingleton def getInstance(cls, *lstArgs, **dctKwArgs): """ Call this to instantiate an instance or retrieve the existing instance. If the singleton requires args to be instantiated, include them the first time you call getInstance. """ if cls._isInstantiated(): if (lstArgs or dctKwArgs) and not hasattr(cls, 'ignoreSubsequent'): raise SingletonException, 'Singleton already instantiated, but getInstance() called with args.' else: _createSingletonInstance(cls, lstArgs, dctKwArgs) return cls.cInstance getInstance = classmethod(getInstance) def _isInstantiated(cls): # Don't use hasattr(cls, 'cInstance'), because that screws things up if there is a singleton that # extends another singleton. hasattr looks in the base class if it doesn't find in subclass. return 'cInstance' in cls.__dict__ _isInstantiated = classmethod(_isInstantiated) # This can be handy for public use also isInstantiated = _isInstantiated def _forgetClassInstanceReferenceForTesting(cls): """ This is designed for convenience in testing -- sometimes you want to get rid of a singleton during test code to see what happens when you call getInstance() under a new situation. To really delete the object, all external references to it also need to be deleted. """ try: if hasattr(cls.cInstance, '_prepareToForgetSingleton'): # tell instance to release anything it might be holding onto. cls.cInstance._prepareToForgetSingleton() del cls.cInstance _removeSingleton(cls) except AttributeError: # run up the chain of base classes until we find the one that has the instance # and then delete it there for baseClass in cls.__bases__: if issubclass(baseClass, Singleton): baseClass._forgetClassInstanceReferenceForTesting() _forgetClassInstanceReferenceForTesting = classmethod(_forgetClassInstanceReferenceForTesting) if __name__ == '__main__': import unittest import time class singletonmixin_Public_TestCase(unittest.TestCase): def testReturnsSameObject(self): """ Demonstrates normal use -- just call getInstance and it returns a singleton instance """ class A(Singleton): def __init__(self): super(A, self).__init__() a1 = A.getInstance() a2 = A.getInstance() self.assertEquals(id(a1), id(a2)) def testInstantiateWithMultiArgConstructor(self): """ If the singleton needs args to construct, include them in the first call to get instances. """ class B(Singleton): def __init__(self, arg1, arg2): super(B, self).__init__() self.arg1 = arg1 self.arg2 = arg2 b1 = B.getInstance('arg1 value', 'arg2 value') b2 = B.getInstance() self.assertEquals(b1.arg1, 'arg1 value') self.assertEquals(b1.arg2, 'arg2 value') self.assertEquals(id(b1), id(b2)) def testInstantiateWithKeywordArg(self): class B(Singleton): def __init__(self, arg1=5): super(B, self).__init__() self.arg1 = arg1 b1 = B.getInstance('arg1 value') b2 = B.getInstance() self.assertEquals(b1.arg1, 'arg1 value') self.assertEquals(id(b1), id(b2)) def testTryToInstantiateWithoutNeededArgs(self): class B(Singleton): def __init__(self, arg1, arg2): super(B, self).__init__() self.arg1 = arg1 self.arg2 = arg2 self.assertRaises(SingletonException, B.getInstance) def testPassTypeErrorIfAllArgsThere(self): """ Make sure the test for capturing missing args doesn't interfere with a normal TypeError. """ class B(Singleton): def __init__(self, arg1, arg2): super(B, self).__init__() self.arg1 = arg1 self.arg2 = arg2 raise TypeError, 'some type error' self.assertRaises(TypeError, B.getInstance, 1, 2) def testTryToInstantiateWithoutGetInstance(self): """ Demonstrates that singletons can ONLY be instantiated through getInstance, as long as they call Singleton.__init__ during construction. If this check is not required, you don't need to call Singleton.__init__(). """ class A(Singleton): def __init__(self): super(A, self).__init__() self.assertRaises(SingletonException, A) def testDontAllowNew(self): def instantiatedAnIllegalClass(): class A(Singleton): def __init__(self): super(A, self).__init__() def __new__(metaclass, strName, tupBases, dct): return super(MetaSingleton, metaclass).__new__(metaclass, strName, tupBases, dct) self.assertRaises(SingletonException, instantiatedAnIllegalClass) def testDontAllowArgsAfterConstruction(self): class B(Singleton): def __init__(self, arg1, arg2): super(B, self).__init__() self.arg1 = arg1 self.arg2 = arg2 B.getInstance('arg1 value', 'arg2 value') self.assertRaises(SingletonException, B, 'arg1 value', 'arg2 value') def test_forgetClassInstanceReferenceForTesting(self): class A(Singleton): def __init__(self): super(A, self).__init__() class B(A): def __init__(self): super(B, self).__init__() # check that changing the class after forgetting the instance produces # an instance of the new class a = A.getInstance() assert a.__class__.__name__ == 'A' A._forgetClassInstanceReferenceForTesting() b = B.getInstance() assert b.__class__.__name__ == 'B' # check that invoking the 'forget' on a subclass still deletes the instance B._forgetClassInstanceReferenceForTesting() a = A.getInstance() B._forgetClassInstanceReferenceForTesting() b = B.getInstance() assert b.__class__.__name__ == 'B' def test_forgetAllSingletons(self): # Should work if there are no singletons forgetAllSingletons() class A(Singleton): ciInitCount = 0 def __init__(self): super(A, self).__init__() A.ciInitCount += 1 A.getInstance() self.assertEqual(A.ciInitCount, 1) A.getInstance() self.assertEqual(A.ciInitCount, 1) forgetAllSingletons() A.getInstance() self.assertEqual(A.ciInitCount, 2) def test_threadedCreation(self): # Check that only one Singleton is created even if multiple # threads try at the same time. If fails, would see assert in _addSingleton class Test_Singleton(Singleton): def __init__(self): super(Test_Singleton, self).__init__() class Test_SingletonThread(threading.Thread): def __init__(self, fTargetTime): super(Test_SingletonThread, self).__init__() self._fTargetTime = fTargetTime self._eException = None def run(self): try: fSleepTime = self._fTargetTime - time.time() if fSleepTime > 0: time.sleep(fSleepTime) Test_Singleton.getInstance() except Exception, e: self._eException = e fTargetTime = time.time() + 0.1 lstThreads = [] for _ in xrange(100): t = Test_SingletonThread(fTargetTime) t.start() lstThreads.append(t) eException = None for t in lstThreads: t.join() if t._eException and not eException: eException = t._eException if eException: raise eException def testNoInit(self): """ Demonstrates use with a class not defining __init__ """ class A(Singleton): pass #INTENTIONALLY UNDEFINED: #def __init__(self): # super(A, self).__init__() A.getInstance() #Make sure no exception is raised def testMultipleGetInstancesWithArgs(self): class A(Singleton): ignoreSubsequent = True def __init__(self, a, b=1): pass a1 = A.getInstance(1) a2 = A.getInstance(2) # ignores the second call because of ignoreSubsequent class B(Singleton): def __init__(self, a, b=1): pass b1 = B.getInstance(1) self.assertRaises(SingletonException, B.getInstance, 2) # No ignoreSubsequent included class C(Singleton): def __init__(self, a=1): pass c1 = C.getInstance(a=1) self.assertRaises(SingletonException, C.getInstance, a=2) # No ignoreSubsequent included def testInheritance(self): """ It's sometimes said that you can't subclass a singleton (see, for instance, http://steve.yegge.googlepages.com/singleton-considered-stupid point e). This test shows that at least rudimentary subclassing works fine for us. """ class A(Singleton): def setX(self, x): self.x = x def setZ(self, z): raise NotImplementedError class B(A): def setX(self, x): self.x = -x def setY(self, y): self.y = y a = A.getInstance() a.setX(5) b = B.getInstance() b.setX(5) b.setY(50) self.assertEqual((a.x, b.x, b.y), (5, -5, 50)) self.assertRaises(AttributeError, eval, 'a.setY', {}, locals()) self.assertRaises(NotImplementedError, b.setZ, 500) unittest.main() # vim:ts=4:sw=4:et: