Python 3 Patterns & Idioms / code / Metaclasses /

# Metaclasses/

def singleton(klass):
    "Simple replacement of object creation operation"
    def getinstance(*args, **kw):
        if not hasattr(klass, 'instance'):
            klass.instance = klass(*args, **kw)
        return klass.instance
    return getinstance

def singleton(klass):
    More powerful approach: Change the behavior
    of the instances AND the class object.
    class Decorated(klass):
        def __init__(self, *args, **kwargs):
            if hasattr(klass, '__init__'):
                klass.__init__(self, *args, **kwargs)
        def __repr__(self) : return klass.__name__ + " obj"
        __str__ = __repr__
    Decorated.__name__ = klass.__name__
    class ClassObject:
        def __init__(cls):
            cls.instance = None
        def __repr__(cls): 
            return klass.__name__
        __str__ = __repr__
        def __call__(cls, *args, **kwargs):
            print str(cls) + " __call__ "
            if not cls.instance:
                cls.instance = Decorated(*args, **kwargs)
            return cls.instance
    return ClassObject()

class ASingleton: pass

a = ASingleton()
b = ASingleton()
print(a, b)
print a.__class__.__name__
print ASingleton
assert a is b

class BSingleton:
    def __init__(self, x):
        self.x = x

c = BSingleton(11)
d = BSingleton(22)
assert c is d
assert c is not a

""" Output:
ASingleton __call__ 
ASingleton __call__ 
(ASingleton obj, ASingleton obj)
BSingleton __call__ 
BSingleton __call__ 
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