WeakSingleton

weaksingleton.py

# Derived from Singleton class by Dan Pascu on python-application package

from new import instancemethod
from weakref import WeakValueDictionary

from application.python.decorator import preserve_signature


class WeakSingleton(type):
    """
    Metaclass for making singletons. Instances are kep in a weak-value 
    dictionary, so they will vanish if only the WeakSingleton is holding
    them.
    """
    def __init__(cls, name, bases, dic):
        from types import UnboundMethodType
        if type(cls.__init__) is UnboundMethodType:
            initializer = cls.__init__
        elif type(cls.__new__) is UnboundMethodType:
            initializer = cls.__new__
        else:
            def initializer(self, *args, **kw): pass
        @preserve_signature(initializer)
        def instance_creator(cls, *args, **kwargs):
            key = (args, tuple(sorted(kwargs.iteritems())))
            try:
                hash(key)
            except TypeError:
                raise TypeError("cannot have singletons for classes with unhashable arguments")
            if key not in cls._instances:
                obj = super(WeakSingleton, cls).__call__(*args, **kwargs)
                cls._instances[key] = obj
            return cls._instances[key]
        super(WeakSingleton, cls).__init__(name, bases, dic)
        cls._instances = WeakValueDictionary()
        cls._instance_creator = instancemethod(instance_creator, cls, type(cls))
    def __call__(cls, *args, **kw):
        return cls._instance_creator(*args, **kw)

# Test
if __name__ == '__main__':
    class Foo(object):
        __metaclass__ = WeakSingleton

    f = Foo()
    print id(f)

    g = Foo()
    print id(g)

    print Foo._instances.items()

    del f
    print Foo._instances.items()

    del g
    print Foo._instances.items()