lazy.py

# Lazy object instantiation

class Lazy(object):

    def __new__(cls, realcls, args, kwargs):
        # Use the real class's __new__ to get a new object instance.
        obj = realcls.__new__(realcls, *args, **kwargs)
        # Replace the class with Lazy until reified.
        obj.__class__ = cls
        # Store info we need to reify object.
        obj.__lazy_info = (realcls, args, kwargs)
        return obj
    #enddef
    
    def __getattr__(self, name):
        print("Getting attribute: {}".format(name))
        (realcls, args, kwargs) = self.__lazy_info
        self.__dict__.clear()
        print("Performing class switch.")
        self.__class__ = realcls
        print("Calling init.")
        self.__init__(*args, **kwargs)
        print("Returning actual value.")
        return getattr(self, name)
    #enddef

    def __repr__(self):
        (realcls, args, kwargs) = self.__lazy_info
        return "Lazy{}{}".format(realcls, (args, kwargs))
    #enddef
        
#endclass

class LazyMeta(type):

    def __call__(cls, *args, **kwargs):
        return Lazy(cls, args, kwargs)
    #enddef
    
#endclass

class Foo(object, metaclass=LazyMeta):
    def __init__(self, a, b):
        self.a = a
        self.b = b
    #enddef
    def add(self): return self.a + self.b
#endclass

#############################################

# >>> f = Foo(10, 20)

# >>> type(f)
# __main__.Lazy

# >>> f.add()
# Getting attribute: add
# Performing class switch.
# Calling init.
# Returning actual value.
# 30

# >>> type(f)
# __main__.Foo