lz.py

"""Example

.. code-block:: python

   In [1]: a = Expensive(1, b=2)

   In [2]: type(a)
   Out[2]: __main__.LazyProxy

   In [3]: a.method(3)
   Out[3]: 6

   In [4]: type(a)
   Out[4]: __main__.Expensive
"""


class LazyProxy:
    def __init__(self, type_, *args, **kwargs):
        self._type = type_
        self._args = args
        self._kwargs = kwargs

    def __getattr__(self, attr):
        type_ = self._type
        del self._type
        args = self._args
        del self._args
        kwargs = self._kwargs
        del self._kwargs

        self.__class__ = type_
        type_.__init__(self, *args, **kwargs)
        return getattr(self, attr)


class Expensive:
    __new__ = LazyProxy

    def __init__(self, a, b):
        self._expensive_thing = a + b

    def method(self, arg):
        return self._expensive_thing + arg