好代码系列（一）：LazyObject

site-packages/django/utils/functional.py

 def new_method_proxy(func):
     def inner(self, *args):
         if self_wrapped is empty:
             self._setup()
         return func(self._wrapped, *args)
     return inner

 class LazyObject(object):
     """
     A wrapper for another class that can be used to delay instantiation of the wrapped class.

     By subclassing, you have the opportunity to intercept and alter the
 instantiation. If you don't need to do that, use SimpleLazyObject.
     """

     # Avoid infinite recursion when tracing __init__ (#19456).
     _wrapped = None

     def __init__(self):
         self._wrapped = empty

     __getattr__ = new_method_proxy(getattr)

     def __setattr__(self, name, value):
         if name == "_wrapped":
             # Assign to __dict__ to avoid infinite __setattr__ loops.
             self.__dict__["_wrapped"] = value
         else:
             if self._wrapped is empty:
                 self._setup()
             setattr(self._wrapped, name, value)

     def __delattr__(self, name):
         if name == "_wrapped":
             raise TypeError("can't delete _wrapped.")
         if self._wrapped is empty:
             self._setup()
         delattr(self._wrapped, name)

     def _setup(self):
         """
         Must be implemented by subclasses to initialize the wrapped object.
         """
         raise NotImplementedError('subclasses of LazyObject must provide a _setup() method')

     # Because we have messed with __class__ below, we confuse pickle as to what
     # class we are pickling. We're going to have to initialize the wrapped
     # object to successfully pickle it, so we might as well just pickle the
     # wrapped object since they're supposed to act the same way.
     #
     # Unfortunately, if we try to simply act like the wrapped object, the ruse
     # will break down when pickle gets our id(). Thus we end up with pickle
     # thinking, in effect, that we are a distinct object from the wrapped
     # object, but with the same __dict__. This can cause problems (see #25389).
     #
     # So instead, we define our own __reduce__ method and custom unpickler. We
     # pickle the wrapped object as the unpickler's argument, so that pickle
     # will pickle it normally, and then the unpickler simply returns its
     # argument.
     def __reduce__(self):
         if self._wrapped is empty:
             self._setup()
         return (unpickle_lazyobject, (self._wrapped,))

     # We have to explicitly override __getstate__ so that older versions of
     # pickle don't try to pickle the __dict__ (which in the case of a
     # SimpleLazyObject may contain a lambda). The value will end up being
     # ignored by our __reduce__ and custom unpickler.
     def __getstate__(self):
         return {}

     def __deepcopy__(self, memo):
         if self._wrapped is empty:
             # We have to use type(self), not self.__class__, because the
             # latter is proxied.
             result = type(self)()
             memo[id(self)] = result
             return result
         return copy.deepcopy(self._wrapped, memo)