def addLazySingleton(self, item_key, value_callable, *pargs, **kwargs):
		"""This is like addLazyItem except value_callable will only be called
		a maximum of 1 time and the result will be cached for future requests."""
		self.lazy_items[item_key] = \
			self._LazyItem(value_callable, pargs, kwargs, True)
		# make it show up in self.keys(), etc...
		UserDict.__setitem__(self, item_key, None)

	def addLazyItem(self, item_key, value_callable, *pargs, **kwargs):
		"""Add a lazy item for the given key.  When the item is requested,
		value_callable will be called with *pargs and **kwargs arguments."""
		self.lazy_items[item_key] = \
			self._LazyItem(value_callable, pargs, kwargs, False)
		# make it show up in self.keys(), etc...
		UserDict.__setitem__(self, item_key, None)

	def __init__(self, loader):
		"""
		@param loader: A class that has a load() that returns two dicts
			the first being a data dict, the second being a dict of errors.
		"""
		UserDict.__init__(self)
		self._loader = loader

	def __deepcopy__(self, memo=None):
		"""
		This forces evaluation of each contained lazy item, and deepcopy of
		the result. A TypeError is raised if any contained lazy item is not
		a singleton, since it is not necessarily possible for the behavior
		of this type of item to be safely preserved.
		"""
		if memo is None:
			memo = {}
		result = self.__class__()
		memo[id(self)] = result
		for k in self:
			k_copy = deepcopy(k, memo)
			lazy_item = self.lazy_items.get(k)
			if lazy_item is not None:
				if not lazy_item.singleton:
					raise TypeError(_unicode_decode("LazyItemsDict " + \
						"deepcopy is unsafe with lazy items that are " + \
						"not singletons: key=%s value=%s") % (k, lazy_item,))
			UserDict.__setitem__(result, k_copy, deepcopy(self[k], memo))
		return result

	def __getitem__(self, item_key):
		if item_key in self.lazy_items:
			lazy_item = self.lazy_items[item_key]
			pargs = lazy_item.pargs
			if pargs is None:
				pargs = ()
			kwargs = lazy_item.kwargs
			if kwargs is None:
				kwargs = {}
			result = lazy_item.func(*pargs, **kwargs)
			if lazy_item.singleton:
				self[item_key] = result
			return result

		else:
			return UserDict.__getitem__(self, item_key)

	def update(self, *args, **kwargs):
		if len(args) > 1:
			raise TypeError(
				"expected at most 1 positional argument, got " + \
				repr(len(args)))
		if args:
			map_obj = args[0]
		else:
			map_obj = None
		if map_obj is None:
			pass
		elif isinstance(map_obj, LazyItemsDict):
			for k in map_obj:
				if k in map_obj.lazy_items:
					UserDict.__setitem__(self, k, None)
				else:
					UserDict.__setitem__(self, k, map_obj[k])
			self.lazy_items.update(map_obj.lazy_items)
		else:
			UserDict.update(self, map_obj)
		if kwargs:
			UserDict.update(self, kwargs)

	def __deepcopy__(self, memo=None):
		"""
		WARNING: If any of the lazy items contains a bound method then it's
		typical for deepcopy() to raise an exception like this:

			File "/usr/lib/python2.5/copy.py", line 189, in deepcopy
				y = _reconstruct(x, rv, 1, memo)
			File "/usr/lib/python2.5/copy.py", line 322, in _reconstruct
				y = callable(*args)
			File "/usr/lib/python2.5/copy_reg.py", line 92, in __newobj__
				return cls.__new__(cls, *args)
			TypeError: instancemethod expected at least 2 arguments, got 0

		If deepcopy() needs to work, this problem can be avoided by
		implementing lazy items with normal (non-bound) functions.

		If deepcopy() raises a TypeError for a lazy item that has been added
		via a call to addLazySingleton(), the singleton will be automatically
		evaluated and deepcopy() will instead be called on the result.
		"""
		if memo is None:
			memo = {}
		result = self.__class__()
		memo[id(self)] = result
		for k in self:
			k_copy = deepcopy(k, memo)
			if k in self.lazy_items:
				lazy_item = self.lazy_items[k]
				try:
					result.lazy_items[k_copy] = deepcopy(lazy_item, memo)
				except TypeError:
					if not lazy_item.singleton:
						raise
					UserDict.__setitem__(result,
						k_copy, deepcopy(self[k], memo))
				else:
					UserDict.__setitem__(result, k_copy, None)
			else:
				UserDict.__setitem__(result, k_copy, deepcopy(self[k], memo))
		return result

	def clear(self):
		self.lazy_items.clear()
		UserDict.clear(self)

	def __delitem__(self, item_key):
		if item_key in self.lazy_items:
			del self.lazy_items[item_key]
		UserDict.__delitem__(self, item_key)

	def __setitem__(self, item_key, value):
		if item_key in self.lazy_items:
			del self.lazy_items[item_key]
		UserDict.__setitem__(self, item_key, value)

	def __init__(self, *args, **kwargs):

		self.lazy_items = {}
		UserDict.__init__(self, *args, **kwargs)