lazy.py

class Uncomputed:
	"""
	A singleton to denote uncomputed values.
	"""
	pass

class lazy(object):
	"""
	Doubles up as a descriptor for lazy properties and a decorator for lazy functions.
	Example usage: 
		
		# === as a descriptor:
		class A(...):
			foo = lazy(lambda self: self._foo())
			def foo(self):
				...
		
		# example:
		a = A()
		print a.foo # computed
		print a.foo # looked up
		
		# === as a decorator:
		@lazy
		def foo():
			...
		
		print foo() # computed
		print foo() # looked up
	"""	
	
	def __init__(self, thunk):
		self.thunk = thunk
		self.cache = {}
		self.value = Uncomputed

	def __get__(self, instance, owner):
		if self.cache.get(instance, Uncomputed) == Uncomputed:
			self.cache[instance] = self.thunk(instance)
		return self.cache[instance]
	
	def __set__(self, instance, value):
		self.cache[instance] = value
	
	def __delete__(self, instance):
		pass
	
	def __call__(self):
		if self.value == Uncomputed:
			self.value = self.thunk()
		return self.value