#
# first solution:
#
class sample(object):
class one(object):
def __get__(self, obj, type=None):
print "computing ..."
obj.one = 1
return 1
one = one()
x=sample()
print x.one
print x.one
#
# other solution:
#
# lazy attribute descriptor
class lazyattr(object):
def __init__(self, fget, doc=''):
self.fget = fget
self.__doc__ = doc
def __appoint__(self, name, cl_name):
if hasattr(self,"name"):
raise SyntaxError, "conflict between "+name+" and "+self.name
self.name = name
def __get__(self, obj, cl=None):
if obj is None:
return self
value = self.fget(obj)
setattr(obj, self.name, value)
return value
# appointer metaclass:
# call the members __appoint__ method
class appointer(type):
def __init__(self, cl_name, bases, namespace):
for name,obj in namespace.iteritems():
try:
obj.__appoint__(name, cl_name)
except AttributeError:
pass
super(appointer, self).__init__(cl_name, bases, namespace)
# base class for lazyattr users
class lazyuser(object):
__metaclass__ = appointer
# usage sample
class sample(lazyuser):
def one(self):
print "computing ..."
return 1
one = lazyattr(one, "one lazyattr")
x=sample()
print x.one
print x.one
del x.one
print x.one
