import weakref
class weakattr(object):
"""
weakattr - a weakly-referenced attribute. When the attribute is no longer
referenced, it 'disappears' from the instance. Great for cyclic references.
"""
__slots__ = ["dict", "errmsg"]
def __init__(self, name = None):
self.dict = weakref.WeakValueDictionary()
if name:
self.errmsg = "%%r has no attribute named %r" % (name,)
else:
self.errmsg = "%r has no such attribute"
def __repr__(self):
return "<weakattr at 0x%08X>" % (id(self),)
def __get__(self, obj, cls):
if obj is None:
return self
try:
return self.dict[id(obj)]
except KeyError:
raise AttributeError(self.errmsg % (obj,))
def __set__(self, obj, value):
self.dict[id(obj)] = value
def __delete__(self, obj):
try:
del self.dict[id(obj)]
except KeyError:
raise AttributeError(self.errmsg % (obj,))
#
# example
#
>>> class x(object):
... next = weakattr()
... def __init__(self):
... self.next = self
... def __del__(self):
... print "g'bye"
...
>>>
>>> y = x()
>>> y.next
<__main__.x object at 0x009EFA50>
>>> del y
>>> gc.collect()
g'bye
0
