Weak reference objects have no attributes or methods, but do allow the referent to be obtained, if it still exists, by calling it:
>>> import weakref >>> class Object: ... pass ... >>> o = Object() >>> r = weakref.ref(o) >>> o2 = r() >>> o is o2 True
If the referent no longer exists, calling the reference object returns None:
>>> del o, o2 >>> print r() None
Testing that a weak reference object is still live should be done
using the expression
ref() is not None. Normally,
application code that needs to use a reference object should follow
# r is a weak reference object o = r() if o is None: # referent has been garbage collected print "Object has been allocated; can't frobnicate." else: print "Object is still live!" o.do_something_useful()
Using a separate test for ``liveness'' creates race conditions in threaded applications; another thread can cause a weak reference to become invalidated before the weak reference is called; the idiom shown above is safe in threaded applications as well as single-threaded applications.
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