PEP 205, Weak References -- initial checkin.

Include/classobject.h

@@ -24,6 +24,7 @@ typedef struct {
     PyObject_HEAD
     PyClassObject *in_class;	/* The class object */
     PyObject	  *in_dict;	/* A dictionary */
+    PyObject	  *in_weakreflist; /* List of weak references */
 } PyInstanceObject;
 
 typedef struct {

Include/object.h

@@ -246,8 +246,8 @@ typedef struct _typeobject {
 	/* rich comparisons */
 	richcmpfunc tp_richcompare;
 
-	/* More spares */
-	long tp_xxx8;
+	/* weak reference enabler */
+	long tp_weaklistoffset;
 
 #ifdef COUNT_ALLOCS
 	/* these must be last */
@@ -284,6 +284,8 @@ extern DL_IMPORT(int) PyCallable_Check(PyObject *);
 extern DL_IMPORT(int) PyNumber_Coerce(PyObject **, PyObject **);
 extern DL_IMPORT(int) PyNumber_CoerceEx(PyObject **, PyObject **);
 
+extern DL_IMPORT(int) (*PyObject_ClearWeakRefs)(PyObject *);
+
 /* Helpers for printing recursive container types */
 extern DL_IMPORT(int) Py_ReprEnter(PyObject *);
 extern DL_IMPORT(void) Py_ReprLeave(PyObject *);
@@ -418,7 +420,7 @@ extern DL_IMPORT(long) _Py_RefTotal;
 
 #define Py_INCREF(op) (_Py_RefTotal++, (op)->ob_refcnt++)
 #define Py_DECREF(op) \
-	if (--_Py_RefTotal, --(op)->ob_refcnt != 0) \
+	if (--_Py_RefTotal, (--((op)->ob_refcnt) != 0)) \
 		; \
 	else \
 		_Py_Dealloc((PyObject *)(op))

Include/objimpl.h

@@ -160,7 +160,11 @@ extern DL_IMPORT(void) _PyObject_Del(PyObject *);
 /* Macros trading binary compatibility for speed. See also pymem.h.
    Note that these macros expect non-NULL object pointers.*/
 #define PyObject_INIT(op, typeobj) \
-	( (op)->ob_type = (typeobj), _Py_NewReference((PyObject *)(op)), (op) )
+	((op)->ob_type = (typeobj), _Py_NewReference((PyObject *)(op)), \
+	 (PyType_SUPPORTS_WEAKREFS((typeobj)) \
+	      ? *(PyObject_GET_WEAKREFS_LISTPTR(op)) = NULL \
+              : NULL), \
+         (op))
 #define PyObject_INIT_VAR(op, typeobj, size) \
 	( (op)->ob_size = (size), PyObject_INIT((op), (typeobj)) )
 
@@ -266,6 +270,12 @@ extern DL_IMPORT(void) _PyGC_Dump(PyGC_Head *);
 
 #endif /* WITH_CYCLE_GC */
 
+/* Test if a type supports weak references */
+#define PyType_SUPPORTS_WEAKREFS(t) ((t)->tp_weaklistoffset > 0)
+
+#define PyObject_GET_WEAKREFS_LISTPTR(o) \
+	((PyObject **) (((char *) (o)) + (o)->ob_type->tp_weaklistoffset))
+
 #ifdef __cplusplus
 }
 #endif

Lib/test/output/test_weakref

+test_weakref
+Basic Weak References
+-- Liveness and referent identity
+-- Reference objects with callbacks
+-- Proxy objects with callbacks
+-- Re-use of weak reference objects
+     reference objects
+     proxy objects
+clearing ref 2
+clearing ref 1
+clearing ref 2
+clearing ref 1
+
+Weak Valued Dictionaries
+objects are stored in weak dict
+weak dict test complete
+
+Non-callable Proxy References
+XXX -- tests not written!
+
+Callable Proxy References

Lib/test/test_weakref.py

+import sys
+import weakref
+
+from test_support import TestFailed, verify
+
+
+class C:
+    pass
+
+
+print "Basic Weak References"
+
+print "-- Liveness and referent identity"
+
+o = C()
+ref = weakref.ref(o)
+verify(ref() is not None, "weak reference to live object should be live")
+o2 = ref()
+verify(ref() is not None, "weak ref should still be live")
+verify(o is o2, "<ref>() should return original object if live")
+del o, o2
+del ref
+
+cbcalled = 0
+def callback(o):
+    global cbcalled
+    cbcalled = 1
+
+o = C()
+ref2 = weakref.ref(o, callback)
+del o
+verify(cbcalled,
+       "callback did not properly set 'cbcalled'")
+verify(ref2() is None,
+       "ref2 should be dead after deleting object reference")
+del ref2
+
+
+print "-- Reference objects with callbacks"
+o = C()
+o.bar = 1
+ref1 = weakref.ref(o, id)
+ref2 = weakref.ref(o, id)
+del o
+verify(ref1() is None,
+       "expected reference to be invalidated")
+verify(ref2() is None,
+       "expected reference to be invalidated")
+
+
+print "-- Proxy objects with callbacks"
+o = C()
+o.bar = 1
+ref1 = weakref.proxy(o, id)
+ref2 = weakref.proxy(o, id)
+del o
+try:
+    ref1.bar
+except weakref.ReferenceError:
+    pass
+else:
+    raise TestFailed("expected ReferenceError exception")
+try:
+    ref2.bar
+except weakref.ReferenceError:
+    pass
+else:
+    raise TestFailed("expected ReferenceError exception")
+
+
+print "-- Re-use of weak reference objects"
+print "     reference objects"
+
+o = C()
+ref1 = weakref.ref(o)
+# create a proxy to make sure that there's an intervening creation
+# between these two; it should make no difference
+proxy = weakref.proxy(o)
+ref2 = weakref.ref(o)
+verify(ref1 is ref2,
+       "reference object w/out callback should have been re-used")
+
+o = C()
+proxy = weakref.proxy(o)
+ref1 = weakref.ref(o)
+ref2 = weakref.ref(o)
+verify(ref1 is ref2,
+       "reference object w/out callback should have been re-used")
+verify(weakref.getweakrefcount(o) == 2,
+       "wrong weak ref count for object")
+del proxy
+verify(weakref.getweakrefcount(o) == 1,
+       "wrong weak ref count for object after deleting proxy")
+
+print "     proxy objects"
+
+o = C()
+ref3 = weakref.proxy(o)
+ref4 = weakref.proxy(o)
+verify(ref3 is ref4,
+       "proxy object w/out callback should have been re-used")
+
+
+def clearing1(r):
+    print "clearing ref 1"
+
+def clearing2(r):
+    print "clearing ref 2"
+
+o = C()
+ref1 = weakref.ref(o, clearing1)
+ref2 = weakref.ref(o, clearing2)
+verify(weakref.getweakrefcount(o) == 2,
+       "got wrong number of weak reference objects")
+del o
+
+o = C()
+ref1 = weakref.ref(o, clearing1)
+ref2 = weakref.ref(o, clearing2)
+del ref1
+verify(weakref.getweakrefs(o) == [ref2],
+       "list of refs does not match")
+del o
+
+o = C()
+ref1 = weakref.ref(o, clearing1)
+ref2 = weakref.ref(o, clearing2)
+del ref2
+verify(weakref.getweakrefs(o) == [ref1],
+       "list of refs does not match")
+del o
+
+print
+print "Weak Valued Dictionaries"
+
+class Object:
+    def __init__(self, arg):
+        self.arg = arg
+    def __repr__(self):
+        return "<Object %r>" % self.arg
+
+dict = weakref.mapping()
+objects = map(Object, range(10))
+for o in objects:
+    dict[o.arg] = o
+print "objects are stored in weak dict"
+for o in objects:
+    verify(weakref.getweakrefcount(o) == 1,
+           "wrong number of weak references to %r!" % o)
+    verify(o is dict[o.arg],
+           "wrong object returned by weak dict!")
+dict.clear()
+print "weak dict test complete"
+
+print
+print "Non-callable Proxy References"
+print "XXX -- tests not written!"
+
+
+def test_proxy(o, proxy):
+    o.foo = 1
+    verify(proxy.foo == 1,
+           "proxy does not reflect attribute addition")
+    o.foo = 2
+    verify(proxy.foo == 2,
+           "proxy does not reflect attribute modification")
+    del o.foo
+    verify(not hasattr(proxy, 'foo'),
+           "proxy does not reflect attribute removal")
+
+    proxy.foo = 1
+    verify(o.foo == 1,
+           "object does not reflect attribute addition via proxy")
+    proxy.foo = 2
+    verify(o.foo == 2,
+           "object does not reflect attribute modification via proxy")
+    del proxy.foo
+    verify(not hasattr(o, 'foo'),
+           "object does not reflect attribute removal via proxy")
+
+
+o = C()
+test_proxy(o, weakref.proxy(o))
+
+print
+print "Callable Proxy References"
+
+class Callable:
+    bar = None
+    def __call__(self, x):
+        self.bar = x
+
+o = Callable()
+ref1 = weakref.proxy(o)
+
+test_proxy(o, ref1)
+
+verify(type(ref1) is weakref.CallableProxyType,
+       "proxy is not of callable type")
+ref1('twinkies!')
+verify(o.bar == 'twinkies!',
+       "call through proxy not passed through to original")
+
+try:
+    ref1()
+except TypeError:
+    # expect due to too few args
+    pass
+else:
+    raise TestFailed("did not catch expected TypeError -- too few args")
+
+try:
+    ref1(1, 2, 3)
+except TypeError:
+    # expect due to too many args
+    pass
+else:
+    raise TestFailed("did not catch expected TypeError -- too many args")

Lib/weakref.py

+"""Weak reference support for Python.
+
+This module is an implementation of PEP 205:
+
+http://python.sourceforge.net/peps/pep-0205.html
+"""
+
+import UserDict
+
+from _weakref import \
+     getweakrefcount, \
+     getweakrefs, \
+     ref, \
+     proxy, \
+     ReferenceError, \
+     CallableProxyType, \
+     ProxyType, \
+     ReferenceType
+
+ProxyTypes = (ProxyType, CallableProxyType)
+
+
+def mapping(dict=None):
+    return WeakDictionary(dict)
+
+
+class WeakDictionary(UserDict.UserDict):
+
+    # We inherit the constructor without worrying about the input
+    # dictionary; since it uses our .update() method, we get the right
+    # checks (if the other dictionary is a WeakDictionary, objects are
+    # unwrapped on the way out, and we always wrap on the way in).
+
+    def __getitem__(self, key):
+        o = self.data.get(key)()
+        if o is None:
+            raise KeyError, key
+        else:
+            return o
+
+    def __repr__(self):
+        return "<WeakDictionary at %s>" % id(self)
+
+    def __setitem__(self, key, value):
+        def remove(o, data=self.data, key=key):
+            del data[key]
+        self.data[key] = ref(value, remove)
+
+    def copy(self):
+        new = WeakDictionary()
+        for key, ref in self.data.items():
+            o = ref()
+            if o is not None:
+                new[key] = o
+
+    def get(self, key, default):
+        try:
+            ref = self.data[key]
+        except KeyError:
+            return default
+        else:
+            o = ref()
+            if o is None:
+                # This should only happen
+                return default
+            else:
+                return o
+
+    def items(self):
+        L = self.data.items()
+        for i in range(len(L)):
+            key, ref = L[i]
+            o = ref()
+            if o is not None:
+                L[i] = key, o
+        return L
+
+    def popitem(self):
+        while 1:
+            key, ref = self.data.popitem()
+            o = ref()
+            if o is not None:
+                return key, o
+
+    def setdefault(self, key, default):
+        try:
+            ref = self.data[key]
+        except KeyError:
+            def remove(o, data=self.data, key=key):
+                del data[key]
+            ref = ref(default, remove)
+            self.data[key] = ref
+            return default
+        else:
+            return ref()
+
+    def update(self, dict):
+        d = self.data
+        L = []
+        for key, o in dict.items():
+            def remove(o, data=d, key=key):
+                del data[key]
+            L.append(key, ref(o, remove))
+        for key, r in L:
+            d[key] = r
+
+    def values(self):
+        L = []
+        for ref in self.data.values():
+            o = ref()
+            if o is not None:
+                L.append(o)
+        return L
+
+
+# no longer needed
+del UserDict

Objects/classobject.c

@@ -515,6 +515,10 @@ instance_dealloc(register PyInstanceObject *inst)
 #ifdef Py_REF_DEBUG
 	extern long _Py_RefTotal;
 #endif
+
+	if (!PyObject_ClearWeakRefs((PyObject *) inst))
+		return;
+
 	/* Temporarily resurrect the object. */
 #ifdef Py_TRACE_REFS
 #ifndef Py_REF_DEBUG
@@ -1771,6 +1775,7 @@ PyTypeObject PyInstance_Type = {
 	(traverseproc)instance_traverse,	/* tp_traverse */
 	0,					/* tp_clear */
 	instance_richcompare,			/* tp_richcompare */
+ 	offsetof(PyInstanceObject, in_weakreflist) /* tp_weaklistoffset */
 };
 
 

Objects/object.c

@@ -100,6 +100,10 @@ PyObject_Init(PyObject *op, PyTypeObject *tp)
 	/* Any changes should be reflected in PyObject_INIT (objimpl.h) */
 	op->ob_type = tp;
 	_Py_NewReference(op);
+	if (PyType_SUPPORTS_WEAKREFS(tp)) {
+		PyObject **weaklist = PyObject_GET_WEAKREFS_LISTPTR(op);
+		*weaklist = NULL;
+	}
 	return op;
 }
 
@@ -119,6 +123,10 @@ PyObject_InitVar(PyVarObject *op, PyTypeObject *tp, int size)
 	op->ob_size = size;
 	op->ob_type = tp;
 	_Py_NewReference((PyObject *)op);
+	if (PyType_SUPPORTS_WEAKREFS(tp)) {
+		PyObject **weaklist = PyObject_GET_WEAKREFS_LISTPTR(op);
+		*weaklist = NULL;
+	}
 	return op;
 }
 
@@ -1458,6 +1466,21 @@ PyObject_Free(void *p)
 }
 
 
+/* Hook to clear up weak references only once the _weakref module is
+   imported.  We use a dummy implementation to simplify the code at each
+   call site instead of requiring a test for NULL.
+*/
+
+static int
+empty_clear_weak_refs(PyObject *o)
+{
+    return 1;
+}
+
+int (*PyObject_ClearWeakRefs)(PyObject *) = empty_clear_weak_refs;
+
+
+
 /* These methods are used to control infinite recursion in repr, str, print,
    etc.  Container objects that may recursively contain themselves,
    e.g. builtin dictionaries and lists, should used Py_ReprEnter() and