Add support for weak references to code objects. This will be used by an optimization in the incoming Python 3 JIT.

Patch by Reid Kleckner!

Doc/library/weakref.rst

@@ -59,13 +59,13 @@ Not all objects can be weakly referenced; those objects which can include class
 instances, functions written in Python (but not in C), methods (both bound and
 unbound), sets, frozensets, file objects, :term:`generator`\s, type objects,
 :class:`DBcursor` objects from the :mod:`bsddb` module, sockets, arrays, deques,
-and regular expression pattern objects.
+regular expression pattern objects, and code objects.
 
 .. versionchanged:: 2.4
    Added support for files, sockets, arrays, and patterns.
 
 .. versionchanged:: 2.7
-   Added support for thread.lock and threading.Lock.
+   Added support for thread.lock, threading.Lock, and code objects.
 
 Several built-in types such as :class:`list` and :class:`dict` do not directly
 support weak references but can add support through subclassing::

Include/code.h

@@ -26,6 +26,7 @@ typedef struct {
     PyObject *co_lnotab;	/* string (encoding addr<->lineno mapping) See
 				   Objects/lnotab_notes.txt for details. */
     void *co_zombieframe;     /* for optimization only (see frameobject.c) */
+    PyObject *co_weakreflist;   /* to support weakrefs to code objects */
 } PyCodeObject;
 
 /* Masks for co_flags above */

Lib/test/test_code.py

@@ -81,8 +81,10 @@ consts: ("'doc string'", 'None')
 """
 
 import unittest
+import weakref
 import _testcapi
 
+
 def consts(t):
     """Yield a doctest-safe sequence of object reprs."""
     for elt in t:
@@ -109,12 +111,37 @@ class CodeTest(unittest.TestCase):
         self.assertEquals(co.co_firstlineno, 15)
 
 
+class CodeWeakRefTest(unittest.TestCase):
+
+    def test_basic(self):
+        # Create a code object in a clean environment so that we know we have
+        # the only reference to it left.
+        namespace = {}
+        exec "def f(): pass" in globals(), namespace
+        f = namespace["f"]
+        del namespace
+
+        self.called = False
+        def callback(code):
+            self.called = True
+
+        # f is now the last reference to the function, and through it, the code
+        # object.  While we hold it, check that we can create a weakref and
+        # deref it.  Then delete it, and check that the callback gets called and
+        # the reference dies.
+        coderef = weakref.ref(f.__code__, callback)
+        self.assertTrue(bool(coderef()))
+        del f
+        self.assertFalse(bool(coderef()))
+        self.assertTrue(self.called)
+
+
 def test_main(verbose=None):
     from test.test_support import run_doctest, run_unittest
     from test import test_code
     run_doctest(test_code, verbose)
-    run_unittest(CodeTest)
+    run_unittest(CodeTest, CodeWeakRefTest)
 
 
-if __name__ == '__main__':
+if __name__ == "__main__":
     test_main()

Lib/test/test_sys.py

@@ -552,7 +552,7 @@ class SizeofTest(unittest.TestCase):
         # complex
         check(complex(0,1), size(h + '2d'))
         # code
-        check(get_cell().func_code, size(h + '4i8Pi2P'))
+        check(get_cell().func_code, size(h + '4i8Pi3P'))
         # BaseException
         check(BaseException(), size(h + '3P'))
         # UnicodeEncodeError

Misc/NEWS

@@ -19,6 +19,8 @@ Core and Builtins
   printed and Python exits.  Initialize the GIL before importing the site
   module.
 
+- Code objects now support weak references.
+
 Library
 -------
 

Objects/codeobject.c

@@ -103,6 +103,7 @@ PyCode_New(int argcount, int nlocals, int stacksize, int flags,
 		Py_INCREF(lnotab);
 		co->co_lnotab = lnotab;
                 co->co_zombieframe = NULL;
+		co->co_weakreflist = NULL;
 	}
 	return co;
 }
@@ -314,6 +315,8 @@ code_dealloc(PyCodeObject *co)
 	Py_XDECREF(co->co_lnotab);
         if (co->co_zombieframe != NULL)
                 PyObject_GC_Del(co->co_zombieframe);
+	if (co->co_weakreflist != NULL)
+		PyObject_ClearWeakRefs((PyObject*)co);
 	PyObject_DEL(co);
 }
 
@@ -491,7 +494,7 @@ PyTypeObject PyCode_Type = {
 	0,				/* tp_traverse */
 	0,				/* tp_clear */
 	code_richcompare,		/* tp_richcompare */
-	0,				/* tp_weaklistoffset */
+	offsetof(PyCodeObject, co_weakreflist), /* tp_weaklistoffset */
 	0,				/* tp_iter */
 	0,				/* tp_iternext */
 	0,				/* tp_methods */