Issue #1445: Fix a SystemError when accessing the ``cell_contents``

attribute of an empty cell object.  Now a ValueError is raised.

Backport of r59170.

Lib/test/test_funcattrs.py

@@ -242,6 +242,17 @@ def test_func_closure():
     verify(c[0].__class__.__name__ == "cell") # don't have a type object handy
     cantset(f, "func_closure", c)
 
+def test_empty_cell():
+    def f(): print a
+    try:
+        f.func_closure[0].cell_contents
+    except ValueError:
+        pass
+    else:
+        raise TestFailed, "shouldn't be able to read an empty cell"
+
+    a = 12
+
 def test_func_doc():
     def f(): pass
     verify(f.__doc__ is None)
@@ -385,6 +396,7 @@ def test_im_name():
 
 def testmore():
     test_func_closure()
+    test_empty_cell()
     test_func_doc()
     test_func_globals()
     test_func_name()

Misc/NEWS

-+++++++++++
++++++++++++
 Python News
 +++++++++++
 
@@ -12,6 +12,9 @@ What's New in Python 2.5.2c1?
 Core and builtins
 -----------------
 
+- Issue #1445: Fix a SystemError when accessing the ``cell_contents`` 
+  attribute of an empty cell object.
+
 - Issue #1265: Fix a problem with sys.settrace, if the tracing function uses a
   generator expression when at the same time the executed code is closing a
   paused generator.

Objects/cellobject.c

@@ -89,7 +89,12 @@ cell_clear(PyCellObject *op)
 static PyObject *
 cell_get_contents(PyCellObject *op, void *closure)
 {
-	Py_XINCREF(op->ob_ref);
+	if (op->ob_ref == NULL)
+	{
+		PyErr_SetString(PyExc_ValueError, "Cell is empty");
+		return NULL;
+	}
+	Py_INCREF(op->ob_ref);
 	return op->ob_ref;
 }