Wrote down the invariants of some common objects whose structure is

exposed in header files.  Fixed a few comments in these headers.

As we might have expected, writing down invariants systematically exposed a
(minor) bug.  In this case, function objects have a writeable func_code
attribute, which could be set to code objects with the wrong number of
free variables.  Calling the resulting function segfaulted the interpreter.
Added a corresponding test.

1 files changed, 2 insertions, 2 deletions

diff --git a/Include/sliceobject.h b/Include/sliceobject.h
index 1fb123a..fc80254 100644
--- a/Include/sliceobject.h
+++ b/Include/sliceobject.h
@@ -16,12 +16,12 @@ PyAPI_DATA(PyObject) _Py_EllipsisObject; /* Don't use this directly */
 
 A slice object containing start, stop, and step data members (the
 names are from range).  After much talk with Guido, it was decided to
-let these be any arbitrary python type. 
+let these be any arbitrary python type.  Py_None stands for omitted values.
 */
 
 typedef struct {
     PyObject_HEAD
-    PyObject *start, *stop, *step;
+    PyObject *start, *stop, *step;	/* not NULL */
 } PySliceObject;
 
 PyAPI_DATA(PyTypeObject) PySlice_Type;