Fix of SF bug #475877 (Mutable subtype instances are hashable).

Rather than tweaking the inheritance of type object slots (which turns
out to be too messy to try), this fix adds a __hash__ to the list and
dict types (the only mutable types I'm aware of) that explicitly
raises an error.  This has the advantage that list.__hash__([]) also
raises an error (previously, this would invoke object.__hash__([]),
returning the argument's address); ditto for dict.__hash__.

The disadvantage for this fix is that 3rd party mutable types aren't
automatically fixed.  This should be added to the rules for creating
subclassable extension types: if you don't want your object to be
hashable, add a tp_hash function that raises an exception.

Also, it's possible that I've forgotten about other mutable types for
which this should be done.

1 files changed, 8 insertions, 1 deletions

diff --git a/Objects/dictobject.c b/Objects/dictobject.c
index 019f112..3b922a9 100644
--- a/Objects/dictobject.c
+++ b/Objects/dictobject.c
@@ -1797,6 +1797,13 @@ dict_init(PyObject *self, PyObject *args, PyObject *kwds)
 	return result;
 }
 
+static long
+dict_nohash(PyObject *self)
+{
+	PyErr_SetString(PyExc_TypeError, "dict objects are unhashable");
+	return -1;
+}
+
 static PyObject *
 dict_iter(dictobject *dict)
 {
@@ -1827,7 +1834,7 @@ PyTypeObject PyDict_Type = {
 	0,					/* tp_as_number */
 	&dict_as_sequence,			/* tp_as_sequence */
 	&dict_as_mapping,			/* tp_as_mapping */
-	0,					/* tp_hash */
+	dict_nohash,				/* tp_hash */
 	0,					/* tp_call */
 	0,					/* tp_str */
 	PyObject_GenericGetAttr,		/* tp_getattro */