From dbb53d99189075522eecc87723292c85aee15388 Mon Sep 17 00:00:00 2001
From: Guido van Rossum <guido@python.org>
Date: Mon, 3 Dec 2001 16:32:18 +0000
Subject: 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.
---
 Lib/test/test_descr.py | 24 ++++++++++++++++++++++++
 Objects/dictobject.c   |  9 ++++++++-
 Objects/listobject.c   | 11 +++++++++--
 3 files changed, 41 insertions(+), 3 deletions(-)

diff --git a/Lib/test/test_descr.py b/Lib/test/test_descr.py
index f3b81ac..d76013e 100644
--- a/Lib/test/test_descr.py
+++ b/Lib/test/test_descr.py
@@ -2571,6 +2571,29 @@ def delhook():
     del c
     vereq(log, [1])
 
+def hashinherit():
+    if verbose: print "Testing hash of mutable subclasses..."
+
+    class mydict(dict):
+        pass
+    d = mydict()
+    try:
+        hash(d)
+    except TypeError:
+        pass
+    else:
+        raise TestFailed, "hash() of dict subclass should fail"
+
+    class mylist(list):
+        pass
+    d = mylist()
+    try:
+        hash(d)
+    except TypeError:
+        pass
+    else:
+        raise TestFailed, "hash() of list subclass should fail"
+
 def test_main():
     class_docstrings()
     lists()
@@ -2623,6 +2646,7 @@ def test_main():
     str_of_str_subclass()
     kwdargs()
     delhook()
+    hashinherit()
     if verbose: print "All OK"
 
 if __name__ == "__main__":
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 */
diff --git a/Objects/listobject.c b/Objects/listobject.c
index b05fe27..dbbc4a9 100644
--- a/Objects/listobject.c
+++ b/Objects/listobject.c
@@ -1617,6 +1617,13 @@ list_init(PyListObject *self, PyObject *args, PyObject *kw)
 	return 0;
 }
 
+static long
+list_nohash(PyObject *self)
+{
+	PyErr_SetString(PyExc_TypeError, "list objects are unhashable");
+	return -1;
+}
+
 static char append_doc[] =
 "L.append(object) -- append object to end";
 static char extend_doc[] =
@@ -1681,7 +1688,7 @@ PyTypeObject PyList_Type = {
 	0,					/* tp_as_number */
 	&list_as_sequence,			/* tp_as_sequence */
 	0,					/* tp_as_mapping */
-	0,					/* tp_hash */
+	list_nohash,				/* tp_hash */
 	0,					/* tp_call */
 	0,					/* tp_str */
 	PyObject_GenericGetAttr,		/* tp_getattro */
@@ -1771,7 +1778,7 @@ static PyTypeObject immutable_list_type = {
 	0,					/* tp_as_number */
 	&immutable_list_as_sequence,		/* tp_as_sequence */
 	0,					/* tp_as_mapping */
-	0,					/* tp_hash */
+	list_nohash,				/* tp_hash */
 	0,					/* tp_call */
 	0,					/* tp_str */
 	PyObject_GenericGetAttr,		/* tp_getattro */
-- 
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