From ab294199b772e47dd86ca665dbc1080715b844d2 Mon Sep 17 00:00:00 2001
From: Raymond Hettinger <python@rcn.com>
Date: Sat, 13 Aug 2005 02:28:54 +0000
Subject: Teach set modules to correctly compute s-=s and s^=s as the empty
 set.

---
 Lib/sets.py           |  4 ++++
 Lib/test/test_set.py  | 12 ++++++++++++
 Lib/test/test_sets.py | 13 +++++++++++++
 Misc/NEWS             |  4 ++++
 Objects/setobject.c   | 23 +++++++++++++----------
 5 files changed, 46 insertions(+), 10 deletions(-)

diff --git a/Lib/sets.py b/Lib/sets.py
index 8ec7e2f..32a0dd6 100644
--- a/Lib/sets.py
+++ b/Lib/sets.py
@@ -480,6 +480,8 @@ class Set(BaseSet):
         value = True
         if not isinstance(other, BaseSet):
             other = Set(other)
+        if self is other:
+            self.clear()
         for elt in other:
             if elt in data:
                 del data[elt]
@@ -497,6 +499,8 @@ class Set(BaseSet):
         data = self._data
         if not isinstance(other, BaseSet):
             other = Set(other)
+        if self is other:
+            self.clear()
         for elt in ifilter(data.has_key, other):
             del data[elt]
 
diff --git a/Lib/test/test_set.py b/Lib/test/test_set.py
index b4c7c4f..354db78 100644
--- a/Lib/test/test_set.py
+++ b/Lib/test/test_set.py
@@ -382,6 +382,18 @@ class TestSet(TestJointOps):
             else:
                 self.assert_(c not in self.s)
 
+    def test_inplace_on_self(self):
+        t = self.s.copy()
+        t |= t
+        self.assertEqual(t, self.s)
+        t &= t
+        self.assertEqual(t, self.s)
+        t -= t
+        self.assertEqual(t, self.thetype())
+        t = self.s.copy()
+        t ^= t
+        self.assertEqual(t, self.thetype())
+
     def test_weakref(self):
         s = self.thetype('gallahad')
         p = proxy(s)
diff --git a/Lib/test/test_sets.py b/Lib/test/test_sets.py
index c5a48b1..ff834e0 100644
--- a/Lib/test/test_sets.py
+++ b/Lib/test/test_sets.py
@@ -243,6 +243,19 @@ class TestBinaryOps(unittest.TestCase):
         self.assertRaises(TypeError, cmp, a, 12)
         self.assertRaises(TypeError, cmp, "abc", a)
 
+    def test_inplace_on_self(self):
+        t = self.set.copy()
+        t |= t
+        self.assertEqual(t, self.set)
+        t &= t
+        self.assertEqual(t, self.set)
+        t -= t
+        self.assertEqual(len(t), 0)
+        t = self.set.copy()
+        t ^= t
+        self.assertEqual(len(t), 0)
+
+
 #==============================================================================
 
 class TestUpdateOps(unittest.TestCase):
diff --git a/Misc/NEWS b/Misc/NEWS
index 0e37a1e..5d58ce9 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -17,6 +17,8 @@ Core and builtins
   __hash__() function.  Also, changed set.__contains__() to have
   identical logic.
 
+- The set() builtin can now properly compute s-=s as an empty set.
+
 - SF bug #1238681:  freed pointer is used in longobject.c:long_pow().
 
 - SF bug #1185883:  Python's small-object memory allocator took over
@@ -58,6 +60,8 @@ Extension Modules
 Library
 -------
 
+- The sets module can now properly compute s-=s and s^=s as an empty set.
+
 - Patch #827386: Support absolute source paths in msvccompiler.py.
 
 - Fix a problem in Tkinter introduced by SF patch #869468: delete bogus
diff --git a/Objects/setobject.c b/Objects/setobject.c
index e77e7cd..0e65320 100644
--- a/Objects/setobject.c
+++ b/Objects/setobject.c
@@ -193,6 +193,16 @@ frozenset_copy(PySetObject *so)
 PyDoc_STRVAR(copy_doc, "Return a shallow copy of a set.");
 
 static PyObject *
+set_clear(PySetObject *so)
+{
+	PyDict_Clear(so->data);
+	so->hash = -1;
+	Py_RETURN_NONE;
+}
+
+PyDoc_STRVAR(clear_doc, "Remove all elements from this set.");
+
+static PyObject *
 set_union(PySetObject *so, PyObject *other)
 {
 	PySetObject *result;
@@ -379,6 +389,9 @@ static PyObject *
 set_difference_update(PySetObject *so, PyObject *other)
 {
 	PyObject *item, *tgtdata, *it;
+
+	if ((PyObject *)so == other)
+		return set_clear(so);
 	
 	it = PyObject_GetIter(other);
 	if (it == NULL)
@@ -758,16 +771,6 @@ set_tp_print(PySetObject *so, FILE *fp, int flags)
 	return 0;
 }
 
-static PyObject *
-set_clear(PySetObject *so)
-{
-	PyDict_Clear(so->data);
-	so->hash = -1;
-	Py_RETURN_NONE;
-}
-
-PyDoc_STRVAR(clear_doc, "Remove all elements from this set.");
-
 static int
 set_tp_clear(PySetObject *so)
 {
-- 
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