#4069: aSet.remove(otherSet) would always report the empty frozenset([]) as the missing key.

Now it correctly refers to the initial otherSet.

Backport of r66836.

Lib/test/test_set.py

@@ -382,6 +382,17 @@ class TestSet(TestJointOps):
             else:
                 self.fail()
 
+    def test_remove_keyerror_set(self):
+        key = self.thetype([3, 4])
+        try:
+            self.s.remove(key)
+        except KeyError as e:
+            self.assert_(e.args[0] is key,
+                         "KeyError should be {0}, not {1}".format(key,
+                                                                  e.args[0]))
+        else:
+            self.fail()
+
     def test_discard(self):
         self.s.discard('a')
         self.assert_('a' not in self.s)

Misc/NEWS

@@ -12,6 +12,11 @@ What's New in Python 2.6.1 alpha 1
 Core and Builtins
 -----------------
 
+- Issue #4069: When set.remove(element) is used with a set element, the element
+  is temporarily replaced with an equivalent frozenset.  But the eventual
+  KeyError would always report the empty frozenset([]) as the missing key. Now
+  it correctly refers to the initial element.
+
 - Fixed C99 style comments in several files. Python is now C89 compatible
   again.
 

Objects/setobject.c

@@ -1874,7 +1874,7 @@ PyDoc_STRVAR(contains_doc, "x.__contains__(y) <==> y in x.");
 static PyObject *
 set_remove(PySetObject *so, PyObject *key)
 {
-	PyObject *tmpkey, *result;
+	PyObject *tmpkey;
 	int rv;
 
 	rv = set_discard_key(so, key);
@@ -1886,11 +1886,14 @@ set_remove(PySetObject *so, PyObject *key)
 		if (tmpkey == NULL)
 			return NULL;
 		set_swap_bodies((PySetObject *)tmpkey, (PySetObject *)key);
-		result = set_remove(so, tmpkey);
+		rv = set_discard_key(so, tmpkey);
 		set_swap_bodies((PySetObject *)tmpkey, (PySetObject *)key);
 		Py_DECREF(tmpkey);
-		return result;
+		if (rv == -1)
-	} else if (rv == DISCARD_NOTFOUND) {
+			return NULL;
+	} 
+
+	if (rv == DISCARD_NOTFOUND) {
 		set_key_error(key);
 		return NULL;
 	}