x**2 should about equal x*x (including for a float x such that the result is

inf) but didn't; added a test to test_float to verify that, and ignored the
ERANGE value for errno in the pow operation to make the new test pass (with
help from Marilyn Davis at the Google Python Sprint -- thanks!).

Lib/test/test_float.py

@@ -99,12 +99,25 @@ def test_float_specials_do_unpack(self):
                               ('<f', LE_FLOAT_NAN)]:
                 struct.unpack(fmt, data)
 
+# on an IEEE platform, "overflowing" operations produce infinity
+
+class IEEEOperationsTestCase(unittest.TestCase):
+    if float.__getformat__("double").startswith("IEEE"):
+        def test_double_infinity(self):
+            big = 4.8e159
+            pro = big*big
+            self.assertEquals(repr(pro), 'inf')
+            sqr = big**2
+            self.assertEquals(repr(sqr), 'inf')
+
 
 def test_main():
     test_support.run_unittest(
         FormatFunctionsTestCase,
         UnknownFormatTestCase,
-        IEEEFormatTestCase)
+        IEEEFormatTestCase,
+        IEEEOperationsTestCase,
+        )
 
 if __name__ == '__main__':
     test_main()

Objects/floatobject.c

@@ -821,12 +821,12 @@ float_pow(PyObject *v, PyObject *w, PyObject *z)
 	ix = pow(iv, iw);
 	PyFPE_END_PROTECT(ix)
 	Py_ADJUST_ERANGE1(ix);
-	if (errno != 0) {
+        /* we need to ignore ERANGE here and just return inf */
+	if (errno != 0 && errno != ERANGE) {
 		/* We don't expect any errno value other than ERANGE, but
 		 * the range of libm bugs appears unbounded.
 		 */
-		PyErr_SetFromErrno(errno == ERANGE ? PyExc_OverflowError :
-						     PyExc_ValueError);
+		PyErr_SetFromErrno(PyExc_ValueError);
 		return NULL;
 	}
 	return PyFloat_FromDouble(ix);