SF bug 705231: Assertion failed, python aborts.

float_pow():  Don't let the platform pow() raise -1.0 to an integer power
anymore; at least glibc gets it wrong in some cases.  Note that
math.pow() will continue to deliver wrong (but platform-native) results
in such cases.

Lib/test/test_pow.py

@@ -101,6 +101,23 @@ class PowTest(unittest.TestCase):
                 return None
         None ** TestRpow() # Won't fail when __rpow__ invoked.  SF bug #643260.
 
+    def test_bug705231(self):
+        # -1.0 raised to an integer should never blow up.  It did if the
+        # platform pow() was buggy, and Python didn't worm around it.
+        eq = self.assertEquals
+        a = -1.0
+        eq(pow(a, 1.23e167), 1.0)
+        eq(pow(a, -1.23e167), 1.0)
+        for b in range(-10, 11):
+            eq(pow(a, float(b)), b & 1 and -1.0 or 1.0)
+        for n in range(0, 100):
+            fiveto = float(5 ** n)
+            # For small n, fiveto will be odd.  Eventually we run out of
+            # mantissa bits, though, and thereafer fiveto will be even.
+            expected = fiveto % 2.0 and -1.0 or 1.0
+            eq(pow(a, fiveto), expected)
+            eq(pow(a, -fiveto), expected)
+        eq(expected, 1.0)   # else we didn't push fiveto to evenness
 
 def test_main():
     test.test_support.run_unittest(PowTest)

Misc/NEWS

@@ -12,6 +12,12 @@ What's New in Python 2.3 beta 2?
 Core and builtins
 -----------------
 
+- SF bug 705231:  builtin pow() no longer lets the platform C pow()
+  raise -1.0 to integer powers, because (at least) glibc gets it wrong
+  in some cases.  The result should be -1.0 if the power is odd and 1.0
+  if the power is even, and any float with a sufficiently large exponent
+  is (mathematically) an exact even integer.
+
 - The encoding attribute has been added for file objects, and set to
   the terminal encoding on Unix and Windows.
 

Objects/floatobject.c

@@ -572,10 +572,39 @@ float_pow(PyObject *v, PyObject *w, PyObject *z)
 		}
 		return PyFloat_FromDouble(0.0);
 	}
-	if (iv < 0.0 && iw != floor(iw)) {
-		PyErr_SetString(PyExc_ValueError,
-				"negative number cannot be raised to a fractional power");
-		return NULL;
+	if (iv < 0.0) {
+		/* Whether this is an error is a mess, and bumps into libm
+		 * bugs so we have to figure it out ourselves.
+		 */
+		if (iw != floor(iw)) {
+			PyErr_SetString(PyExc_ValueError, "negative number "
+				"cannot be raised to a fractional power");
+			return NULL;
+		}
+		/* iw is an exact integer, albeit perhaps a very large one.
+		 * -1 raised to an exact integer should never be exceptional.
+		 * Alas, some libms (chiefly glibc as of early 2003) return
+		 * NaN and set EDOM on pow(-1, large_int) if the int doesn't
+		 * happen to be representable in a *C* integer.  That's a
+		 * bug; we let that slide in math.pow() (which currently
+		 * reflects all platform accidents), but not for Python's **.
+		 */
+		 if (iv == -1.0 && !Py_IS_INFINITY(iw) && iw == iw) {
+		 	/* XXX the "iw == iw" was to weed out NaNs.  This
+		 	 * XXX doesn't actually work on all platforms.
+		 	 */
+		 	/* Return 1 if iw is even, -1 if iw is odd; there's
+		 	 * no guarantee that any C integral type is big
+		 	 * enough to hold iw, so we have to check this
+		 	 * indirectly.
+		 	 */
+		 	ix = floor(iw * 0.5) * 2.0;
+			return PyFloat_FromDouble(ix == iw ? 1.0 : -1.0);
+		}
+		/* Else iv != -1.0, and overflow or underflow are possible.
+		 * Unless we're to write pow() ourselves, we have to trust
+		 * the platform to do this correctly.
+		 */
 	}
 	errno = 0;
 	PyFPE_START_PROTECT("pow", return NULL)
@@ -583,8 +612,11 @@ float_pow(PyObject *v, PyObject *w, PyObject *z)
 	PyFPE_END_PROTECT(ix)
 	Py_ADJUST_ERANGE1(ix);
 	if (errno != 0) {
-		assert(errno == ERANGE);
-		PyErr_SetFromErrno(PyExc_OverflowError);
+		/* 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);
 		return NULL;
 	}
 	return PyFloat_FromDouble(ix);