diff options
| -rw-r--r-- | Lib/test/test_float.py | 25 | ||||
| -rw-r--r-- | Misc/NEWS | 4 | ||||
| -rw-r--r-- | Objects/floatobject.c | 18 |
3 files changed, 43 insertions, 4 deletions
diff --git a/Lib/test/test_float.py b/Lib/test/test_float.py index b4897c9..b5b6b65 100644 --- a/Lib/test/test_float.py +++ b/Lib/test/test_float.py @@ -240,6 +240,31 @@ class GeneralFloatCases(unittest.TestCase): self.assertTrue(s == s, "{%r} not equal to itself" % f) self.assertTrue(d == d, "{%r : None} not equal to itself" % f) + def assertEqualAndEqualSign(self, a, b): + # fail unless a == b and a and b have the same sign bit; + # the only difference from assertEqual is that this test + # distingishes -0.0 and 0.0. + self.assertEqual((a, copysign(1.0, a)), (b, copysign(1.0, b))) + + @requires_IEEE_754 + def test_float_mod(self): + # Check behaviour of % operator for IEEE 754 special cases. + # In particular, check signs of zeros. + mod = operator.mod + + self.assertEqualAndEqualSign(mod(-1.0, 1.0), 0.0) + self.assertEqualAndEqualSign(mod(-1e-100, 1.0), 1.0) + self.assertEqualAndEqualSign(mod(-0.0, 1.0), 0.0) + self.assertEqualAndEqualSign(mod(0.0, 1.0), 0.0) + self.assertEqualAndEqualSign(mod(1e-100, 1.0), 1e-100) + self.assertEqualAndEqualSign(mod(1.0, 1.0), 0.0) + + self.assertEqualAndEqualSign(mod(-1.0, -1.0), -0.0) + self.assertEqualAndEqualSign(mod(-1e-100, -1.0), -1e-100) + self.assertEqualAndEqualSign(mod(-0.0, -1.0), -0.0) + self.assertEqualAndEqualSign(mod(0.0, -1.0), -0.0) + self.assertEqualAndEqualSign(mod(1e-100, -1.0), -1.0) + self.assertEqualAndEqualSign(mod(1.0, -1.0), -0.0) class FormatFunctionsTestCase(unittest.TestCase): @@ -10,6 +10,10 @@ What's New in Python 3.1.4? Core and Builtins ----------------- +- Issue #10596: Fix float.__mod__ to have the same behaviour as + float.__divmod__ with respect to signed zeros. -4.0 % 4.0 should be + 0.0, not -0.0. + Library ------- diff --git a/Objects/floatobject.c b/Objects/floatobject.c index 865c960..6a2af74 100644 --- a/Objects/floatobject.c +++ b/Objects/floatobject.c @@ -600,10 +600,20 @@ float_rem(PyObject *v, PyObject *w) #endif PyFPE_START_PROTECT("modulo", return 0) mod = fmod(vx, wx); - /* note: checking mod*wx < 0 is incorrect -- underflows to - 0 if wx < sqrt(smallest nonzero double) */ - if (mod && ((wx < 0) != (mod < 0))) { - mod += wx; + if (mod) { + /* ensure the remainder has the same sign as the denominator */ + if ((wx < 0) != (mod < 0)) { + mod += wx; + } + } + else { + /* the remainder is zero, and in the presence of signed zeroes + fmod returns different results across platforms; ensure + it has the same sign as the denominator; we'd like to do + "mod = wx * 0.0", but that may get optimized away */ + mod *= mod; /* hide "mod = +0" from optimizer */ + if (wx < 0.0) + mod = -mod; } PyFPE_END_PROTECT(mod) return PyFloat_FromDouble(mod); |