Issue #18739: Fix inconsistent results from math.log(n) and math.log(long(n))
diff --git a/Lib/test/test_math.py b/Lib/test/test_math.py
index ac4475e..a8677ce 100644
--- a/Lib/test/test_math.py
+++ b/Lib/test/test_math.py
@@ -599,6 +599,9 @@
self.assertEqual(math.log(INF), INF)
self.assertRaises(ValueError, math.log, NINF)
self.assertTrue(math.isnan(math.log(NAN)))
+ # Log values should match for int and long (issue #18739).
+ for n in range(1, 1000):
+ self.assertEqual(math.log(n), math.log(long(n)))
def testLog1p(self):
self.assertRaises(TypeError, math.log1p)
@@ -621,6 +624,9 @@
self.assertEqual(math.log(INF), INF)
self.assertRaises(ValueError, math.log10, NINF)
self.assertTrue(math.isnan(math.log10(NAN)))
+ # Log values should match for int and long (issue #18739).
+ for n in range(1, 1000):
+ self.assertEqual(math.log10(n), math.log10(long(n)))
def testModf(self):
self.assertRaises(TypeError, math.modf)
diff --git a/Misc/NEWS b/Misc/NEWS
index b7402e1..1500b15 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -9,6 +9,9 @@
Core and Builtins
-----------------
+- Issue #18739: Fix an inconsistency between math.log(n) and math.log(long(n));
+ the results could be off from one another by a ulp or two.
+
- Issue #13461: Fix a crash in the "replace" error handler on 64-bit platforms.
Patch by Yogesh Chaudhari.
diff --git a/Modules/mathmodule.c b/Modules/mathmodule.c
index cd74b0d..845fb9b 100644
--- a/Modules/mathmodule.c
+++ b/Modules/mathmodule.c
@@ -1277,23 +1277,33 @@
{
/* If it is long, do it ourselves. */
if (PyLong_Check(arg)) {
- double x;
+ double x, result;
Py_ssize_t e;
- x = _PyLong_Frexp((PyLongObject *)arg, &e);
- if (x == -1.0 && PyErr_Occurred())
- return NULL;
- if (x <= 0.0) {
+
+ /* Negative or zero inputs give a ValueError. */
+ if (Py_SIZE(arg) <= 0) {
PyErr_SetString(PyExc_ValueError,
"math domain error");
return NULL;
}
- /* Special case for log(1), to make sure we get an
- exact result there. */
- if (e == 1 && x == 0.5)
- return PyFloat_FromDouble(0.0);
- /* Value is ~= x * 2**e, so the log ~= log(x) + log(2) * e. */
- x = func(x) + func(2.0) * e;
- return PyFloat_FromDouble(x);
+
+ x = PyLong_AsDouble(arg);
+ if (x == -1.0 && PyErr_Occurred()) {
+ if (!PyErr_ExceptionMatches(PyExc_OverflowError))
+ return NULL;
+ /* Here the conversion to double overflowed, but it's possible
+ to compute the log anyway. Clear the exception and continue. */
+ PyErr_Clear();
+ x = _PyLong_Frexp((PyLongObject *)arg, &e);
+ if (x == -1.0 && PyErr_Occurred())
+ return NULL;
+ /* Value is ~= x * 2**e, so the log ~= log(x) + log(2) * e. */
+ result = func(x) + func(2.0) * e;
+ }
+ else
+ /* Successfully converted x to a double. */
+ result = func(x);
+ return PyFloat_FromDouble(result);
}
/* Else let libm handle it by itself. */