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gh-120950: Fix overflow in math.log() with large int-like argument #121011

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Merged
serhiy-storchaka merged 7 commits into from
Nov 11, 2025
Merged

gh-120950: Fix overflow in math.log() with large int-like argument #121011

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ca00e3b
gh-120950: Fix overflow in math.log() with large int-like argument
serhiy-storchaka Jun 25, 2024
6d10cc6
Add a NEWS entry and more tests.
serhiy-storchaka Jun 26, 2024
48f3e27
Merge branch 'main' into math-log-int-like-overflow
serhiy-storchaka Jul 13, 2024
1d7449c
Add tests for types that have both `__float__` and `__index__`.
serhiy-storchaka Jul 13, 2024
c0d903f
Merge branch 'main' into math-log-int-like-overflow
serhiy-storchaka Jul 25, 2024
1b491d5
Merge branch 'main' into math-log-int-like-overflow
serhiy-storchaka May 4, 2025
4c37a25
Merge branch 'main' into math-log-int-like-overflow
serhiy-storchaka Nov 11, 2025
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33 changes: 33 additions & 0 deletions Lib/test/test_math.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1208,8 +1208,23 @@ def testLog(self):
self.ftest('log(10**40, 10**20)', math.log(10**40, 10**20), 2)
self.ftest('log(10**1000)', math.log(10**1000),
2302.5850929940457)
self.ftest('log(10**2000, 10**1000)', math.log(10**2000, 10**1000), 2)
self.ftest('log(MyIndexable(32), MyIndexable(2))',
math.log(MyIndexable(32), MyIndexable(2)), 5)
self.ftest('log(MyIndexable(10**1000))',
math.log(MyIndexable(10**1000)),
2302.5850929940457)
self.ftest('log(MyIndexable(10**2000), MyIndexable(10**1000))',
math.log(MyIndexable(10**2000), MyIndexable(10**1000)),
2)
self.assertRaises(ValueError, math.log, 0.0)
self.assertRaises(ValueError, math.log, 0)
self.assertRaises(ValueError, math.log, MyIndexable(0))
self.assertRaises(ValueError, math.log, -1.5)
self.assertRaises(ValueError, math.log, -1)
self.assertRaises(ValueError, math.log, MyIndexable(-1))
self.assertRaises(ValueError, math.log, -10**1000)
self.assertRaises(ValueError, math.log, MyIndexable(-10**1000))
self.assertRaises(ValueError, math.log, 10, -10)
self.assertRaises(ValueError, math.log, NINF)
self.assertEqual(math.log(INF), INF)
Expand All @@ -1230,13 +1245,22 @@ def testLog2(self):
self.assertEqual(math.log2(1), 0.0)
self.assertEqual(math.log2(2), 1.0)
self.assertEqual(math.log2(4), 2.0)
self.assertEqual(math.log2(MyIndexable(4)), 2.0)

# Large integer values
self.assertEqual(math.log2(2**1023), 1023.0)
self.assertEqual(math.log2(2**1024), 1024.0)
self.assertEqual(math.log2(2**2000), 2000.0)
self.assertEqual(math.log2(MyIndexable(2**2000)), 2000.0)

self.assertRaises(ValueError, math.log2, 0.0)
self.assertRaises(ValueError, math.log2, 0)
self.assertRaises(ValueError, math.log2, MyIndexable(0))
self.assertRaises(ValueError, math.log2, -1.5)
self.assertRaises(ValueError, math.log2, -1)
self.assertRaises(ValueError, math.log2, MyIndexable(-1))
self.assertRaises(ValueError, math.log2, -2**2000)
self.assertRaises(ValueError, math.log2, MyIndexable(-2**2000))
self.assertRaises(ValueError, math.log2, NINF)
self.assertTrue(math.isnan(math.log2(NAN)))

Expand All @@ -1255,8 +1279,17 @@ def testLog10(self):
self.ftest('log10(1)', math.log10(1), 0)
self.ftest('log10(10)', math.log10(10), 1)
self.ftest('log10(10**1000)', math.log10(10**1000), 1000.0)
self.ftest('log10(MyIndexable(10))', math.log10(MyIndexable(10)), 1)
self.ftest('log10(MyIndexable(10**1000))',
math.log10(MyIndexable(10**1000)), 1000.0)
self.assertRaises(ValueError, math.log10, 0.0)
self.assertRaises(ValueError, math.log10, 0)
self.assertRaises(ValueError, math.log10, MyIndexable(0))
self.assertRaises(ValueError, math.log10, -1.5)
self.assertRaises(ValueError, math.log10, -1)
self.assertRaises(ValueError, math.log10, MyIndexable(-1))
self.assertRaises(ValueError, math.log10, -10**1000)
self.assertRaises(ValueError, math.log10, MyIndexable(-10**1000))
self.assertRaises(ValueError, math.log10, NINF)
self.assertEqual(math.log(INF), INF)
self.assertTrue(math.isnan(math.log10(NAN)))
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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Library/2024-06-26-16-16-43.gh-issue-121011.qW54eh.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
:func:`math.log` now supports arbitrary large integer-like arguments in the
same way as arbitrary large integer arguments.
78 changes: 50 additions & 28 deletions Modules/mathmodule.c
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Original file line number Diff line number Diff line change
Expand Up @@ -2217,41 +2217,63 @@ math_modf_impl(PyObject *module, double x)
in that int is larger than PY_SSIZE_T_MAX. */

static PyObject*
loghelper(PyObject* arg, double (*func)(double))
loghelper_int(PyObject* arg, double (*func)(double))
{
/* If it is int, do it ourselves. */
if (PyLong_Check(arg)) {
double x, result;
Py_ssize_t e;
double x, result;
Py_ssize_t e;

/* Negative or zero inputs give a ValueError. */
if (!_PyLong_IsPositive((PyLongObject *)arg)) {
PyErr_SetString(PyExc_ValueError,
"math domain error");
return NULL;
}
/* Negative or zero inputs give a ValueError. */
if (!_PyLong_IsPositive((PyLongObject *)arg)) {
PyErr_SetString(PyExc_ValueError,
"math domain error");
return NULL;
}

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);
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);
}

static PyObject*
loghelper(PyObject* arg, double (*func)(double))
{
/* If it is int, do it ourselves. */
if (PyLong_Check(arg)) {
return loghelper_int(arg, func);
}
/* Else let libm handle it by itself. */
return math_1(arg, func, 0);
PyObject *res = math_1(arg, func, 0);
if (res == NULL &&
PyErr_ExceptionMatches(PyExc_OverflowError) &&
PyIndex_Check(arg))
{
/* Here the conversion to double overflowed, but it's possible
to compute the log anyway. Clear the exception, convert to
integer and continue. */
PyErr_Clear();
arg = _PyNumber_Index(arg);
if (arg == NULL) {
return NULL;
}
res = loghelper_int(arg, func);
Py_DECREF(arg);
}
return res;
}


Expand Down