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[3.15] gh-153200: Fix math.isqrt() for int subclasses with overridden comparison operators (GH-153203) #153223

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Merged
serhiy-storchaka merged 1 commit into from
Jul 6, 2026
+28 −3
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[3.15] gh-153200: Fix math.isqrt() for int subclasses with overridden comparison operators (GH-153203) #153223

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18 changes: 18 additions & 0 deletions Lib/test/test_math_integer.py
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Original file line number Diff line number Diff line change
Expand Up @@ -15,6 +15,19 @@ def __init__(self, value):
def __index__(self):
return self.value

# int subclass with broken arithmetic operators; implementations must
# convert their arguments to exact ints instead of using these.
class BadIntSubclass(int):
def _binop(self, other='ignored', mod=None):
return 42
__add__ = __radd__ = __sub__ = __rsub__ = _binop
__mul__ = __rmul__ = __mod__ = __rmod__ = _binop
__divmod__ = __rdivmod__ = __pow__ = __rpow__ = _binop
__floordiv__ = __rfloordiv__ = _binop
__lshift__ = __rlshift__ = __rshift__ = __rrshift__ = _binop
__and__ = __rand__ = __or__ = __ror__ = __xor__ = __rxor__ = _binop
__lt__ = __le__ = __gt__ = __ge__ = _binop

# Here's a pure Python version of the math.integer.factorial algorithm, for
# documentation and comparison purposes.
#
Expand Down Expand Up @@ -226,6 +239,11 @@ def test_isqrt(self):
self.assertIntEqual(isqrt(False), 0)
self.assertIntEqual(isqrt(MyIndexable(1729)), 41)

# Overridden operators of an int subclass must not affect the
# result.
self.assertIntEqual(isqrt(BadIntSubclass(10**20)), 10**10)
self.assertIntEqual(isqrt(BadIntSubclass(10**20 - 1)), 10**10 - 1)

with self.assertRaises(ValueError):
isqrt(MyIndexable(-3))

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3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/Library/2026-07-06-12-00-00.gh-issue-153200.isqrtLt.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,3 @@
Fix :func:`math.isqrt` returning an incorrect result for arguments not
less than 2**64 that are instances of an :class:`int` subclass with an
overridden comparison operator.
10 changes: 7 additions & 3 deletions Modules/mathintegermodule.c
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Expand Up @@ -454,16 +454,20 @@ math_integer_isqrt(PyObject *module, PyObject *n)
/* The correct result is either a or a - 1. Figure out which, and
decrement a if necessary. */

/* a_too_large = n < a * a */
/* a_too_large = n < a * a. Compare by value: n can be an instance
of an int subclass with an overridden __lt__ method. */
b = PyNumber_Multiply(a, a);
if (b == NULL) {
goto error;
}
a_too_large = PyObject_RichCompareBool(n, b, Py_LT);
PyObject *cmp = PyLong_Type.tp_richcompare(n, b, Py_LT);
Py_DECREF(b);
if (a_too_large == -1) {
if (cmp == NULL) {
goto error;
}
assert(PyBool_Check(cmp));
a_too_large = (cmp == Py_True);
Py_DECREF(cmp);

if (a_too_large) {
Py_SETREF(a, PyNumber_Subtract(a, _PyLong_GetOne()));
Expand Down
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