gh-145300: Add __length_hint__ for itertools.islice

Lib/test/test_itertools.py

@@ -1213,6 +1213,35 @@ def __index__(self):
         self.assertEqual(list(islice(range(100), IntLike(10), IntLike(50), IntLike(5))),
                          list(range(10,50,5)))
 
+        # Test __length_hint__
+        self.assertEqual(islice(range(10), 1).__length_hint__(), 1)
+        self.assertEqual(islice(range(10), 1, 2).__length_hint__(), 1)
+        self.assertEqual(islice(range(10), 1, 2, 2).__length_hint__(), 1)
+        self.assertEqual(islice(range(10), 0, 4, 2).__length_hint__(), 2)
+        self.assertEqual(islice(range(10), 2, None, 1).__length_hint__(), 8)
+        self.assertEqual(islice(range(10), 2, None, 2).__length_hint__(), 4)
+        self.assertEqual(islice(range(10), 3, None, 2).__length_hint__(), 4)
+        self.assertEqual(islice(range(10), 2, 2, 1).__length_hint__(), 0)
+
+        it = islice(iter(range(5)), None)
+        list(it)  # exhaust
+        self.assertEqual(it.__length_hint__(), 0)
+
+        it = islice(iter(range(5)), 3)
+        list(it)  # exhaust
+        self.assertEqual(it.__length_hint__(), 0)
+
+        it = islice(iter(range(10)), 2, None, 2)
+        self.assertEqual(it.__length_hint__(), 4)  # indices 2, 4, 6, 8
+        next(it)  # yields 2
+        self.assertEqual(it.__length_hint__(), 3)  # indices 4, 6, 8
+        next(it)  # yields 4
+        self.assertEqual(it.__length_hint__(), 2)  # indices 6, 8
+        next(it)  # yields 6
+        self.assertEqual(it.__length_hint__(), 1)  # index 8
+        next(it)  # yields 8
+        self.assertEqual(it.__length_hint__(), 0)
+
     def test_takewhile(self):
         data = [1, 3, 5, 20, 2, 4, 6, 8]
         self.assertEqual(list(takewhile(underten, data)), [1, 3, 5])

Misc/NEWS.d/next/Library/2026-02-28-09-23-41.gh-issue-145300.JlCq0D.rst

@@ -0,0 +1 @@
+Add ``__length_hint__`` to :class:`itertools.islice`.

Modules/itertoolsmodule.c

@@ -1660,6 +1660,35 @@ islice_next(PyObject *op)
     return NULL;
 }
 
+static PyObject *
+islice_length_hint(PyObject *op, PyObject *Py_UNUSED(dummy))
+{
+    isliceobject *lz = isliceobject_CAST(op);
+
+    if (lz->it == NULL || (lz->stop >= 0 && lz->stop <= lz->next)) {
+        return PyLong_FromSsize_t(0);
+    }
+
+    Py_ssize_t remaining;
+    if (lz->stop == -1) {
+        Py_ssize_t hint = PyObject_LengthHint(lz->it, 0);
+        if (hint < 0) {
+            /* propagate exception */
+            return NULL;
+        }
+        remaining = hint - (lz->next - lz->cnt);
+    } else {
+        remaining = lz->stop - lz->next;
+    }
+
+    if (remaining <= 0) {
+        return PyLong_FromSsize_t(0);
+    }
+
+    Py_ssize_t steps = 1 + (remaining - 1) / lz->step;
+    return PyLong_FromSsize_t(steps);
+}
+
 PyDoc_STRVAR(islice_doc,
 "islice(iterable, stop) --> islice object\n\
 islice(iterable, start, stop[, step]) --> islice object\n\
@@ -1671,6 +1700,11 @@ specified as another value, step determines how many values are\n\
 skipped between successive calls.  Works like a slice() on a list\n\
 but returns an iterator.");
 
+static PyMethodDef islice_methods[] = {
+    {"__length_hint__", islice_length_hint, METH_NOARGS, NULL},
+    {NULL, NULL},
+};
+
 static PyType_Slot islice_slots[] = {
     {Py_tp_dealloc, islice_dealloc},
     {Py_tp_getattro, PyObject_GenericGetAttr},
@@ -1680,6 +1714,7 @@ static PyType_Slot islice_slots[] = {
     {Py_tp_iternext, islice_next},
     {Py_tp_new, islice_new},
     {Py_tp_free, PyObject_GC_Del},
+    {Py_tp_methods, islice_methods},
     {0, NULL},
 };