python · kristjanvalur · Jan 6, 2024 · Jan 6, 2024 · Jan 6, 2024 · Jan 7, 2024
diff --git a/Lib/test/list_tests.py b/Lib/test/list_tests.py
@@ -380,6 +380,27 @@ def __eq__(self, other):
         # This used to seg fault before patch #1005778
         self.assertRaises(ValueError, a.index, None)
 
+    def test_index_key(self):
+        data = [(i, chr(i)) for i in range(100)]
+        idx = data.index("a", key=lambda i:i[1])
+        self.assertEqual(data[idx][1], "a")
+
+        with self.assertRaises(ValueError):
+            data.index("foo", key=lambda i:i[1])
+
+        # verify that errors propagate
+        with self.assertRaises(ZeroDivisionError):
+            data.index("a", key=lambda i: 1/0)
+
+        # verify that key method can modify the list without crashing
+        def evil_key(i):
+            del data[:]
+            return i[1]
+
+        with self.assertRaises(ValueError):
+            idx = data.index("a", key=evil_key)
+        self.assertEqual(data, [])
+
     def test_reverse(self):
         u = self.type2test([-2, -1, 0, 1, 2])
         u2 = u[:]

diff --git a/Lib/test/test_call.py b/Lib/test/test_call.py
@@ -98,10 +98,6 @@ def test_varargs3_kw(self):
         msg = r"bool\(\) takes no keyword arguments"
         self.assertRaisesRegex(TypeError, msg, bool, x=2)
 
-    def test_varargs4_kw(self):
-        msg = r"^(list[.])?index\(\) takes no keyword arguments$"
-        self.assertRaisesRegex(TypeError, msg, [].index, x=2)
-
     def test_varargs5_kw(self):
         msg = r"^hasattr\(\) takes no keyword arguments$"
         self.assertRaisesRegex(TypeError, msg, hasattr, x=2)

diff --git a/Misc/NEWS.d/next/Core and Builtins/2024-01-07-12-53-28.gh-issue-113773.uUjnBu.rst b/Misc/NEWS.d/next/Core and Builtins/2024-01-07-12-53-28.gh-issue-113773.uUjnBu.rst
@@ -0,0 +1 @@
+:func:`list.index` now supports the *key* keyword-only argument. When provided, each list item is first transformed by it before comparint it with *value*.
diff --git a/Objects/clinic/listobject.c.h b/Objects/clinic/listobject.c.h
diff --git a/Objects/listobject.c b/Objects/listobject.c
@@ -2659,38 +2659,56 @@ list.index
     start: slice_index(accept={int}) = 0
     stop: slice_index(accept={int}, c_default="PY_SSIZE_T_MAX") = sys.maxsize
     /
+    *
+    key: object=NULL
 
 Return first index of value.
 
 Raises ValueError if the value is not present.
+A callable 'key' can be provided to transform each item before comparison with 'value'.
 [clinic start generated code]*/
 
 static PyObject *
 list_index_impl(PyListObject *self, PyObject *value, Py_ssize_t start,
-                Py_ssize_t stop)
-/*[clinic end generated code: output=ec51b88787e4e481 input=40ec5826303a0eb1]*/
+                Py_ssize_t stop, PyObject *key)
+/*[clinic end generated code: output=23a18266d9a3a9b3 input=7115b8d01b704c54]*/
 {
     Py_ssize_t i;
 
     if (start < 0) {
         start += Py_SIZE(self);
-        if (start < 0)
+        if (start < 0) {
             start = 0;
+        }
     }
     if (stop < 0) {
         stop += Py_SIZE(self);
-        if (stop < 0)
+        if (stop < 0) {
             stop = 0;
+        }
     }
     for (i = start; i < stop && i < Py_SIZE(self); i++) {
-        PyObject *obj = self->ob_item[i];
-        Py_INCREF(obj);
+        PyObject *obj, *item = self->ob_item[i];
+        /* take reference to item, in case list modified by key or comparison */
+        Py_INCREF(item);
+        if (key == NULL) {
+            obj = item;
+        }
+        else {
+            obj = PyObject_CallFunctionObjArgs(key, item, NULL);
+            Py_DECREF(item);
+            if (obj == NULL) {
+                return NULL;
+            }
+        }
         int cmp = PyObject_RichCompareBool(obj, value, Py_EQ);
         Py_DECREF(obj);
-        if (cmp > 0)
+        if (cmp > 0) {
             return PyLong_FromSsize_t(i);
-        else if (cmp < 0)
+        }
+        else if (cmp < 0) {
             return NULL;
+        }
     }
     PyErr_Format(PyExc_ValueError, "%R is not in list", value);
     return NULL;
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1 @@
		:func:`list.index` now supports the key keyword-only argument. When provided, each list item is first transformed by it before comparint it with value.