gh-124294: str.format() raises misleading error when using negative indices

Lib/test/test_str.py

@@ -1388,6 +1388,11 @@ def __repr__(self):
         self.assertEqual("{!s}".format(n), 'N(data)')
         self.assertRaises(TypeError, "{}".format, n)
 
+        # String index for sequences
+        index_msg = 'Index must be a sequence of digits'
+        self.assertRaisesRegex(TypeError, index_msg, "{[s]}".format, "abc")
+        self.assertRaisesRegex(TypeError, index_msg, "{[-1]}".format, "abc")
+
     def test_format_map(self):
         self.assertEqual(''.format_map({}), '')
         self.assertEqual('a'.format_map({}), 'a')

Misc/NEWS.d/next/Core_and_Builtins/2024-09-24-11-12-09.gh-issue-124294.jBPOm1.rst

@@ -0,0 +1,2 @@
+More informative error message in :func:`str.format` when passing negative
+integer or string index to sequence.

Objects/stringlib/unicode_format.h

@@ -462,8 +462,21 @@ get_field_object(SubString *input, PyObject *args, PyObject *kwargs,
             tmp = getattr(obj, &name);
         else
             /* getitem lookup "[]" */
-            if (index == -1)
+            if (index == -1) {
+                if (PySequence_Check(obj)) {
+                    /* string index can't be passed to sequence */
+                    PyObject *str = SubString_new_object(&name);
+                    if (str) {
+                        PyErr_Format(PyExc_TypeError,
+                             "Index must be a sequence of digits, string %R "
+                             "is not allowed",
+                             str);
+                    }
+                    Py_XDECREF(str);
+                    goto error;
+                }
                 tmp = getitem_str(obj, &name);
+            }
             else
                 if (PySequence_Check(obj))
                     tmp = getitem_sequence(obj, index);