[ Patch #102852 ] Make % error a bit more informative by indicates the

index at which an unknown %-escape was found

Objects/stringobject.c

@@ -2939,8 +2939,9 @@ PyString_Format(PyObject *format, PyObject *args)
 				break;
 			default:
 				PyErr_Format(PyExc_ValueError,
-				"unsupported format character '%c' (0x%x)",
-					c, c);
+				  "unsupported format character '%c' (0x%x) "
+				  "at index %i",
+				  c, c, fmt - 1 - PyString_AsString(format));
 				goto error;
 			}
 			if (sign) {

Objects/unicodeobject.c

@@ -5067,8 +5067,9 @@ PyObject *PyUnicode_Format(PyObject *format,
 
 	    default:
 		PyErr_Format(PyExc_ValueError,
-			     "unsupported format character '%c' (0x%x)",
-			     c, c);
+			     "unsupported format character '%c' (0x%x) "
+			     "at index %i",
+			     c, c, fmt -1 - PyUnicode_AS_UNICODE(uformat));
 		goto onError;
 	    }
 	    if (sign) {