Issue #7928: Document str.format element_index better.

diff --git a/Doc/library/string.rst b/Doc/library/string.rst
index 9352666a0cf47bbaa8e2c4ea715988c60f24008f..731484813a071ec1587a9742ad68c74e24b7ca61 100644 (file)
--- a/Doc/library/string.rst
+++ b/Doc/library/string.rst
@@ -226,7 +226,8 @@ The grammar for a replacement field is as follows:
       field_name: arg_name ("." `attribute_name` | "[" `element_index` "]")*
       arg_name: (`identifier` | `integer`)?
       attribute_name: `identifier`
-      element_index: `integer`
+      element_index: `integer` | `index_string`
+      index_string: <any source character except "]"> +
       conversion: "r" | "s"
       format_spec: <described in the next section>