Merged revisions 78444 via svnmerge from

svn+ssh://pythondev@svn.python.org/python/trunk

........
  r78444 | eric.smith | 2010-02-25 09:26:33 -0500 (Thu, 25 Feb 2010) | 1 line

  Issue #7928: Document str.format element_index better.
........

Doc/library/string.rst

@@ -223,7 +223,8 @@ The grammar for a replacement field is as follows:
       replacement_field: "{" `field_name` ["!" `conversion`] [":" `format_spec`] "}"
       field_name: (`identifier` | `integer`) ("." `attribute_name` | "[" `element_index` "]")*
       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>