Document that format string don’t support arbitrary dictonary keys.

Text adapted from the PEP.  Addition requested by Terry J. Reedy on
2011-02-23 on python-dev.

diff --git a/Doc/library/string.rst b/Doc/library/string.rst
index 2443180f315fcc2ab1b329a0bfe07cf17a437a56..78f2b4ded36b540aadfb6c85b74ce0f71b0995fb 100644 (file)
--- a/Doc/library/string.rst
+++ b/Doc/library/string.rst
@@ -216,6 +216,8 @@ keyword.  If it's a number, it refers to a positional argument, and if it's a ke
 it refers to a named keyword argument.  If the numerical arg_names in a format string
 are 0, 1, 2, ... in sequence, they can all be omitted (not just some)
 and the numbers 0, 1, 2, ... will be automatically inserted in that order.
+Because *arg_name* is not quote-delimited, it is not possible to specify arbitrary
+dictionary keys (e.g., the strings ``'10'`` or ``':-]'``) within a format string.
 The *arg_name* can be followed by any number of index or
 attribute expressions. An expression of the form ``'.name'`` selects the named
 attribute using :func:`getattr`, while an expression of the form ``'[index]'``