If you use safe_substitute and try to use a variable reference style other than ${foo}, you will find that it assumes ${foo} style. In particular, when evaluating $[foo] (square braces) and 'foo' is not defined, safe_substitute will put the string back together using ${foo} (curly braces).

See <http://stackoverflow.com/questions/14573230/pythons-string-template-changes-brackets-when-variable-is-unset> for details and test case.

See PEP 292 and the section titled "Why `$' and Braces?"

http://www.python.org/dev/peps/pep-0292/

Barry, I read that passage in the PEP to explain why the default style uses ${}, but the bug here is about a Template class with an overriden placeholder pattern.

Florent Xicluna already fixed this in r84888 for 3.2+; I've tested that the patch applies cleanly to 2.7 and tests pass. Someone with the commit bit should transplant that commit into 2.7 as it does not change the public API to this module.

Done in 8a98ee6baa1e. Thanks!