Be Careful with Python's New-Style String Format
http://lucumr.pocoo.org/2016/12/29/careful-with-str-format/ [lucumr.pocoo.org]
2016-12-31 19:27
However the bigger feature is that you can access attributes and items of objects. The latter is what is causing the problem here.
This includes globals that you may not expect to be accessible.
{event.__init__.__globals__[CONFIG][SECRET_KEY]}
source: HN