Dustincollections.ChainMap from #python is pretty cool. One place I use it when I want to merge configuration from CLI, environment vars, and config files. You can do that like this...
Ivan Sagalaev 
@dmwyatt it's pretty much *the* use case for ChainMap, I always thought!
That might be! The first time I ever used extensively was when I was writing a template engine. (Think Jinja2 or Django)
@dmwyatt ah, stacked contexts, that too.