I implemented the ECS-"Pattern" in python before, but at that variant is very optionated when it comes to all things systems, i.e. the "S" in "ECS". This time around I kept it simple and punted all the "filtering" out the user of the module. In turn this version is terser, for better or for worse.
Another sidenote, that I find important enough to actually mention, is that this implementation is actually more or less a cross between my prior version and esper. The later of which influenced the whole lru caching this time around...