Google App Engine originally used a custom containment strategy.

Back in the day, I was on the team that added Python 2.7 support to App Engine and we were experimenting with a different containment approach.

But Python is a complex language to support - you need to support WSGI, to support dynamic loading (for C extensions), a reasonably performant file system (Python calls `stat` about a billion times before actually importing a file), etc.

So our original runtime was actually Brainf#ck. So, at once point, if you had guessed that Google supported it, you could have written your (simple) webapp in Brainf#ck and Google would have scaled it up to hundreds of machines if needed ;-)