I like your idea. As I understand it, since SQLite performs better on most benchmarks anyway, why not have an engine subclass that utilizes SQLite itself?

This would be a little trickier than your envisioning. The engine class is the lowest layer but doesn't handle the slow bits like the finds, managing the indicies, etc. So to really get the benefit of SQLite you would have to pull those slow bits in.

I do really like the idea of offering a third option so that you have - Memory (fastest) - SQLite (almost as fast but compromises on perfect MongoDB reproduction) - Disk (slower but is faithful to MongoDB)

Happy to discuss it more if you want to email me.

> How do you feel about type annotations in* the code (as opposed to just the comments, as far as I can see)

I used type annotations for a while when they came out but didn't find them to offer advantages over docstrings. The world might have moved over to type annotations though and I've not been aware.

> PyPy? I wonder if you'd get a ~free speedup

I'm getting a lot of suspicion right now from other commenters who are convinced I have my thumb on the benchmarks and I don't want to give them any more excuses :tears-of-joy:. But joking aside, you're probably right and it's not something I had considered.