Mongita is a lightweight embedded document database that implements a commonly-used subset of the MongoDB/PyMongo interface. Use it when a MongoDB installation is too much or when SQLite is not the right choice.
Hope this belongs here since Mongita is not actually a "product". Nevertheless, it was a sizable project that I think will be useful to others. I'm surprised at how well it turned out - especially the performance relative to MongoDB and SQLite given that it's written in pure Python.
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Mongita
Mongita