Ask HN: What are some good open sourced mobile back ends

Hey all, exploring the mobile world here. I've been through the udemy ios complete course and found Baas such as parse as really convenient. The only caveat is cost. Are there any open source backend solutions that you guys can suggest? I'm most familiar with python, but couldn't find any pre-existing solutions. Let me know!

(NOTE: I'm currently trying out datakit/parsekit, seems promising, but I'm looking for a solution that you guys can vouch for) Thanks!