In need of a light, changing database/storage solution
You might look at mongoengine we use it in production with flask(there's an extension) and it has suited our needs well, there's also mongoalchemy which I haven't tried but seems to be decently popular. 
The downside to using mongo is that there is no expire automatically, having said that you might take a look at using redis which has the ability to auto expire items. There are a few ORMs out there that might suit your needs.
I have a Python Flask app I'm writing, and I'm about to start on the backend. The main part of it involves users POSTing data to the backend, usually a small piece of data every second or so, to later be retrieved by other users. The data will always be retrieved within under an hour, and could be retrieved in as low as a minute. I need a database or storage solution that can constantly take in and store the data, purge all data that was retrieved, and also perform a purge on data that's been in storage for longer than an hour.
I do not need any relational system; JSON/key-value should be able to handle both incoming and outgoing data. And also, there will be very constant reading, writing, and deleting.
Should I go with something like MongoDB? Should I use a database system at all, and instead write to a directory full of .json files constantly, or something? (Using only files is probably a bad idea, but it's kind of the extent of what I need.)