Getting or deleting cache entries in Flask with the key starting with (or containing) a substring
It's not supported because Memcache is designed to be a distributed hash. There's no index of keys stored to search in. 
Ideally you should know what suffixes a key may have.
If not, you could maintain an index yourself in a special key for the user.
Like user_id + '_keys' which contains a list of keys.
This way you can cycle key by key and delete all the cache for the user.
You can override the .set function to manage this new key.
I'm trying to delete all entries in the cache store that contain (in this case start with) a substring of the cache key, but I don't see any easy way of doing this. I'm using Memcache as backend. 
If I understand the code correctly, I need to pass the full cache key when calling delete or delete_many. Is there any other way of doing this?
I'll explain what I'm trying to do in case there is a better way: I need to clear the cache for certain users when they modify their settings. Clearing the cache with clear() will remove the cache entries for all the users, which are some 110K, so I don't want to use that. 
I am generating key_prefix with the ID of the user, the request's path, and other variables. The cache keys always start with the ID of the authenticated user. So ideally I would use something like delete_many(user_id + ".*")