How to ensure several Python processes access the data base one by one?
You could create a function that each process must call in order to talk to the DB. You could re-write the scripts so that they must call that function rather than talk directly to the DB. Within that function, you could have a scope-based lock so that only one process would be talking to the DB at a time.
I got a lot scripts running: scrappers, checkers, cleaners, etc. They have some things in common:

they are forever running;
they have no time constrain to finish their job;
they all access the same MYSQL DB, writting and reading.

Accumulating them, it's starting to slow down the website, which runs on the same system, but depends on these scripts.
I can use queues with Kombu to inline all writtings.
But do you know a way to make the same with reading ? 
E.G: if one script need to read from the DB, his request is sent to a blocking queue, et it resumes when it got the answer ? This way everybody is making request to one process, and the process is the only one talking to the DB, making one request at the time.
I have no idea how to do this.
Of course, in the end I may have to add more servers to the mix, but before that, is there something I can do at the software level ?