I’m working on a personal project using SQLite and a whole lot of data. To speed it up, I want to do multiprocessing. What I’ve found when trying to implement this is that I get a pickling error if I have the sqlite connection created within the __init__, but it doesn’t happen when created outside of __init__. Why is this? Sample code showing the error below.

import sqlite3
import multiprocessing

class db_class:
    def __init__(self):
        self.conn = sqlite3.connect("test.db")
        self.conn.execute("create table if not exists mytable (field INT)")
        self.conn.commit()
        self.conn.execute("insert into mytable VALUES (1)")
        self.conn.commit()
    
    def test(self, q):
        self.conn.execute(q)    
    
 
if __name__ == '__main__': 
    query = "select * from mytable"
    db = db_class()
    procA = multiprocessing.Process(target=db.test, args=(query,))
    procB = multiprocessing.Process(target=db.test, args=(query,))

    procA.start()
    procB.start()

    procA.join()
    procB.join()