Moving back and forth between an on-disk database and a fast in-memory database?

Question

Python\'s sqlite3 :memory: option provides speedier queries and updates than the equivalent on-disk database. How can I load a disk-based database in

Answer 1

The answer at How to load existing db file to memory in Python sqlite3? provided the important clues. Building on that answer, here is a simplification and generalization of that code.

It eliminates eliminate the unnecessary use of StringIO and is packaged into reusable form for both reading into and writing from an in-memory database.

import sqlite3

def copy_database(source_connection, dest_dbname=':memory:'):
    '''Return a connection to a new copy of an existing database.                        
       Raises an sqlite3.OperationalError if the destination already exists.             
    '''
    script = ''.join(source_connection.iterdump())
    dest_conn = sqlite3.connect(dest_dbname)
    dest_conn.executescript(script)
    return dest_conn

if __name__ == '__main__':
    from contextlib import closing

    with closing(sqlite3.connect('pepsearch.db')) as disk_db:
        mem_db = copy_database(disk_db)

    mem_db.execute('DELETE FROM documents WHERE uri="pep-3154"')
    mem_db.commit()

    copy_database(mem_db, 'changed.db').close()