Question

Is it okay to use a single MySQLdb connection for multiple transactions without closing the connection between them? In other words, something like this:

conn = MySQLdb.connect(host="1.2.3.4", port=1234, user="root", passwd="x", db="test")

for i in range(10):
    try:
        cur = conn.cursor()
        query = "DELETE FROM SomeTable WHERE ID = %d" % i
        cur.execute(query)
        cur.close()
        conn.commit()

    except Exception:
        conn.rollback()

conn.close()

It seems to work okay, but I just wanted to double check.

Answer 1

I think there is a misunderstanding about what constitutes a transaction here.

Your example opens up one connection, then executes one transaction on it. You execute multiple SQL statements in that transaction, but you close it completely after committing. Of course that’s more than fine.

Executing multiple transactions (as opposed to just SQL statements), looks like this:

conn = MySQLdb.connect(host="1.2.3.4", port=1234, user="root", passwd="x", db="test")

for j in range(10):
    try:
        for i in range(10):
            cur = conn.cursor()
            query = "DELETE FROM SomeTable WHERE ID = %d" % i
            cur.execute(query)
            cur.close()
        conn.commit()
    except Exception:
        conn.rollback()

conn.close()

The above code commits 10 transactions, each consisting of 10 individual delete statements.

And yes, you should be able to re-use the open connection for that without problems, as long as you don’t share that connection between threads.

For example, SQLAlchemy re-uses connections by pooling them, handing out open connections as needed to the application. New transactions and new statements are executed on these connections throughout the lifetime of an application, without needing to be closed until the application is shut down.