Why still has "commands out of sync; you can't run this command now" error

Question

I am using Python mysqldb library to connect mysql db. I have a web server with 4 worker process which has 1 conn and 1 cursor to mysql db. so every worker process will use its connection/cursor to execute sql sentence.

Now, I am have several client to simultaneously to send request to server, and server will query mysql db, and return some result to client. I encounter error. 2014, "Commands out of sync; you can't run this command now"
I have check sql, it just simple as SELECT a, b, c from table WHERE a = 1. There is no semicolon, or store procedure, and I also try below code as Python, "commands out of sync; you can't run this command now" suggest. but it still same error.

self.cursor.execute(sql, data)
self.conn.commit()
result = result + self.cursor.fetchall()
self.cursor.close()
self.cursor = self.conn.cursor() 

Answer 1

Finally, I fixed this issue. My app has multithread to use the same connection, it seems is not a proper way to access mysql, so when I do not share connection, the issue is gone.

Under 'threadSafety' in the MySQLdb User Guide:

The MySQL protocol can not handle multiple threads using the same connection at once. Some earlier versions of MySQLdb utilized locking to achieve a threadsafety of 2. While this is not terribly hard to accomplish using the standard Cursor class (which uses mysql_store_result()), it is complicated by SSCursor (which uses mysql_use_result(); with the latter you must ensure all the rows have been read before another query can be executed. It is further complicated by the addition of transactions, since transactions start when a cursor execute a query, but end when COMMIT or ROLLBACK is executed by the Connection object. Two threads simply cannot share a connection while a transaction is in progress, in addition to not being able to share it during query execution. This excessively complicated the code to the point where it just isn't worth it.

The general upshot of this is: Don't share connections between threads. It's really not worth your effort or mine, and in the end, will probably hurt performance, since the MySQL server runs a separate thread for each connection. You can certainly do things like cache connections in a pool, and give those connections to one thread at a time. If you let two threads use a connection simultaneously, the MySQL client library will probably upchuck and die. You have been warned.