SQLAlchemy and SQLite: database is locked

SQLite locks the database when a write is made to it, such as when an UPDATE, INSERT or DELETE is sent. When using the ORM, these get sent on flush. The database will remain locked until there is a COMMIT or ROLLBACK.

I've mostly seen the "database is locked" error in multi-threading situations. One thread will lock the database and another thread will attempt a write of its own. If the first thread doesn't release the lock within the timeout period (4-5 seconds by default, if I recall) the OperationalError is raised on the second thread.

It can be tricky to know when a flush, and therefore a write is made to the database when the session has autoflush=True (the default setting) since any query will cause a flush. Sometimes turning on the SQL logging can help clarify when things are happening:

logging.getLogger('sqlalchemy.engine').setLevel(logging.INFO)

There is some relevant documentation here: http://docs.sqlalchemy.org/en/rel_0_9/dialects/sqlite.html#database-locking-behavior-concurrency

sqlite databases only allow one process to access it at a time. Perhaps you have a separate process using the database?

you should use a single session across all objects in a thread. sqlite really doesn't like multiple connections, and sqlalchemy is effectively a connection per session (it looks like you may have a session for each class, which implies multiple sessions in a single thread).