Question

there’s something I’m struggling to understand with SQLAlchamy from it’s documentation and tutorials.

I see how to autoload classes from a DB table, and I see how to design a class and create from it (declaratively or using the mapper()) a table that is added to the DB.

My question is how does one write code that both creates the table (e.g. on first run) and then reuses it?

I don’t want to have to create the database with one tool or one piece of code and have separate code to use the database.

Thanks in advance,
Peter

Answer 1

create_all() does not do anything if a table exists already, so just call it as soon as you set up your engine or connection.

(Note that if you change your table schema, create_all() will not update it! So you still need “another program” to do that.)

This is the usual pattern:

def createEngine(metadata, dsn, **args):
    engine  = create_engine(dsn, **args)
    metadata.create_all(engine)
    return engine

def doStuff(engine):
    res = engine.execute('select * from mytable')
    # etc etc

def main():
    engine = createEngine(metadata, 'sqlite:///:memory:')
    doStuff(engine)

if __name__=='__main__':
    main()