Why does this not update my table in sqlalchemy?

I have the following code:

engine = sqlalchemy.create_engine(connectstring, echo=_echo).connect()
md = sqlalchemy.MetaData(engine)
table = sqlalchemy.Table('table_name', md, autoload=True, autoload_with=engine)
_Session = sessionmaker(bind=engine)
session = _Session()

for row in reader:
    table.insert({'key': 'value'})
session.commit()

Why does nothing get added to my table? The code runs, but the table is never updated.