sqlalchemy beginner

[Jabba Laci]

Hi,

I'm reading the Essential SQLAlchemy book from O'Reilly. It explains
SqlAlch 0.4 but my current version is 0.7 and there are some
differences.

Here is an example from the book:

user_table = Table('tf_user', metadata,
        Column('id', Integer, primary_key=True),
        Column('user_name', Unicode(16), unique=True, nullable=False),
        Column('password', Unicode(40), nullable=False),
        Column('display_name', Unicode(255), default=''),
        Column('created', DateTime, default=datetime.now)
    )

Here I get the following warning:

SAWarning: Unicode column received non-unicode default value.
  Column('display_name', Unicode(255), default=''),

Changing Unicode(255) to String(255) makes the warning disappear but
I'm not sure if it's the correct solution.

For table names, the book uses the prefix convention 'tf_' but what
does it mean? 't' is table, but what is 'f'?

Thanks,

Laszlo