I asked this question in sqlalchemy mail list, and got the answer, you can find in sqlalchemy FAQ.

I copied the example in here:

    from sqlalchemy import *

    from StringIO import StringIO

    buf = StringIO()
    engine = create_engine('postgres://', strategy='mock', executor=lambda s, p='': buf.write(s + p))
    meta = MetaData()
    t1 = Table('sometable', meta, Column('foo', String(30)))
    meta.create_all(engine)

    print buf.getvalue()

The most important thing in above example is the create_engine invoking.

The example uses 'postgress://', and I've tested it with 'sqlite://', and it's different in some column definition. And the strategy='mock' will prevent the tables creation exactly.

So with this help I also added an 'sql' command in Uliweb, so that you can see the creation SQL.