title: Sqlalchemy CAST equivalent of Double precision
body: <p>i am interested to know the Sqlalchemy equivalent of the following command</p>
  
  <p>update tablename set field1 = cast(field2 as double precision[])</p>
  
  <p>so far, i tried the following with no luck</p>
  
  <pre><code>from sqlalchemy import create_engine
  metadata = sqlalchemy.MetaData(engine)
  table = sqlalchemy.Table('tablename',md,autoload=true)
  update_stmt = table.update(values={table.c.field1:cast(table.c.field2, **double precision[]**)})
  engine.execute(update_stmt)
  </code></pre>
  
  <p>please help to resolve double precision equivalent in sqlalchemy. this is running update statement from python to postgresql</p>