title: how to get column handle from key in sqlalchemy
body: <p>Consider a function </p>
  
  <p>def filterGT(session, stmt, attr, value):</p>
  
  <p>which filters rows with values greater than value on attr. So far I have the following 
  code:</p>
  
  <pre><code>def filterGT(session, stmt, attr, value):  
      stmt=session.query(stmt).filter(stmt.c.agep&gt;value).subquery()
      return stmt
  </code></pre>
  
  <p>My problem is that I am not able construct handle to the column given its label i.e.
  If I pass "agep" as attr I want to filter on stmt.c.agep where agep 
  is one of the columns.</p>
  
  <p>Thanks  </p>