title: Cannot find col function in pyspark
body: <p>In pyspark 1.6.2, I can import <code>col</code> function by</p>
  
  <pre><code>from pyspark.sql.functions import col
  </code></pre>
  
  <p>but when I try to look it up in the <a href="https://github.com/apache/spark/blob/branch-1.6/python/pyspark/sql/functions.py" rel="noreferrer">Github source code</a> I find no <code>col</code> function in <code>functions.py</code> file, how can python import a function that doesn't exist?</p>