title: Unresolved reference while trying to import col from pyspark.sql.functions in python 3.5
body: <p>Refer to the post here:
  <a href="https://stackoverflow.com/questions/43284380/spark-structured-streaming-with-python">Spark structured streaming with python</a>
  I would like to import 'col' in python 3.5</p>
  
  <pre><code>from pyspark.sql.functions import col
  </code></pre>
  
  <p>However I got an error saying unresolved reference to col. I've installed pyspark library, so just wondering has the 'col' been removed from pyspark library? And how can I import the 'col' then.</p>