title: get value out of dataframe
body: <p>In Scala I can do <code>get(#)</code> or <code>getAs[Type](#)</code> to get values out of a dataframe. How should I do it in <code>pyspark</code>?</p>
  
  <p>I have a two columns DataFrame: <code>item(string)</code> and <code>salesNum(integers)</code>. I do a <code>groupby</code> and <code>mean</code> to get a mean of those numbers like this: </p>
  
  <p><code>saleDF.groupBy("salesNum").mean()).collect()</code></p>
  
  <p>and it works. Now I have the mean in a dataframe with one value.</p>
  
  <p>How can I get that value out of the dataframe to get the mean as a float number?</p>