title: DynamoDB boto3 : How to query if a key has some value
body: <p>I'm writing a python code to query a table on dynamo DB. Along with other conditions I'd also like to get rows where there exists some value for the key I'm specifying. </p>
  
  <pre><code>KeyConditionExpression = Key('Status').eq('Done') &amp; Key('Name').begins_with('A') &amp; Key('Error').exists()
  </code></pre>
  
  <p>In the code above, I'd like to display rows where the column "Error" has some value be it anything.
  but the last condition is throwing an error. 
  AttributeError: 'Key' object has no attribute 'exists'. </p>
  
  <p>How can modify the code to incorporate the third query?</p>