title: update list element in dynamodb
body: <p>I have a list of maps as one field of a DynamoDB table. How can I update a specific element (or, rather element field ?)
  Trying something like </p>
  
  <pre><code>rc = table.update_item(Key={ 'username' : user }, 
      UpdateExpression="set list[:i].field = :nd",
      ExpressionAttributeValues={
          ':i' : itemnum,
          ':nd': data,
      },
      ReturnValues="UPDATED_NEW"
  );
  </code></pre>
  
  <p>But I am getting an error: </p>
  
  <pre><code>Invalid UpdateExpression: Syntax error; token: ":i", near: "[:i]"
  </code></pre>
  
  <p>Any ideas how can I reference list element with variable number. Thanks.</p>