title: Boto3 list_append a value to overwrite a null value in a list
body: <p>I am writing a small lambda function to update a <code>game</code> item in my DynamoDB. My <code>game</code> item has a <code>participants</code> attribute which is a list consisting of the <code>participant_id</code> of each participant in the game. When a game doesn't have any participants yet (i.e. a round later in the bracket) the list will hold 2 <code>null</code> values:</p>
  
  <p><a href="https://i.stack.imgur.com/Rtosn.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Rtosn.png" alt="Empty participant list"></a></p>
  
  <p>I would like to overwrite the <code>null</code> values with a string each time I add a new participant. Currently I am able to append a new String to the list but the <code>null</code> values remain:</p>
  
  <p><a href="https://i.stack.imgur.com/b9xGa.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/b9xGa.png" alt="After adding a participant"></a></p>
  
  <p><strong><em>How can I overwrite one of the 2 null values with a string using boto3?</em></strong> 
  (i.e. to where it looks like <code>["participantOneId", Null]</code> after adding one participant and <code>["participantOneId", "participantTwoId"]</code> after adding both participants)
  This is my current implementation:</p>
  
  <pre><code>import boto3
  import json
  
  dynamodb = boto3.resource("dynamodb")
  gamesTable = dynamodb.Table('games')
  
  def lambda_handler(event, context):
    # get "next_game_id" and "winner_id" from the event body
    updated_next_game = gamesTable.update_item(
        Key={'id': next_game_id },
        UpdateExpression="SET participants = list_append(participants, :i)",
        ExpressionAttributeValues={
          ':i': [winner_id],
        },
        ReturnValues="ALL_NEW"
    )
  </code></pre>