title: How to delete boto3 DynamoDB key?
body: <p>This is my DynamoDB JSON</p>
  
  <pre><code>{
    "follow_count": 3,
    "followed_back_count": 4,
    "followed_back_users": [
      32432
    ],
    "login": "login1",
    "target": "target1",
    "target_followed_users": [
      234232342,
      453453434,
      241413422
    ]
  }
  </code></pre>
  
  <p>I need to delete the <code>target_followed_users</code> from the db so I tired this...</p>
  
  <pre><code># Update for followed_back_users
  table.update_item(
      Key={
          'login': login,
          'target': target,
      },
      UpdateExpression='DELETE followed_back_users = :followed_back_users',
      ExpressionAttributeValues={
          ':followed_back_users': db_followed_back_users
      }
  )
  </code></pre>
  
  <p>I got below error..</p>
  
  <blockquote>
    <p>botocore.exceptions.ClientError: An error occurred (ValidationException) when calling the UpdateItem operation: Invalid UpdateExpression: Syntax error; token: "=", near: "followed_back_users = :followed_back_users"</p>
  </blockquote>