Add or remove an entry from a List type attribute in a DynamoDB table item

The accepted answer is incorrect, the correct way of doing this is listed in the DynamoDB UpdateExpression documentation

You need to read the list then get the index of the item you're looking to remove then run REMOVE list[index] as an UpdateExpression

You can use SET operator to add element in the attribute list. But for that you have to retrieve the existing list first then append new element in the list. Suppose you have a attribute named active_user which contain the list of active users.

previous_list = "#a"
query = "SET %s = list_append(%s, :new_user)" % (previous_list, previous_list)
user_table.update_item(
    Key={
        # primary key
    },
    UpdateExpression=query,
    ExpressionAttributeNames={
        '#a': 'active_user',
    },
    ExpressionAttributeValues={
        ':new_user': new_user_model
    }
)

You can use REMOVE operator to remove or delete an element of the list. But you have to find the index of the element because REMOVE operator removes the given index of the list.

#user_index is the position of the target user in the list

query = "REMOVE active_user[%d]" % (user_index)
user_table.update_item(
    Key={
        # primary key
    },
    UpdateExpression=query
)

Here is REMOVE operator doc and SET operator doc.