add_col_ddb.py

import boto3

dynamodb = boto3.resource('dynamodb')
table = dynamodb.Table('TABLE_NAME')


def update_items(items):
    for item in items:
        table.update_item(
            Key={
                'KEY': item['KEY']
            },
            UpdateExpression="SET FIELD = :val",
            ExpressionAttributeValues={
                ':val': 'VALUE'
            }
        )


response = table.scan()
update_items(response['Items'])

while 'LastEvaluatedKey' in response:
    response = table.scan(ExclusiveStartKey=response['LastEvaluatedKey'])
    update_items(response['Items'])