DynamoDB Expression builder for update_item method in Boto3 (Python)

dynamo_expression_builder.md

The update_item method in Boto3 requires preparing something like this:

try:
    result = table.update_item(
        Key={
            'id': event['pathParameters']['id']
        },
        ExpressionAttributeNames=attr_names,
        ExpressionAttributeValues=expression_vals,
        UpdateExpression=update_expression,
        ReturnValues='ALL_NEW',
    )
    return respond(None,result)
except Exception as err:
    print(err) # to logs
    return respond(err)

This is for an AWS Lambda function where event['pathParameters']['id'] is the id parameter from the PUT call.

To merely pass in the event['body'] data and prepare for an update, where the assumption is to update any/all of the fields present in the data JSON object, the following is useful:

def buildExpression(data):
    pf = 'prefix'
    timestamp = int(time.time() * 1000)
    data['updatedAt'] = timestamp
    vals = {}
    exp = 'SET '
    attr_names = {}
    for key,value in data.items():
        vals[':{}'.format(key)] = value
        attr_names['#pf_{}'.format(key)] = key
        exp += '#pf_{} = :{},'.format(key, key)
    exp = exp.rstrip(",")
    return vals, exp, attr_names

pf is a useful prefix to label the aliases (in ExpressionAttributeNames) for logging/debugging. It seems safer merely to rename all fields with a #pf prefix in order to avoid any collisions with reserved keywords without having to catch them explicitly.