What is the wildcard equivalent for boto3 dynamodb ProjectionExpression

June 10, 2023

I have a function that calls boto3’s query operation. My data object for each record is rather large so I have created an optional parameter that can be passed to reduce the fields that are returned. Here is the function:

def get_leads(type,leadTable,**kwargs):
    attributes = kwargs.get('attributes', None)
    client = boto3.resource('dynamodb')
    table = client.Table(leadTable)
    try:
        if attributes != None:
            response = table.query(ProjectionExpression=attributes,KeyConditionExpression=Key('lead_type').eq(type))
        else:
            response = table.query(KeyConditionExpression=Key('lead_type').eq(type))
        return response
    except Exception as e:
        print(e)
        return 0

This way, i can optionally pass attributes as a parameter
e.g.,

get_leads("user","table1",attributes="first_name,last_name,title,image_link")

returns only first_name,last_name,title,image_link fields

get_leads("user","table1")

returns all fields

Ideally, I would like to simplify this function and set attributes to a wildcard ProjectionExpression value when it is not defined.
e.g.,

def get_leads(type,leadTable,**kwargs):
    attributes = kwargs.get('attributes', "*")
    client = boto3.resource('dynamodb')
    table = client.Table(leadTable)
    try:
        return table.query(ProjectionExpression=attributes,KeyConditionExpression=Key('lead_type').eq(type))
    except Exception as e:
        print(e)
        return 0

Sadly, I can’t seem to find documentation anywhere stating what this wildcard value is or if it even exists.

>Solution :

There is no wild card, the absence of a ProjectipnExpression signifies DynamoDB to return all attributes.

Another tip is not to create a client in your function, create it globally and share its state across invocations, you’ll get better latency and reduced costs if you use KMS at scale.