How to pass a parameter from CloudFormation to an AWS Lambda function stored in an S3 bucket

Question

My Lambda code is stored in a S3 bucket. I use CloudFormation to deploy in the child account.

Lambda code snippet:

def lambda_handler(event,context):
  ids = ["${id}"]

The CloudFormation uses the parameters to take in the id as:

Parameters:

    id:  
      Type: String
      Description: Name of id. 
      Default: abc

However the id doesn't get populated in the lambda because the code is stored in the S3 bucket.

How to go about it?

Answer 1

From your description, it seems your requirement is:

• A CloudFormation template takes an input parameter
• You wish to make the value of that input parameter available to all invocations of an AWS Lambda function that is defined in the same template

You could achieve this by passing the parameter to the Lambda function via an Environment Variable.

When defining the Lambda function in the CloudFormation template, environment variables can be provided:

Type: AWS::Lambda::Function
Properties: 
  Code: 
    Code
  Environment:
    Variables:
      id:
        Ref: PARAMETER-NAME
...

In this case, the value of the variable is coming from a parameter.

Within your Lambda code, you would then access the environment variable. Here's a Python example:

import os

def lambda_handler(event, context):
    print("environment variable: " + os.environ['id'])

References:

• Dynamically passing in ENV vars to Lambda functions created by Cloudformation | Christopher Diehl's blog
• How to access an AWS Lambda environment variable from Python

Answer 2

Another approach is storing parameters in parameter store of system manager, which can be managed in cloudformation script as well.