Testing an AWS Lambda function locally

Never had to test AWS Lambda functions locally (test in production YOLO). I’ve always ended up writing toy functions that I test using the console. For functions that are non-trivial and have to process different kinds of input events, it’s probably best to test them locally to discover any regressions early on.

While walking through the docs with a colleague, we found that the nifty way to test a lambda function created using SAM is to run:

$ sam local start-lambda

In the directory where you have your SAM template.yml. This is LambdaInvoking the function, as opposed to passing it an event via API Gateway. From the docs, write a quick script like this to then invoke the Lambda.

import boto3
import botocore

# Set "running_locally" flag if you are running the integration test locally
running_locally = True

if running_locally:

    # Create Lambda SDK client to connect to appropriate Lambda endpoint
    lambda_client = boto3.client('lambda',
        region_name="eu-west-1",
        endpoint_url="http://127.0.0.1:3001",
        use_ssl=False,
        verify=False,
        config=botocore.client.Config(
            signature_version=botocore.UNSIGNED,
            read_timeout=30,
            retries={'max_attempts': 0},
        )
    )
else:
    lambda_client = boto3.client('lambda')


# Invoke your Lambda function as you normally usually do. The function will run
# locally if it is configured to do so
response = lambda_client.invoke(FunctionName="HelloFunction")

# Verify the response
assert response == "Hello World"

This is from the docs, but it took me a while to figure out that FunctionName above needs to refer to the logical name of your Lambda function in the CloudFormation template.

The first run will be slow as SAM will fetch the container for your function runtime, subsequent runs should be quicker (adjust read_timeout accordingly).

If you’re a security nerd, the bits above that say use_ssl=False and verify=False should give you pause. In this instance, you’re connecting to a service locally and not reaching out to the internet, so you’re OK.