Cross-post from: Medium
This article explains how to locally test, with AWS SAM (Serverless Application Model), a Lambda function + API Gateway created with AWS CDK (Cloud Development Kit). Python AWS CDK and SAM are a match made in heaven, or at least a match made in the cloud. This example uses the AWS CDK in Python.
Using Python AWS CDK and SAM
When using AWS CDK for your infrastructure as code, it can be a pain to figure out how to efficiently test your Lambda functions. Deploying your code to AWS with
cdk deploy for every change is slow, having to run a deploy for every code change is not ideal. Luckily it is straightforward to use AWS SAM to test your function locally.
For scenarios where local dev isn’t feasible, get faster deployments to Lambda with
cdk deploy --hotswap
Steps 0–2 help you set up a fresh CDK project. If you already have a CDK project which creates a Lambda function and API Gateway, skip ahead to step 3.
0) Prerequisites, Assumptions, and Environment
- NPM Installed
- Python Installed
- Docker Installed
- If you don’t have AWS CDK installed follow this guide
- If you don’t have AWS SAM installed, follow this guide
1) Initialize a Python CDK project
Note: If this is your first time using AWS CDK with Python, I recommend you try out the CDK Workshop from AWS: https://cdkworkshop.com/30-python/20-create-project/100-cdk-init.html
Create a directory for your project and
cd into it
$ mkdir aws-sam-cdk-demo $ cd aws-sam-cdk-demo
Then, initialize your CDK project
$ cdk init app --language python
Create and source a python virtual environment
$ python3 -m venv .venv $ source .venv/bin/activate
Install CDK requirements
$ pip install -r requirements.txt
Run CDK synth to ensure everything is setup correctly
$ cdk synth
You should see a small CloudFormation template get created with only the
CDKMetadata resource and some conditions:
2) Create a Lambda function
In this example, I’ll be making an API that provides a greeting from a list of greetings I have hard coded.
Create a directory for the lambda code and create a file for the code
$ mkdir lambda$ touch lambda/greeting_generator.py
Go ahead and add some code to your function. You can copy the code from my function and tweak it as you see fit: https://github.com/chrishart0/aws-cdk-sam-demo/blob/master/lambda/greeting_generator.py
Install the lambda and API Gateway constructs
aws-cdk.aws-apigateway as lines to
$ pip install -r requirements.txt
Time to add lambda and API Gateway to your stack
The stack file is where you will define your infrastructure for CDK.
The stack file was created by
cdk init, it follows the naming scheme
Example: Parent folder which init was run in is
- Example in GitHub: https://github.com/chrishart0/aws-cdk-sam-demo/blob/master/aws_sam_cdk_demo/aws_sam_cdk_demo_stack.py
Import lambda at the top of the stack file referenced above. How this file should look at the end is provided below or check the repo.
from aws_cdk import (
aws_lambda as _lambda,
aws_apigateway as apigateway,
Add code to your stack to generate a lambda function
greeting_function = _lambda.Function(
Add code for the API Gateway. Ensure the handler is the same as the variable for the lambda function.
greeting_apg = apigateway.LambdaRestApi(
description="Greeting API endpoint",
It should now look like the image below:
Tip: Add links back to the CDK docs in your code; this will save you a lot of time hunting for the right page in the future.
cdk synth to ensure everything compiles. You should now see more resources in the CloudFormation output in your terminal.
3) Test that lambda function locally with SAM
Here is where things start to get a little… nutty 🐿️ (get it, like the SAM squirrel mascot???)
If you already have a CDK application with a lambda function start here!
Synthesize a template and write it to
$ cdk synth --no-staging > template.yaml
Test the API with SAM
$ sam local start-api
In your browser or in another terminal hit the endpoint
You can change the code of your lambda function on the fly, no need to restart SAM. Edit the source code in the
lambda/ dir and call the endpoint again.
You can also test individual invocations with simulated events, this is ideal if you are not using API Gateway.
$ sam local invoke functionName -e event.json