AWS SAM Python Setup and Debugging

process.md

This changes often and may not work as is for you.

Last checked on 06-25-2020

Step - 0 (What)

The steps below show how to setup an AWS Lambda function using AWS SAM CLI for Python. The setup takes one through the process of initialization, building, deploying and debugging a AWS Lambda function. The default SAM template also creates an API endpoint for testing the Lambda function. This testing can be done locally, and after deploying the function to AWS. While most of the instructions are easily found on the AWS docs, the instructions for debugging through VS Code and testing using Postman are not well-documented. Please install SAM CLI and read the AWS SAM python deployment guide.

Step - 1

sam init

This initializes the AWS SAM CLI using the configuration wizard that asks questions related to the runtime, name, deployment region etc. The file template.yaml in the directory created can be edited reflect your requirements, like changing GET to POST, change the function name, API endpoint path etc.

Step - 2

sam build

This create a deployment package ready for deployment. You will need to build everytime you make change to the app.py file in the function directory. This creates a directory .aws-sam/build with the required python packages, and deploy copy of app.py. This file is ephemeral and make changes to file only for debugging. Any changes will be lost of running sam build again. If you plan to debug your function, then add the python package ptvsd on a new line in the requirements.txt in the function directory.

Step - 3

sam deploy --guided

This will deploy the AWS Lambda function to AWS and create an endpoint.

Step - 4

Debugging in VS Code

• Use the following as the launch.json

{
    "version": "0.2.0",
    "configurations": [
        {
           "name": "SAM CLI Python Hello World",
           "type": "python",
           "request": "attach",
           "port": 5890,
           "host": "localhost",
           "pathMappings": [
               {
                   "localRoot": "${workspaceFolder}/your_function_name/.aws-sam/build/HelloWorldFunction",
                   "remoteRoot": "/var/task"
               }
           ]
       }
   ]
 }

Replace your_function_name above with the name of the function.

• Now add the following code in the app.py in the .aws-sam/build/HelloWorldFunction directory.

import ptvsd

# Enable ptvsd on 0.0.0.0 address and on port 5890 that we'll connect later with our IDE
ptvsd.enable_attach(address=('0.0.0.0', 5890), redirect_output=True)
ptvsd.wait_for_attach()

• Then start the API locally in debug mode using:

sam local start-api -d 5890 --debug

This will need docker desktop to be working locally.

• Now visit the API endpoint using the browser or Postman.

localhost:3000/your_function_name

• Now press the "Run" button from the Run tab in VS code. This will pause the code at the breakpoint in your app.py and you can proceed by stepping through the code.