AWS Lambda + Python + External Libraries the Easy Way


As a systems administrator systems engineeer devops engineer SRE? I’ve come to love AWS Lambda. Even though there are limitations (duration and resource caps) most of the server-side daemon code you end up developing can be ported over without too much hassle. There is one hitch that can make your serverless life painful: external libraries.

Why External Libraries

The Python standard library has just about everything you need to write elegant software that can do just about everything. But sometimes you just want to do things the simple way and incorporate the work some genius engineer has done so that getting the job done takes less code and less time.

This is where most people (including me) hit a wall with AWS Lambda. If you take a look at the documentation on the AWS Lambda execution environment you’ll see that you have very few tools to play with. For Python developers this means you have Python 2.7 or 3.6 and Boto3 (the AWS SDK for Python) library and that’s about it. If you need to take advantage of other libraries, Lambda can be unforgiving.

How External Libraries are Used

When you want to integrate an external library with your Lambda code, you’ll need to build a virtual environment, install your libraries, then package them together with your Python code. Unfortunately there are a few gotchas that you need to look out for when you’re doing this.

The Workaround: Docker

LambCI is a Continuous Integration service built on top of AWS Lambda. While the project is pretty neat, waht they’ve done one the Docker side of the house is of direct benefit to those of us that are in the position of needed to use (and compile) external libraries. Take a look at the LambCI Docker Hub registry. They’ve created several containers that mimic the runtime environment of AWS Lambda. Using these containers, you can easily install and package your external libraries knowing that they’ll be 100% compatible when uploaded to AWS Lambda.

A Quick Example

I recently had to create a Lambda to generate Confluence pages based on a CloudWatch event. In order to make life easier I decided to use the Requests library (which is fantastic if you’re doing anything with HTTP in Python). Unfortunately this meant that I had to go through the process of adding external libraries to my code.

  1. To start off, create a requiremnts.txt file in the root of your project. List any external libraries you’ll need (this is also good practice no matter what you’re doing with Python). In addition to any external libraries, it’s reccomended that you update boto3 by adding it into your package list.

    Example requirements.txt:

  2. We need to get an image from LambCI that matches our execution environment. For my example I’m using Python 3.6 so the image I want to get is the build-python3.6 image.

    docker pull lambci/lambda:build-python3.6
  3. Once I have the image locally we need to cd to the root of our project and run the container in interactive mode. In this example I’m going to fire up the container and drop into a BASH shell.

    docker run -v "$PWD":/var/task -it lambci/lambda:build-python3.6 bash
    • -v "$PWD":/var/task: We’re creating a mount from the current directory to the /var/task directory on the container. This mirrors the location where your code will run in an AWS Lambda container execution environment
    • -it: This combination of flags starts the container in interactive mode with a pseudo-TTY.
    • lambci/lambda:build-python3.6 bash: The first part of this just tells Docker which image we want to run. The bash at the end starts a shell session. Paired with the it flag we’re dropped into a shell on the running container.
  4. Start up a virtual environment inside your container. If there is a pre-existing virtual environment in your project root, create a new one. As a general rule, I name them venv (when doing local development) and aws if using Docker. Create the virtual environment, activate it, and install your external libraries.

    virtualenv aws
    . ./aws/bin/activate
    pip install -r requirements.txt
  5. At this point you can start working on your Lambda code inside the container. Since we’ve mounted our project directory to the container, we can use our favorite editor to write with while executing inside the container to make sure everything works.

  6. When your code is doing all the neat things you want it to do, we need to zip up all the external libraries (and thier dependencies) and package it together with our existing code. One thing that will get you is that some libraries may be 32-bit or 64-bit only. The libraries for a virtual environment are located in the venv/lib/python3.6/site-packages and venv/lib/python3.6/site-packages folders. In order to make sure you have all of the code you’ll need, make sure to get site-packages from both the lib and lib64 directories then exit your conatiner.

    cd venv/lib/python3.6/site-packages
    zip -r ../../../../ *
    cd ../../../lib64/python3.6/site-packages
    zip -r ../../../../ *
  7. Add your lambda code to the zip file then upload it to AWS Lambda via the console or the aws cli command. The only downside is that you will no longer be able to edit your code inline in the console.



I hope this blog post helps others who find themselves in the same situation. AWS Lambda can be fun to work with but making complex code with external libraries can cause a few headaches. After reading this article you should be able to avoid the learning curve that I had in figuring this out.