Possum – A packaging tool for Python AWS Serverless Applications

The applications I build on AWS are all written in Python using the Serverless Application Model (SAM). Building my applications using a template and Lambda functions, I quickly ran into a limitation of the aws command line tools: external dependencies.

If your Lambda functions have no dependencies (not including the AWS SDKs), or you pre-download and embed them alongside your code, the standard package command works:

serverless_04

However, if you want to install dependencies at the time of packaging the application, you are left in a position where you need to roll your own build system. Amazon provides instructions on creating a Python deployment package, but it would be nice if running the aws cloudformation command did this for us.

Possum

I wrote a packaging tool to fill in the gap left by Amazon’s. Possum (an amalgamation of “Python AWS SAM”) processes a SAM template file just as aws cloudformation package but creates per-function Lambda deployment packages if it detects a requirements file within the function’s directory (Pipfile or requirements.txt).

Possum can be installed from the Python Package Index:

possum_01

Once installed, Possum becomes available as a command line tool (it is loaded into your Python installation’s /bin directory):

possum_02.png

What Possum does is iterate over the Resources section of your SAM template and find all the objects of the AWS:Serverless:Function type, determine the location of their code using the Properties:CodeUri value, and through the magic of Pipenv create individual virtual environments to download the external dependencies, if any, and zip the files together into a Lambda package. Once the package and upload process is complete, Possum will either print your updated deployment template on the screen or write it out to a filename that you specified.

possum_03.png

In the above example, my HelloWorld function didn’t have any defined dependencies within it’s directory so the contents were zipped up as they were. For the Authorizer, there was a Pipfile present which triggered the build process. The approach to Lambda function dependencies with Possum is to handle them on a per-function basis. This creates artifacts that only include the required packages for that function (or none).

Pipenv is not installed with Possum. Instead, Possum will shell-out to run the Pipenv commands (so you will need to have Pipenv installed separately).

After Possum has finished, I can take the deployment.yaml file and deploy the application using aws cloudformation deploy or the AWS console.

Try It Out

If you’re working with Python Lambda functions, please give Possum a try! If you encounter an issue, or have a feature request, you can open an issue on the GitHub page.

Possum’s GitHub Page
https://github.com/brysontyrrell/Possum

Possum on the Python Package Index
https://pypi.org/project/possum/

 

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