Easy settings management for Python functions with Pydantic

File this under quick tips and tricks for Python functions in OpenFaas.

I just learned about Pydantic’s support for reading settings from secret files and it fits perfectly with secrets in OpenFaaS. If you love Python or you are writing Python functions in OpenFaas, this is a great way to simplify your configuration parsing.

I am going to keep this short and sweet. Let’s say you have an OpenFaas function that needs to access a database. Typically, this requires the db_host, db_user, and db_password to be configured. In OpenFaas, we would expect the db_host and db_user as environment variables, but the db_password is sensitive and should be supplied as a secret, see the docs for a quick intro to secrets.

Your function stack.yaml might look like this

  functions:
    cool_function:
      lang: python3
      image: functions/cool_function:latest
      env:
        db_host: cool.rds.aws.com
        db_user: super_cool_person
      secrets:
      - db_password

If you use pydantic, then the only code you need for to configure your function looks like this

from pydantic import BaseSettings
class Settings(BaseSettings):
    db_password: str
    db_host: str
    db_user: str

    class Config:
        secrets_dir = '/var/openfaas/secrets'

settings = Settings()

When Settings is instantiated, it will automatically read the values from the environment variables and from our secrets, because we set secrets_dir. Given our stack.yaml above, this means

• settings.db_host == cool.rds.aws.com,
• db_user == super_cool_person, and
• db_password will be the value from /var/openfaas/secrets/db_password,

because OpenFaaS puts all of the secrets into /var/openfaas/secrets.

That’s it, 10 lines of code and you get simple type safe configuration.

Next steps

I am a big fan of type hinting in Python. Once, you are using Pydantic, then the next step is to configure mypy to get static type checking before you deploy your functions. But that is a blog post for another day.