Example of a settings.py file with validation & different context {dev, prod, test} | Pydantic settings.py

settings.py

"""
In order to share and have easy access on to
how a settings.py file should be written.

Advantages:
- in a manner of Django project
- with validation
- with state-of-the-art validation
- even for several context, e.g. dev, prod

Aknowlegment
- https://medium.com/swlh/cool-things-you-can-do-with-pydantic-fc1c948fbde0
- Check out, e.g. PostgresDSN: https://pydantic-docs.helpmanual.io/usage/settings/
"""

import os
from typing import Union

import pytest
from pydantic import BaseSettings, HttpUrl, ValidationError, parse_obj_as
from typing_extensions import Literal


class LocalContext(BaseSettings):
    env: Literal["local"]
    echo_server_url: str


class ProdContext(BaseSettings):
    env: Literal["prod"]
    external_server_url: HttpUrl
    external_server_port: int


Context = Union[LocalContext, ProdContext]


def test_reads_local_context():
    os.environ.clear()
    os.environ["ENV"] = "local"
    os.environ["ECHO_SERVER_URL"] = "http://localhost"

    context = parse_obj_as(Context, {})

    assert context.env == "local"
    assert context.echo_server_url == "http://localhost"
    assert not hasattr(context, "external_server_url")
    assert not hasattr(context, "external_server_port")


def test_reads_prod_context():
    os.environ.clear()
    os.environ["ENV"] = "prod"
    os.environ["EXTERNAL_SERVER_URL"] = "http://some.echo.serv.er"
    os.environ["EXTERNAL_SERVER_PORT"] = "8080"

    context = parse_obj_as(Context, {})

    assert context.env == "prod"
    assert context.external_server_url == "http://some.echo.serv.er"
    assert not hasattr(context, "echo_server_url")


def test_only_given_literals_are_supported():
    os.environ.clear()
    os.environ["ENV"] = "staging"

    with pytest.raises(ValidationError):
        parse_obj_as(Context, {})