Application configuration and testability

helpers.py

# tests/helpers.py

from contextlib import contextmanager
from typing import TYPE_CHECKING, Any, Iterator

from defmain.settings import get_settings

if TYPE_CHECKING:
    from defmain.settings import Settings


@contextmanager
def override_settings(**overrides: Any) -> Iterator["Settings"]:
    try:
        settings = get_settings()
        for name, value in overrides.items():
            if not hasattr(settings, name):
                raise AttributeError(
                    f'The Settings class does not define the "{name}" setting'
                )
            setattr(settings, name, value)
        yield settings
    finally:
        get_settings.cache_clear()

settings.py

# defmain/settings.py

from functools import lru_cache
from typing import List

from pydantic import BaseSettings

__all__ = ("Settings", "get_settings")


@lru_cache()
def get_settings() -> "Settings":
    return Settings()


class Settings(BaseSettings):
    """Application settings.

    The application defines a set of configuration parameters.

    The application is configured as follows, in descending order of priority:
        - environment variables with the `DEFMAIN_` prefix,
        - variables with the `DEFMAIN_` prefix in the` .env` file
          in the current working directory (if any),
        - default values specified in the code.
    """

    debug: bool = False

    # CORS middleware
    allow_origins: List[str] = ["*"]
    allow_methods: List[str] = ["*"]
    allow_headers: List[str] = ["*"]

    class Config:
        env_file = ".env"
        env_prefix = "DEFMAIN_"

test_helpers.py

# tests/instrumental/test_helpers.py

from defmain.settings import get_settings
from tests.helpers import override_settings

def test_override_settings():
    original_settings = get_settings()
    debug = not original_settings.debug

    with override_settings(debug=debug) as settings:
        assert settings.debug is debug
        assert get_settings().debug is debug

    settings = get_settings()

    assert settings.debug is not debug