Gavin Wiggins

Testing a Python Configuration Module

Written on September 21, 2025

A configuration (config) module is useful for storing configuration settings in a Python package or application. Values such as API keys and passwords are often defined as environment variables which are used in the module. An example of a config module is shown below. For this example, the module is part of a Python package named mypkg.

# config.py in mypkg

import os

api_key = os.getenv("API_KEY")
api_url = os.getenv("API_URL", "http://localhost:8090")
save_path = "~/Desktop/downloads"

my-project/
├── src/
│   └── mypkg/
│       ├── __init__.py
│       ├── config.py
│       └── downloader.py
├── tests/
│   ├── test_config.py
│   └── test_downloader.py
└── pyproject.toml

Other modules in the Python package or app can import the config module as shown next. Notice the api_key is checked to make sure the associated environment variable was set otherwise an error is raised.

# downloader.py in mypkg

from . import config


def download_file():
    if config.api_key is None:
        raise RuntimeError("API_KEY environment variable is not set")

    print("API_KEY is", config.api_key)
    print("Saved file to", config.save_path)

The default values for the config module are not difficult to test. But testing with environment variables requires the monkeypatch fixture in pytest. This is demonstrated in the test functions given here. Notice the config module must be reloaded to pick up the new environment variables.

# test_config.py

import importlib
from mypkg import config


def test_config_defaults():
    assert config.api_key is None
    assert config.api_url == "http://localhost:8090"
    assert config.save_path == "~/Desktop/downloads"


def test_config_env(monkeypatch):
    monkeypatch.setenv("API_KEY", "12345")
    monkeypatch.setenv("API_URL", "https://www.apple.com")

    importlib.reload(config)

    assert config.api_key == "12345"
    assert config.api_url == "https://www.apple.com"
    assert config.save_path == "~/Desktop/downloads"

And here are the tests for the download_file function. To make sure the environment variables are reset, the config module is reloaded in these tests too.

# test_downloader.py

import importlib

import pytest

import mypkg
from mypkg import config


def test_downloader_defaults():
    importlib.reload(config)

    err = "API_KEY environment variable is not set"
    with pytest.raises(RuntimeError, match=err):
        mypkg.download_file()


def test_downloader_env(monkeypatch, capsys):
    monkeypatch.setenv("API_KEY", "12345")

    importlib.reload(config)
    mypkg.download_file()

    captured = capsys.readouterr()
    assert "Saved file to ~/Desktop/downloads\n" in captured.out

The example code discussed in this article is available in the GitHub pythonic repo at pythonic/projects/test-config/.