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<html>
<meta content="width=device-width, initial-scale=1.0" name="viewport">
<head>
<title>
leontrolski - stop doing mad config
</title>
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<a href="index.html">
<img src="pic.png" style="height:2em">
⇦
</a>
<p><em>Part of a series - <a href="index.html#stop">Stop doing things</a></em></p>
<br>
<p><i>2020-12-17</i></p>
<h1>
Stop doing mad config
</h1>
<p>
People do all sorts of wild and wonderful things in Python to get configuration from files/the environment, then even stranger things attaching said config to applications.
</p>
<p>
Here's how everyone should do it, in a way that's (in vague order of importance):
</p>
<ul>
<li>
Easy to test.
</li>
<li>
Can be used from web apps, cron jobs, message processors, you name it.
</li>
<li>
Well typed.
</li>
<li>
Simple and well-understood.
</li>
<li>
Quick.
</li>
</ul>
<pre class="language-python"><code>from dataclasses import dataclass
from functools import lru_cache
import os
from pathlib import Path
@dataclass
class Config:
DB_URL: str
NUMBER_OF_WORKERS: int
COMPLICATED_THING: dict
@lru_cache(None)
def get_config() -> Config:
complicated_path = Path(__file__).parent / "some-file.json"
return Config(
DB_URL=os.environ["DB_URL"],
NUMBER_OF_WORKERS=int(os.environ.get("N_WORKERS", 4)),
COMPLICATED_THING=json.loads(complicated_path.read_bytes()),
)</code>
</pre>
<p>
When you want to use it, do:
</p>
<pre class="language-python"><code>from my_service import config
def create_application():
engine = Engine(url=config.get_config().DB_URL)
...</code>
</pre>
<p>
When you want to test it, do:
</p>
<pre class="language-python"><code>@pytest.fixture
def dummy_config(monkeypatch):
monkeypatch.setattr(config, "get_config", lambda: Config(...))
def test_thingie(dummy_config):
...</code>
</pre>
<p>
Now no more funny stuff OK.
</p>
</body>
</html>