Missing env var

missing-env-var

os.environ[...] is dict-style indexing. Missing keys raise KeyError with no context — the traceback shows an indexing line, not a message about which variable was expected or why the program needs it. In production logs, this reads as a mystery crash:

KeyError: 'APP_API_KEY'
  File "app/config.py", line 12, in get_api_key
    return os.environ["APP_API_KEY"]

Example

Wrong — opaque KeyError; caller has no idea what to fix:

import os

def get_api_key() -> str:
    return os.environ["APP_API_KEY"]

Right — explicit check, specific error, actionable message:

import os

class ConfigError(Exception):
    pass

def get_api_key() -> str:
    value = os.environ.get("APP_API_KEY")
    if not value:
        raise ConfigError(
            "APP_API_KEY is not set. Export it before running."
        )
    return value

Prefer failing at startup

Read configuration once at process boot, not on every request. A missing key should surface before the first user hits the path:

REQUIRED = ("APP_API_KEY", "APP_DB_URL")

def load_config() -> dict[str, str]:
    missing = [k for k in REQUIRED if not os.environ.get(k)]
    if missing:
        raise ConfigError(f"missing env vars: {', '.join(missing)}")
    return {k: os.environ[k] for k in REQUIRED}

This turns a 500 during a user request into a crash at boot — loud, visible, and immediately fixable.

Treat empty strings as missing

.get() returns the empty string if the var is exported but empty. Use if not value: rather than if value is None: — an empty string is almost never what you want for an API key or URL.