Env Config Loader

Use this skill when a script reads configuration from the environment and you want it to fail fast at startup with a clear message - "missing required keys: API_BASE, RUN_LIMIT" - instead of a KeyError halfway through, or a silent string-vs-int bug. It parses a simple .env, overlays the real environment, checks a required-key list once, and exposes typed getters (get_int, get_bool, get_float, get_list) that raise readable errors on bad values. No python-dotenv, no pydantic.

When to invoke

• User says: "load my .env", "validate config at startup", "fail fast if an env var is missing", "read this env var as an int / bool / list".
• Code in the conversation calls os.environ[...] scattered around with no central validation.

When NOT to invoke

• A large nested config with deep schemas and cross-field rules - reach for pydantic-settings.
• Secrets live in a vault/secret manager, not env/.env; load from there instead.

Concrete example

User input:

Read API_BASE, RUN_LIMIT (int), and DRY_RUN (bool) from .env, and crash early with a clear message if any are missing.

Output:

# Copy assets/config.py into your project, then:
from config import Config

cfg = Config.load(".env", required=["API_BASE", "RUN_LIMIT"])

base = cfg.get("API_BASE")                       # str, guaranteed present
limit = cfg.get_int("RUN_LIMIT")                 # ValueError if not an int
dry_run = cfg.get_bool("DRY_RUN", default=False) # true/1/yes/on -> True
tags = cfg.get_list("TAGS", default=[])          # "a,b,c" -> ["a", "b", "c"]

If API_BASE or RUN_LIMIT is absent, Config.load raises ConfigError: missing required keys: ... at startup, before any work begins. .env ships as .env.example with placeholders only - never real values.

Pattern to apply

1. Parse .env permissively: skip blanks/# comments, support export KEY=val and quoted values.
2. Let the real process environment win over .env, so production overrides the file.
3. Validate the required list once in load() and raise a single error listing every missing key.
4. Use typed getters; coerce explicitly and raise a clear ConfigError on a bad value, never a silent default.
5. Commit only .env.example with obvious placeholders; keep the real .env gitignored.

Reference: assets/config.py.

Source

Distilled from production use across the author's automation projects. v1.0.0. See also: [[config-audit-checklist]], [[telegram-alerter]], [[slack-webhook-alerter]].

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