feat: configuration loader with env var overrides

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.env.example

@@ -0,0 +1,6 @@
+# Copy to .env for Docker / local development secrets
+# These override values in config.yaml
+
+FABLEDNETMON_SECRET_KEY=change-me
+FABLEDNETMON_DATABASE__URL=postgresql+asyncpg://fablednetmon:password@localhost/fablednetmon
+FABLEDNETMON_SMTP__PASSWORD=

config.example.yaml

@@ -0,0 +1,36 @@
+# FabledNetMon configuration example
+# Copy to config.yaml and fill in your values.
+# Secrets can be set via env vars: FABLEDNETMON_<UPPER__PATH>
+
+secret_key: "change-me-to-a-random-string"
+
+database:
+  url: "postgresql+asyncpg://fablednetmon:password@localhost/fablednetmon"
+
+session:
+  lifetime_hours: 8
+
+data:
+  retention_days: 90
+
+# plugin_dir: ./plugins   # optional; defaults to ./plugins relative to config.yaml
+
+plugins: {}
+  # traefik:
+  #   enabled: true
+  #   metrics_url: "http://localhost:8080/metrics"
+  #   scrape_interval_seconds: 60
+
+smtp:
+  host: ""
+  port: 587
+  tls: true
+  username: ""
+  password: ""        # or: FABLEDNETMON_SMTP__PASSWORD env var
+  recipients: []
+  #   - admin@example.com
+
+webhook:
+  url: ""             # leave empty to disable
+  template: |
+    {"content": "**{{ alert.state }}** — {{ alert.resource }} — {{ alert.rule_name }} ({{ alert.metric }} = {{ alert.value }})"}

fablednetmon/config.py

@@ -0,0 +1,63 @@
+from __future__ import annotations
+import os
+from pathlib import Path
+from typing import Any
+import yaml
+from dotenv import load_dotenv
+
+REQUIRED_KEYS = ["secret_key", "database.url"]
+_PREFIX = "FABLEDNETMON_"
+
+
+def load_config(config_path: Path | str | None = None) -> dict[str, Any]:
+    """Load config from YAML file with env var overrides.
+
+    Env var naming: FABLEDNETMON_ + uppercased dotted path, dots -> __
+    Example: database.url -> FABLEDNETMON_DATABASE__URL
+    List values are never overridden by env vars.
+    """
+    load_dotenv()
+
+    if config_path is None:
+        config_path = Path("config.yaml")
+    config_path = Path(config_path)
+
+    if config_path.exists():
+        with config_path.open() as f:
+            cfg: dict[str, Any] = yaml.safe_load(f) or {}
+    else:
+        cfg = {}
+
+    # Apply env var overrides (scalars only)
+    for key, value in os.environ.items():
+        if not key.startswith(_PREFIX):
+            continue
+        path = key[len(_PREFIX):].lower().split("__")
+        _set_nested(cfg, path, value)
+
+    # Validate required keys
+    for dotted in REQUIRED_KEYS:
+        parts = dotted.split(".")
+        node = cfg
+        for part in parts:
+            if not isinstance(node, dict) or part not in node:
+                raise ValueError(
+                    f"Required config key '{dotted}' is missing. "
+                    f"Set it in config.yaml or via env var "
+                    f"FABLEDNETMON_{dotted.upper().replace('.', '__')}"
+                )
+            node = node[part]
+
+    return cfg
+
+
+def _set_nested(cfg: dict, path: list[str], value: str) -> None:
+    """Set a scalar value at a nested dict path. Skips if target is a list."""
+    node = cfg
+    for part in path[:-1]:
+        node = node.setdefault(part, {})
+    key = path[-1]
+    existing = node.get(key)
+    if isinstance(existing, list):
+        return  # never override list values
+    node[key] = value

tests/test_config.py

@@ -0,0 +1,35 @@
+import os
+import textwrap
+import pytest
+from fablednetmon.config import load_config
+
+def test_load_minimal_config(tmp_path):
+    cfg_file = tmp_path / "config.yaml"
+    cfg_file.write_text(textwrap.dedent("""\
+        database:
+          url: postgresql+asyncpg://user:pass@localhost/fablednetmon
+        secret_key: test-secret
+    """))
+    cfg = load_config(cfg_file)
+    assert cfg["database"]["url"] == "postgresql+asyncpg://user:pass@localhost/fablednetmon"
+    assert cfg["secret_key"] == "test-secret"
+
+def test_env_var_overrides_scalar(tmp_path, monkeypatch):
+    cfg_file = tmp_path / "config.yaml"
+    cfg_file.write_text("database:\n  url: original\nsecret_key: s\n")
+    monkeypatch.setenv("FABLEDNETMON_DATABASE__URL", "overridden")
+    cfg = load_config(cfg_file)
+    assert cfg["database"]["url"] == "overridden"
+
+def test_env_var_does_not_override_list(tmp_path, monkeypatch):
+    cfg_file = tmp_path / "config.yaml"
+    cfg_file.write_text("smtp:\n  recipients:\n    - a@b.com\nsecret_key: s\ndatabase:\n  url: x\n")
+    monkeypatch.setenv("FABLEDNETMON_SMTP__RECIPIENTS", "bad")
+    cfg = load_config(cfg_file)
+    assert cfg["smtp"]["recipients"] == ["a@b.com"]
+
+def test_missing_required_key_raises(tmp_path):
+    cfg_file = tmp_path / "config.yaml"
+    cfg_file.write_text("{}")
+    with pytest.raises(ValueError, match="secret_key"):
+        load_config(cfg_file)