python 60 lines · 7 steps

A hot-reloading config file watcher

A Settings class loads JSON config and watches its file to reload automatically on change, guarded by a lock for thread safety.

Explained by highlit
Intermediate file-watching thread-safety hot-reload configuration observer-pattern
1import json
2import logging
3import threading
4from pathlib import Path
5 
6from watchdog.events import FileSystemEventHandler
7from watchdog.observers import Observer
8 
9logger = logging.getLogger(__name__)
10 
11 
12class Settings:
13 def __init__(self, path):
14 self._path = Path(path).resolve()
15 self._lock = threading.RLock()
16 self._values = {}
17 self._reload()
18 
19 self._observer = Observer()
20 self._observer.schedule(
21 _ConfigReloadHandler(self),
22 str(self._path.parent),
23 recursive=False,
24 )
25 self._observer.start()
26 
27 def get(self, key, default=None):
28 with self._lock:
29 return self._values.get(key, default)
30 
31 def as_dict(self):
32 with self._lock:
33 return dict(self._values)
34 
35 def _reload(self):
36 try:
37 raw = self._path.read_text(encoding="utf-8")
38 parsed = json.loads(raw)
39 except (OSError, json.JSONDecodeError) as exc:
40 logger.warning("Ignoring bad config %s: %s", self._path, exc)
41 return
42 
43 with self._lock:
44 self._values = parsed
45 logger.info("Reloaded config from %s", self._path)
46 
47 def stop(self):
48 self._observer.stop()
49 self._observer.join()
50 
51 
52class _ConfigReloadHandler(FileSystemEventHandler):
53 def __init__(self, settings):
54 self._settings = settings
55 
56 def on_modified(self, event):
57 if not event.is_directory and Path(event.src_path).resolve() == self._settings._path:
58 self._settings._reload()
59 
60 on_created = on_modified
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1A reentrant lock keeps reads and writes consistent while a background thread swaps config in.
  2. 2Filesystem watchers let you reload config without restarting the process.
  3. 3Failing safe on parse errors keeps a running app alive despite a broken config file.

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