feat: add structured JSON logging

New JsonFormatter emits one JSON object per log line (JSONL). Activated
via format = "json" in [logging] config. Default remains "text".

src/derp/cli.py

@@ -10,6 +10,7 @@ import sys
 from derp import __version__
 from derp.bot import Bot
 from derp.config import resolve_config
+from derp.log import JsonFormatter
 from derp.plugin import PluginRegistry
 
 LOG_FORMAT = "%(asctime)s %(levelname)-5s %(name)s  %(message)s"
@@ -60,11 +61,17 @@ def main(argv: list[str] | None = None) -> int:
     parser = build_parser()
     args = parser.parse_args(argv)
 
-    level = logging.DEBUG if args.verbose else logging.INFO
-    logging.basicConfig(format=LOG_FORMAT, datefmt=LOG_DATE, level=level)
-
     config = resolve_config(args.config)
 
+    level = logging.DEBUG if args.verbose else logging.INFO
+    log_fmt = config.get("logging", {}).get("format", "text")
+    if log_fmt == "json":
+        handler = logging.StreamHandler()
+        handler.setFormatter(JsonFormatter())
+        logging.basicConfig(handlers=[handler], level=level)
+    else:
+        logging.basicConfig(format=LOG_FORMAT, datefmt=LOG_DATE, level=level)
+
     log = logging.getLogger("derp")
     log.info("derp %s starting", __version__)
 

src/derp/log.py

@@ -0,0 +1,21 @@
+"""Structured logging formatters."""
+
+from __future__ import annotations
+
+import json
+import logging
+
+
+class JsonFormatter(logging.Formatter):
+    """Emit log records as single-line JSON objects."""
+
+    def format(self, record: logging.LogRecord) -> str:
+        entry: dict[str, str] = {
+            "ts": self.formatTime(record, "%Y-%m-%dT%H:%M:%S"),
+            "level": record.levelname.lower(),
+            "logger": record.name,
+            "msg": record.getMessage(),
+        }
+        if record.exc_info and not isinstance(record.exc_info, bool) and record.exc_info[0]:
+            entry["exc"] = self.formatException(record.exc_info)
+        return json.dumps(entry, ensure_ascii=False, separators=(",", ":"))