Split logs by severity

Author: jcastro

src/stopliga/logging_utils.py

@@ -6,6 +6,7 @@
 from contextvars import ContextVar
 import json
 import logging
+import sys
 from typing import Any
 
 
@@ -159,7 +160,7 @@ def _visible_fields(event: str | None, fields: dict[str, Any], levelno: int) ->
 
 
 class KeyValueFormatter(logging.Formatter):
-    """Simple key=value formatter that stays readable in Docker stdout."""
+    """Simple key=value formatter that stays readable in container logs."""
 
     def format(self, record: logging.LogRecord) -> str:
         fields = getattr(record, "fields", {})
@@ -193,12 +194,20 @@ def format(self, record: logging.LogRecord) -> str:
 def configure_logging(level_name: str) -> None:
     """Configure application-wide logging."""
 
-    handler = logging.StreamHandler()
-    handler.setFormatter(KeyValueFormatter())
+    formatter = KeyValueFormatter()
+    stdout_handler = logging.StreamHandler(sys.stdout)
+    stdout_handler.setFormatter(formatter)
+    stdout_handler.addFilter(lambda record: record.levelno < logging.ERROR)
+
+    stderr_handler = logging.StreamHandler(sys.stderr)
+    stderr_handler.setFormatter(formatter)
+    stderr_handler.addFilter(lambda record: record.levelno >= logging.ERROR)
+
     root = logging.getLogger()
     root.handlers.clear()
     root.setLevel(getattr(logging, level_name.upper(), logging.INFO))
-    root.addHandler(handler)
+    root.addHandler(stdout_handler)
+    root.addHandler(stderr_handler)
 
 
 def log_event(logger: logging.Logger, level: int, event: str, **fields: Any) -> None:

tests/test_logging.py

@@ -1,9 +1,11 @@
 from __future__ import annotations
 
+import io
 import logging
 import unittest
 from pathlib import Path
 import sys
+from unittest.mock import patch
 
 
 ROOT = Path(__file__).resolve().parents[1]
@@ -12,7 +14,7 @@
 if str(SRC) not in sys.path:
     sys.path.insert(0, str(SRC))
 
-from stopliga.logging_utils import KeyValueFormatter  # noqa: E402
+from stopliga.logging_utils import KeyValueFormatter, configure_logging  # noqa: E402
 
 
 class LoggingFormatterTests(unittest.TestCase):
@@ -91,3 +93,38 @@ def test_missing_vpn_client_network_log_includes_docs_url(self) -> None:
             'docs_url="https://github.com/jcastro/stopliga/blob/main/README.md#vpn-client-network-required"',
             output,
         )
+
+
+class LoggingConfigurationTests(unittest.TestCase):
+    def setUp(self) -> None:
+        root = logging.getLogger()
+        self._original_handlers = list(root.handlers)
+        self._original_level = root.level
+
+    def tearDown(self) -> None:
+        root = logging.getLogger()
+        root.handlers.clear()
+        root.handlers.extend(self._original_handlers)
+        root.setLevel(self._original_level)
+
+    def test_logs_are_split_between_stdout_and_stderr_by_severity(self) -> None:
+        stdout = io.StringIO()
+        stderr = io.StringIO()
+
+        with patch("sys.stdout", stdout), patch("sys.stderr", stderr):
+            configure_logging("DEBUG")
+            logger = logging.getLogger("stopliga.test")
+            logger.info("info_message")
+            logger.warning("warning_message")
+            logger.error("error_message")
+
+        stdout_output = stdout.getvalue()
+        stderr_output = stderr.getvalue()
+
+        self.assertIn("INFO info_message", stdout_output)
+        self.assertIn("WARNING warning_message", stdout_output)
+        self.assertNotIn("ERROR error_message", stdout_output)
+
+        self.assertIn("ERROR error_message", stderr_output)
+        self.assertNotIn("INFO info_message", stderr_output)
+        self.assertNotIn("WARNING warning_message", stderr_output)