feat(stdlib): add add_syslog_level processor

src/structlog/stdlib.py

@@ -60,6 +60,7 @@
     "add_log_level",
     "add_log_level_number",
     "add_logger_name",
+    "add_syslog_level",
     "filter_by_level",
     "get_logger",
     "recreate_defaults",
@@ -865,6 +866,38 @@ def add_log_level_number(
     return event_dict
 
 
+# syslog(3) / logging.handlers.SysLogHandler priority values (0-7).
+_NAME_TO_SYSLOG_LEVEL = {
+    "critical": 2,  # LOG_CRIT
+    "error": 3,  # LOG_ERR
+    "exception": 3,  # LOG_ERR
+    "warn": 4,  # LOG_WARNING
+    "warning": 4,  # LOG_WARNING
+    "info": 6,  # LOG_INFO
+    "debug": 7,  # LOG_DEBUG
+    "notset": 7,  # LOG_DEBUG
+}
+
+
+def add_syslog_level(
+    logger: logging.Logger, method_name: str, event_dict: EventDict
+) -> EventDict:
+    """
+    Add the syslog severity (0–7) to the event dict under ``syslog_level``.
+
+    Useful when feeding plain stdout/stderr to *systemd* with
+    ``SyslogLevelPrefix=yes`` (see :manpage:`systemd.exec(5)` / *sd-daemon*),
+    or when writing processors that talk to the stdlib ``syslog`` module.
+
+    The mapping follows :class:`logging.handlers.SysLogHandler` and
+    :manpage:`syslog(3)`.
+
+    .. versionadded:: NEXT
+    """
+    event_dict["syslog_level"] = _NAME_TO_SYSLOG_LEVEL[method_name]
+    return event_dict
+
+
 def add_logger_name(
     logger: logging.Logger, method_name: str, event_dict: EventDict
 ) -> EventDict:

tests/test_stdlib.py

@@ -43,6 +43,7 @@
     add_log_level,
     add_log_level_number,
     add_logger_name,
+    add_syslog_level,
     filter_by_level,
     get_logger,
     recreate_defaults,
@@ -530,6 +531,29 @@ def test_log_level_number_added(self, level, number):
         assert number == event_dict["level_number"]
 
 
+class TestAddSyslogLevel:
+    @pytest.mark.parametrize(
+        ("level", "priority"),
+        [
+            ("critical", 2),
+            ("error", 3),
+            ("exception", 3),
+            ("warn", 4),
+            ("warning", 4),
+            ("info", 6),
+            ("debug", 7),
+            ("notset", 7),
+        ],
+    )
+    def test_syslog_level_added(self, level, priority):
+        """
+        The syslog severity (0-7) is added to the event dict (#249).
+        """
+        event_dict = add_syslog_level(None, level, {})
+
+        assert priority == event_dict["syslog_level"]
+
+
 class TestAddLogLevel:
     def test_log_level_added(self):
         """