Add optional default_extra param to formatter initializer.

README.md

@@ -213,18 +213,13 @@ logging.error({"example": True}) # at=ERROR example=yes
 
 **Default Key/Value Pairs**
 
-Instead of providing key/value pairs at each log call, you can override
-the log record factory to provide defaults:
+Instead of providing key/value pairs at each log call, you can setup
+the formatter to provide defaults:
 
 ```py
-_record_factory = logging.getLogRecordFactory()
-
-def record_factory(*args, **kwargs):
-    record = _record_factory(*args, **kwargs)
-    record.trace_id = 123
-    return record
+from logfmter import Logfmter
 
-logging.setLogRecordFactory(record_factory)
+formatter = Logfmter(default_extra={"trace_id": 123})
 ```
 
 This will cause all logs to have the `trace_id=123` pair regardless of including

src/logfmter/formatter.py

@@ -143,12 +143,18 @@ def __init__(
         self,
         keys: List[str] = ["at"],
         mapping: Dict[str, str] = {"at": "levelname"},
+        default_extra: Optional[Dict[str, str]] = None,
         datefmt: Optional[str] = None,
     ):
         self.keys = [self.normalize_key(key) for key in keys]
         self.mapping = {
             self.normalize_key(key): value for key, value in mapping.items()
         }
+        self.default_extra = (
+            {self.normalize_key(key): value for key, value in default_extra.items()}
+            if default_extra is not None
+            else {}
+        )
         self.datefmt = datefmt
 
     def format(self, record: logging.LogRecord) -> str:
@@ -163,6 +169,7 @@ def format(self, record: logging.LogRecord) -> str:
         else:
             params = {"msg": record.getMessage()}
 
+        params.update(self.default_extra)
         params.update(self.get_extra(record))
 
         tokens = []

tests/test_formatter.py

@@ -266,3 +266,34 @@ def test_extra_keys(record):
     assert (
         Logfmter(keys=["at", "attr"]).format(record) == "at=INFO msg=alpha attr=value"
     )
+
+
+@pytest.mark.parametrize(
+    "default_extra,record,expected",
+    [
+        # When a parameter is present in the default_extra it should be emitted
+        (
+            {"extra1": "from_formatter"},
+            {"levelname": "INFO", "msg": "test"},
+            "at=INFO msg=test extra1=from_formatter",
+        ),
+        # When a parameter is present in the default_extra and also in the record
+        # the record value should take precedence
+        (
+            {"extra1": "from_formatter"},
+            {"levelname": "INFO", "extra1": "from_record", "msg": "test"},
+            "at=INFO msg=test extra1=from_record",
+        ),
+    ],
+)
+def test_format_default_extra(default_extra, record, expected):
+    """
+    If the `default_extra` parameter is specified in the formatter then all log
+    records must include such parameters.
+    """
+
+    # Generate a real `logging.LogRecord` from the provided dictionary.
+    record = logging.makeLogRecord(record)
+    formatter = Logfmter(keys=["at", "extra1"], default_extra=default_extra)
+
+    assert formatter.format(record) == expected