Add support for default key/value pairs; Fix duplicate keys issue

Author: jteppinette

README.md

@@ -208,6 +208,25 @@ logging.basicConfig(handlers=[handler])
 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:
+
+```py
+_record_factory = logging.getLogRecordFactory()
+
+def record_factory(*args, **kwargs):
+    record = _record_factory(*args, **kwargs)
+    record.trace_id = 123
+    return record
+
+logging.setLogRecordFactory(record_factory)
+```
+
+This will cause all logs to have the `trace_id=123` pair regardless of including
+`trace_id` in keys or manually adding `trace_id` to the `extra` parameter or the `msg` object.
+
 ## Development
 
 ### Required Software

src/logfmter/formatter.py

@@ -161,8 +161,9 @@ def format(self, record: logging.LogRecord) -> str:
                 self.normalize_key(key): value for key, value in record.msg.items()
             }
         else:
-            extra = self.get_extra(record)
-            params = {"msg": record.getMessage(), **extra}
+            params = {"msg": record.getMessage()}
+
+        params.update(self.get_extra(record))
 
         tokens = []
 
@@ -181,6 +182,11 @@ def format(self, record: logging.LogRecord) -> str:
             if key in self.mapping:
                 attribute = self.mapping[key]
 
+            # If this key is in params, then skip it, because it was manually passed in
+            # will be added via the params system.
+            if attribute in params:
+                continue
+
             # If the attribute doesn't exist on the log record, then skip it.
             if not hasattr(record, attribute):
                 continue

tests/test_formatter.py

@@ -245,3 +245,24 @@ def test_format_datefmt():
 
     asctime = re.search(r'asctime="(.*)"', value).group(1)
     datetime.strptime(asctime, " %H ")
+
+
+@pytest.mark.parametrize(
+    "record",
+    [
+        {"msg": "alpha", "levelname": "INFO"},
+        {"msg": {"msg": "alpha"}, "levelname": "INFO"},
+    ],
+)
+def test_extra_keys(record):
+    """
+    When attributes are added directly to the `logging.LogRecord` object, they should
+    be included in the output and not be duplicated, regardless of a str or dict based
+    msg object.
+    """
+    record = logging.makeLogRecord(record)
+    record.attr = "value"
+
+    assert (
+        Logfmter(keys=["at", "attr"]).format(record) == "at=INFO msg=alpha attr=value"
+    )