feat(logging): allow for custom json order

  Changes:
  * add `format_key` to allow for a custom order
  * remove unessarly verbose `json_formatter` with just `str`
  * update tests

Author: Michael Brewer

aws_lambda_powertools/logging/formatter.py

@@ -1,27 +1,5 @@
 import json
 import logging
-from typing import Any
-
-
-def json_formatter(unserializable_value: Any):
-    """JSON custom serializer to cast unserializable values to strings.
-
-    Example
-    -------
-
-    **Serialize unserializable value to string**
-
-        class X: pass
-        value = {"x": X()}
-
-        json.dumps(value, default=json_formatter)
-
-    Parameters
-    ----------
-    unserializable_value: Any
-        Python object unserializable by JSON
-    """
-    return str(unserializable_value)
 
 
 class JsonFormatter(logging.Formatter):
@@ -44,17 +22,15 @@ def __init__(self, **kwargs):
 
         Other kwargs are used to specify log field format strings.
         """
-        self.default_json_formatter = kwargs.pop("json_default", json_formatter)
+        self.default_json_formatter = kwargs.pop("json_default", str)
         datefmt = kwargs.pop("datefmt", None)
 
         super(JsonFormatter, self).__init__(datefmt=datefmt)
         self.reserved_keys = ["timestamp", "level", "location"]
-        self.format_dict = {
-            "timestamp": "%(asctime)s",
-            "level": "%(levelname)s",
-            "location": "%(funcName)s:%(lineno)d",
-            "message": None,
-        }
+        self.format_dict = dict.fromkeys(kwargs.pop("format_key", ["level", "location", "message", "timestamp"]))
+        self.format_dict.update(
+            {"level": "%(levelname)s", "location": "%(funcName)s:%(lineno)d", "timestamp": "%(asctime)s"}
+        )
         self.format_dict.update(kwargs)
 
     def update_formatter(self, **kwargs):

tests/functional/test_aws_lambda_logging.py

@@ -127,5 +127,18 @@ def test_log_dict_key_seq(stdout):
 
     log_dict: dict = json.loads(stdout.getvalue())
 
-    # THEN the key sequence should be `timestamp,level,location,message`
-    assert ",".join(list(log_dict.keys())[:4]) == "timestamp,level,location,message"
+    # THEN the key sequence should be `level,location,message,timestamp`
+    assert ",".join(list(log_dict.keys())[:4]) == "level,location,message,timestamp"
+
+
+def test_log_dict_key_custom_seq(stdout):
+    # GIVEN any logger configuration
+    logger = Logger(level="INFO", stream=stdout, another="xxx", format_key=["message"])
+
+    # WHEN logging a message
+    logger.info("Message")
+
+    log_dict: dict = json.loads(stdout.getvalue())
+
+    # THEN the key sequence should be `level,location,message,timestamp`
+    assert ",".join(list(log_dict.keys())[:4]) == "message,level,location,timestamp"