fix: Logger string interpolation to main reserved keys only

aws_lambda_powertools/helper/models.py

@@ -65,7 +65,7 @@ def build_lambda_context_model(context: object) -> LambdaContextModel:
 
     context = {
         "function_name": context.function_name,
-        "function_memory_size": str(context.memory_limit_in_mb),
+        "function_memory_size": context.memory_limit_in_mb,
         "function_arn": context.invoked_function_arn,
         "function_request_id": context.aws_request_id,
     }

aws_lambda_powertools/logging/logger.py

@@ -62,6 +62,7 @@ def __init__(self, **kwargs):
         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",
@@ -76,10 +77,12 @@ def format(self, record):  # noqa: A003
 
         log_dict = {}
         for key, value in self.format_dict.items():
-            if value:
+            if value and key in self.reserved_keys:
                 # converts default logging expr to its record value
                 # e.g. '%(asctime)s' to '2020-04-24 09:35:40,698'
                 log_dict[key] = value % record_dict
+            else:
+                log_dict[key] = value
 
         if isinstance(record_dict["msg"], dict):
             log_dict["message"] = record_dict["msg"]
@@ -149,20 +152,19 @@ def handler(evt, ctx):
     raise DeprecationWarning("Use Logger instead - This method will be removed when GA")
 
 
-def _is_cold_start() -> str:
-    """Verifies whether is cold start and return a string used for struct logging
+def _is_cold_start() -> bool:
+    """Verifies whether is cold start
 
     Returns
     -------
-    str
-        lower case bool as a string
-        aws_lambda_logging doesn't support bool; cast cold start value to string
+    bool
+        cold start bool value
     """
-    cold_start = "false"
+    cold_start = False
 
     global is_cold_start
     if is_cold_start:
-        cold_start = str(is_cold_start).lower()
+        cold_start = is_cold_start
         is_cold_start = False
 
     return cold_start

tests/functional/test_logger.py

@@ -228,12 +228,12 @@ def handler(event, context):
     first_log, second_log, third_log, fourth_log = logs
 
     # First execution
-    assert "true" == first_log["cold_start"]
-    assert "true" == second_log["cold_start"]
+    assert first_log["cold_start"] is True
+    assert second_log["cold_start"] is True
 
     # Second execution
-    assert "false" == third_log["cold_start"]
-    assert "false" == fourth_log["cold_start"]
+    assert third_log["cold_start"] is False
+    assert fourth_log["cold_start"] is False
 
 
 def test_log_metric(capsys):