Support .info("foo %s", "bar") formatting in logger (#426)

Author: huonw

aws_lambda_powertools/logging/formatter.py

@@ -176,6 +176,9 @@ def _extract_log_message(self, log_record: logging.LogRecord) -> Union[Dict[str,
         if isinstance(message, dict):
             return message
 
+        if log_record.args:  # logger.info("foo %s", "bar") requires formatting
+            return log_record.getMessage()
+
         if isinstance(message, str):  # could be a JSON string
             try:
                 message = self.json_deserializer(message)

tests/functional/test_logger_powertools_formatter.py

@@ -275,3 +275,16 @@ def test_logging_various_primitives(stdout, service_name, message):
     # THEN it should raise no serialization/deserialization error
     logger.info(message)
     json.loads(stdout.getvalue())
+
+
+def test_log_formatting(stdout, service_name):
+    # GIVEN a logger with default settings
+    logger = Logger(service=service_name, stream=stdout)
+
+    # WHEN logging a message with formatting
+    logger.info('["foo %s %d %s", null]', "bar", 123, [1, None])
+
+    log_dict: dict = json.loads(stdout.getvalue())
+
+    # THEN the formatting should be applied (NB. this is valid json, but hasn't be parsed)
+    assert log_dict["message"] == '["foo bar 123 [1, None]", null]'