fix: initial work on handler fix

Author: heitorlessa

aws_lambda_powertools/logging/logger.py

@@ -233,14 +233,14 @@ def __init__(
         self.child = child
         self.logger_formatter = logger_formatter
         self._stream = stream or sys.stdout
-        self.logger_handler = logger_handler or logging.StreamHandler(self._stream)
         self.log_uncaught_exceptions = log_uncaught_exceptions
 
         self._is_deduplication_disabled = resolve_truthy_env_var_choice(
             env=os.getenv(constants.LOGGER_LOG_DEDUPLICATION_ENV, "false"),
         )
         self._default_log_keys = {"service": self.service, "sampling_rate": self.sampling_rate}
         self._logger = self._get_logger()
+        self.logger_handler = self._get_handler(custom_handler=logger_handler)
 
         # NOTE: This is primarily to improve UX, so IDEs can autocomplete LambdaPowertoolsFormatter options
         # previously, we masked all of them as kwargs thus limiting feature discovery
@@ -279,6 +279,15 @@ def _get_logger(self) -> logging.Logger:
 
         return logging.getLogger(logger_name)
 
+    def _get_handler(self, custom_handler: logging.Handler):
+        if self.child:
+            return self._logger.parent.powertools_handler
+
+        if custom_handler:
+            return custom_handler
+
+        return logging.StreamHandler(self._stream)
+
     def _init_logger(
         self,
         formatter_options: Optional[Dict] = None,
@@ -317,6 +326,7 @@ def _init_logger(
         # std logging will return the same Logger with our attribute if name is reused
         logger.debug(f"Marking logger {self.service} as preconfigured")
         self._logger.init = True  # type: ignore[attr-defined]
+        self._logger.powertools_handler = self.logger_handler  # type: ignore[attr-defined]
 
     def _configure_sampling(self) -> None:
         """Dynamically set log level based on sampling rate
@@ -672,10 +682,7 @@ def removeFilter(self, filter: logging._FilterType) -> None:  # noqa: A002 # fil
     @property
     def registered_handler(self) -> logging.Handler:
         """Convenience property to access the first logger handler"""
-        # We ignore mypy here because self.child encodes whether or not self._logger.parent is
-        # None, mypy can't see this from context but we can
-        handlers = self._logger.parent.handlers if self.child else self._logger.handlers  # type: ignore[union-attr]
-        return handlers[0]
+        return self.logger_handler
 
     @property
     def registered_formatter(self) -> BasePowertoolsFormatter:

tests/functional/test_logger.py

@@ -1176,3 +1176,40 @@ def test_logger_json_unicode(stdout, service_name):
 
     assert log["message"] == non_ascii_chars
     assert log[japanese_field] == japanese_string
+
+
+def test_logger_registered_handler_is_custom_handler(service_name):
+    # GIVEN a library or environment pre-setup a logger for us using the same name (see #4277)
+    class ForeignHandler(logging.StreamHandler): ...
+
+    foreign_handler = ForeignHandler()
+    logging.getLogger(service_name).addHandler(foreign_handler)
+
+    # WHEN Logger init with a custom handler
+    custom_handler = logging.StreamHandler()
+    logger = Logger(service=service_name, logger_handler=custom_handler)
+
+    # THEN registered handler should always return what we provided
+    assert logger.registered_handler is not foreign_handler
+    assert logger.registered_handler is custom_handler
+    assert logger.logger_handler is custom_handler
+
+
+def test_child_logger_registered_handler_is_custom_handler(service_name):
+    # GIVEN
+    class ForeignHandler(logging.StreamHandler): ...
+
+    foreign_handler = ForeignHandler()
+    logging.getLogger(service_name).addHandler(foreign_handler)
+
+    custom_handler = logging.StreamHandler()
+    custom_handler.name = "CUSTOM HANDLER"
+    parent = Logger(service=service_name, logger_handler=custom_handler)
+
+    # WHEN a child Logger init
+    child = Logger(service=service_name, child=True)
+
+    # THEN child registered handler should always return what we provided in the parent
+    assert child.registered_handler is not foreign_handler
+    assert child.registered_handler is custom_handler
+    assert child.registered_handler is parent.registered_handler