fix(logger): reverting logger child modification

aws_lambda_powertools/logging/constants.py

@@ -1,5 +1,5 @@
 # logger.powertools_handler is set with Powertools Logger handler; useful when there are many handlers
 LOGGER_ATTRIBUTE_POWERTOOLS_HANDLER = "powertools_handler"
-# logger.init attribute is set when Logger has been configured 
+# logger.init attribute is set when Logger has been configured
 LOGGER_ATTRIBUTE_PRECONFIGURED = "init"
 LOGGER_ATTRIBUTE_HANDLER = "logger_handler"

aws_lambda_powertools/logging/exceptions.py

@@ -1,6 +1,2 @@
 class InvalidLoggerSamplingRateError(Exception):
     pass
-
-
-class OrphanedChildLoggerError(Exception):
-    pass

aws_lambda_powertools/logging/logger.py

@@ -19,13 +19,10 @@
     Optional,
     TypeVar,
     Union,
-    cast,
     overload,
 )
 
 from aws_lambda_powertools.logging.constants import (
-    LOGGER_ATTRIBUTE_HANDLER,
-    LOGGER_ATTRIBUTE_POWERTOOLS_HANDLER,
     LOGGER_ATTRIBUTE_PRECONFIGURED,
 )
 from aws_lambda_powertools.shared import constants
@@ -37,7 +34,7 @@
 from aws_lambda_powertools.utilities import jmespath_utils
 
 from ..shared.types import AnyCallableT
-from .exceptions import InvalidLoggerSamplingRateError, OrphanedChildLoggerError
+from .exceptions import InvalidLoggerSamplingRateError
 from .filters import SuppressFilter
 from .formatter import (
     RESERVED_FORMATTER_CUSTOM_KEYS,
@@ -239,14 +236,14 @@ 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 = logger_handler or self._get_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
@@ -285,18 +282,6 @@ def _get_logger(self) -> logging.Logger:
 
         return logging.getLogger(logger_name)
 
-    def _get_handler(self) -> logging.Handler:
-        # is a logger handler already configured?
-        if getattr(self, LOGGER_ATTRIBUTE_HANDLER, None):
-            return self.logger_handler
-
-        # for children, use parent's handler
-        if self.child:
-            return getattr(self._logger.parent, LOGGER_ATTRIBUTE_POWERTOOLS_HANDLER, None)  # type: ignore[return-value] # always checked in formatting
-
-        # otherwise, create a new stream handler (first time init)
-        return logging.StreamHandler(self._stream)
-
     def _init_logger(
         self,
         formatter_options: Optional[Dict] = None,
@@ -335,7 +320,6 @@ 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
@@ -691,20 +675,15 @@ 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"""
-        return self._get_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]
 
     @property
     def registered_formatter(self) -> BasePowertoolsFormatter:
         """Convenience property to access the first logger formatter"""
-        handler = self.registered_handler
-        if handler is None:
-            raise OrphanedChildLoggerError(
-                "Orphan child loggers cannot append nor remove keys until a parent is initialized first. "
-                "To solve this issue, you can A) make sure a parent logger is initialized first, or B) move append/remove keys operations to a later stage."  # noqa: E501
-                "Reference: https://docs.powertools.aws.dev/lambda/python/latest/core/logger/#reusing-logger-across-your-code",
-            )
-
-        return cast(BasePowertoolsFormatter, handler.formatter)
+        return self.registered_handler.formatter  # type: ignore[return-value]
 
     @property
     def log_level(self) -> int:

docs/core/logger.md

@@ -496,6 +496,16 @@ Notice in the CloudWatch Logs output how `payment_id` appears as expected when l
     ```json hl_lines="12"
     --8<-- "examples/logger/src/logger_reuse_output.json"
     ```
+???+ note "Note: About Child Loggers"
+    Coming from standard library, you might be used to use `logging.getLogger(__name__)`. This will create a new instance of a Logger with a different name.
+
+    In Powertools, you can have the same effect by using `child=True` parameter: `Logger(child=True)`. This creates a new Logger instance named after `service.<module>`. All state changes will be propagated bi-directionally between Child and Parent.
+
+    For that reason, there could be side effects depending on the order the Child Logger is instantiated, because Child Loggers don't have a handler.
+
+    For example, if you instantiated a Child Logger and immediately used `logger.append_keys/remove_keys/set_correlation_id` to update logging state, this might fail if the Parent Logger wasn't instantiated.
+
+    In this scenario, you can either ensure any calls manipulating state are only called when a Parent Logger is instantiated (example above), or refrain from using `child=True` parameter altogether.
 
 ### Sampling debug logs
 
@@ -571,56 +581,48 @@ You can use import and use them as any other Logger formatter via `logger_format
 
 ### Migrating from other Loggers
 
-If you're migrating from other Loggers, there are few key points to be aware of: [Service parameter](#the-service-parameter), [Child Loggers](#child-loggers), [Overriding Log records](#overriding-log-records), and [Logging exceptions](#logging-exceptions).
+If you're migrating from other Loggers, there are few key points to be aware of: [Service parameter](#the-service-parameter), [Inheriting Loggers](#inheriting-loggers), [Overriding Log records](#overriding-log-records), and [Logging exceptions](#logging-exceptions).
 
 #### The service parameter
 
 Service is what defines the Logger name, including what the Lambda function is responsible for, or part of (e.g payment service).
 
 For Logger, the `service` is the logging key customers can use to search log operations for one or more functions - For example, **search for all errors, or messages like X, where service is payment**.
 
-#### Child Loggers
+#### Inheriting Loggers
 
-<center>
-```mermaid
-stateDiagram-v2
-    direction LR
-    Parent: Logger()
-    Child: Logger(child=True)
-    Parent --> Child: bi-directional updates
-    Note right of Child
-        Both have the same service
-    end note
-```
-</center>
+??? tip "Tip: Prefer [Logger Reuse feature](#reusing-logger-across-your-code) over inheritance unless strictly necessary, [see caveats.](#reusing-logger-across-your-code)"
 
-For inheritance, Logger uses `child` parameter to ensure we don't compete with its parents config. We name child Loggers following Python's convention: _`{service}`.`{filename}`_.
+> Python Logging hierarchy happens via the dot notation: `service`, `service.child`, `service.child_2`
+For inheritance, Logger uses a `child=True` parameter along with `service` being the same value across Loggers.
 
-Changes are bidirectional between parents and loggers. That is, appending a key in a child or parent will ensure both have them. This means, having the same `service` name is important when instantiating them.
+For child Loggers, we introspect the name of your module where `Logger(child=True, service="name")` is called, and we name your Logger as **{service}.{filename}**.
 
-=== "logging_inheritance_good.py"
+???+ danger
+    A common issue when migrating from other Loggers is that `service` might be defined in the parent Logger (no child param), and not defined in the child Logger:
+
+=== "logging_inheritance_bad.py"
 
     ```python hl_lines="1 9"
-    --8<-- "examples/logger/src/logging_inheritance_good.py"
+    --8<-- "examples/logger/src/logging_inheritance_bad.py"
     ```
 
 === "logging_inheritance_module.py"
-
     ```python hl_lines="1 9"
     --8<-- "examples/logger/src/logging_inheritance_module.py"
     ```
 
-There are two important side effects when using child loggers:
+In this case, Logger will register a Logger named `payment`, and a Logger named `service_undefined`. The latter isn't inheriting from the parent, and will have no handler, resulting in no message being logged to standard output.
 
-1. **Service name mismatch**. Logging messages will be dropped as child loggers don't have logging handlers.
-    * Solution: use `POWERTOOLS_SERVICE_NAME` env var. Alternatively, use the same service explicit value.
-2. **Changing state before a parent instantiate**. Using `logger.append_keys` or `logger.remove_keys` without a parent Logger will lead to `OrphanedChildLoggerError` exception.
-    * Solution: always initialize parent Loggers first. Alternatively, move calls to `append_keys`/`remove_keys` from the child at a later stage.
+???+ tip
+    This can be fixed by either ensuring both has the `service` value as `payment`, or simply use the environment variable `POWERTOOLS_SERVICE_NAME` to ensure service value will be the same across all Loggers when not explicitly set.
 
-=== "logging_inheritance_bad.py"
+Do this instead:
+
+=== "logging_inheritance_good.py"
 
     ```python hl_lines="1 9"
-    --8<-- "examples/logger/src/logging_inheritance_bad.py"
+    --8<-- "examples/logger/src/logging_inheritance_good.py"
     ```
 
 === "logging_inheritance_module.py"

tests/functional/test_logger.py

@@ -17,7 +17,7 @@
 
 from aws_lambda_powertools import Logger, Tracer, set_package_logger_handler
 from aws_lambda_powertools.logging import correlation_paths
-from aws_lambda_powertools.logging.exceptions import InvalidLoggerSamplingRateError, OrphanedChildLoggerError
+from aws_lambda_powertools.logging.exceptions import InvalidLoggerSamplingRateError
 from aws_lambda_powertools.logging.formatter import (
     BasePowertoolsFormatter,
     LambdaPowertoolsFormatter,
@@ -1176,63 +1176,3 @@ 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
-    assert logger.handlers == [foreign_handler, 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
-
-
-def test_logger_handler_is_created_despite_env_pre_setup(service_name):
-    # GIVEN a library or environment pre-setup a logger for us using the same name
-    environment_handler = logging.StreamHandler()
-    logging.getLogger(service_name).addHandler(environment_handler)
-
-    # WHEN Logger init without a custom handler
-    logger = Logger(service=service_name)
-
-    # THEN registered handler should be Powertools default handler, not env
-    assert logger.registered_handler is not environment_handler
-
-
-def test_child_logger_append_keys_before_parent(stdout, service_name):
-    # GIVEN a child Logger is initialized before its/without parent
-    child = Logger(stream=stdout, service=service_name, child=True)
-
-    # WHEN a child Logger appends a key
-    # THEN it will raise an AttributeError
-    with pytest.raises(OrphanedChildLoggerError):
-        child.append_keys(customer_id="value")