Change stderr prints to use the logging module instead

Author: Rapptz

discord/app_commands/tree.py

@@ -23,9 +23,8 @@
 """
 
 from __future__ import annotations
+import logging
 import inspect
-import sys
-import traceback
 
 from typing import (
     Any,
@@ -79,6 +78,7 @@
 
 ClientT = TypeVar('ClientT', bound='Client')
 
+_log = logging.getLogger(__name__)
 
 def _retrieve_guild_ids(
     command: Any, guild: Optional[Snowflake] = MISSING, guilds: Sequence[Snowflake] = MISSING
@@ -775,8 +775,8 @@ async def on_error(self, interaction: Interaction, error: AppCommandError) -> No
 
         A callback that is called when any command raises an :exc:`AppCommandError`.
 
-        The default implementation prints the traceback to stderr if the command does
-        not have any error handlers attached to it.
+        The default implementation logs the exception using the library logger
+        if the command does not have any error handlers attached to it.
 
         To get the command that failed, :attr:`discord.Interaction.command` should
         be used.
@@ -794,11 +794,9 @@ async def on_error(self, interaction: Interaction, error: AppCommandError) -> No
             if command._has_any_error_handlers():
                 return
 
-            print(f'Ignoring exception in command {command.name!r}:', file=sys.stderr)
+            _log.error('Ignoring exception in command %r', command.name, exc_info=error)
         else:
-            print(f'Ignoring exception in command tree:', file=sys.stderr)
-
-        traceback.print_exception(error.__class__, error, error.__traceback__, file=sys.stderr)
+            _log.error('Ignoring exception in command tree', exc_info=error)
 
     def error(self, coro: ErrorFunc) -> ErrorFunc:
         """A decorator that registers a coroutine as a local error handler.

discord/client.py

@@ -29,7 +29,6 @@
 import logging
 import sys
 import os
-import traceback
 from typing import (
     Any,
     AsyncIterator,
@@ -519,16 +518,16 @@ async def on_error(self, event_method: str, /, *args: Any, **kwargs: Any) -> Non
 
         The default error handler provided by the client.
 
-        By default this prints to :data:`sys.stderr` however it could be
+        By default this logs to the library logger however it could be
         overridden to have a different implementation.
         Check :func:`~discord.on_error` for more details.
 
         .. versionchanged:: 2.0
 
-            ``event_method`` parameter is now positional-only.
+            ``event_method`` parameter is now positional-only
+            and instead of writing to ``sys.stderr`` it logs instead.
         """
-        print(f'Ignoring exception in {event_method}', file=sys.stderr)
-        traceback.print_exc()
+        _log.exception('Ignoring exception in %s', event_method)
 
     # hooks
 

discord/ext/commands/bot.py

@@ -31,7 +31,7 @@
 import inspect
 import importlib.util
 import sys
-import traceback
+import logging
 import types
 from typing import (
     Any,
@@ -95,6 +95,7 @@
 T = TypeVar('T')
 CFT = TypeVar('CFT', bound='CoroFunc')
 
+_log = logging.getLogger(__name__)
 
 def when_mentioned(bot: _Bot, msg: Message, /) -> List[str]:
     """A callable that implements a command prefix equivalent to being mentioned.
@@ -304,14 +305,15 @@ async def on_command_error(self, context: Context[BotT], exception: errors.Comma
 
         The default command error handler provided by the bot.
 
-        By default this prints to :data:`sys.stderr` however it could be
+        By default this logs to the library logger, however it could be
         overridden to have a different implementation.
 
         This only fires if you do not specify any listeners for command error.
 
         .. versionchanged:: 2.0
 
             ``context`` and ``exception`` parameters are now positional-only.
+            Instead of writing to ``sys.stderr`` this now uses the library logger.
         """
         if self.extra_events.get('on_command_error', None):
             return
@@ -324,8 +326,7 @@ async def on_command_error(self, context: Context[BotT], exception: errors.Comma
         if cog and cog.has_error_handler():
             return
 
-        print(f'Ignoring exception in command {context.command}:', file=sys.stderr)
-        traceback.print_exception(type(exception), exception, exception.__traceback__, file=sys.stderr)
+        _log.error('Ignoring exception in command %s', command, exc_info=exception)
 
     # global check registration
 

discord/ext/tasks/__init__.py

@@ -42,8 +42,6 @@
 import aiohttp
 import discord
 import inspect
-import sys
-import traceback
 
 from collections.abc import Sequence
 from discord.backoff import ExponentialBackoff
@@ -536,8 +534,7 @@ def is_running(self) -> bool:
 
     async def _error(self, *args: Any) -> None:
         exception: Exception = args[-1]
-        print(f'Unhandled exception in internal background task {self.coro.__name__!r}.', file=sys.stderr)
-        traceback.print_exception(type(exception), exception, exception.__traceback__, file=sys.stderr)
+        _log.error('Unhandled exception in internal background task %r.', self.coro.__name__, exc_info=exception)
 
     def before_loop(self, coro: FT) -> FT:
         """A decorator that registers a coroutine to be called before the loop starts running.
@@ -600,11 +597,15 @@ def error(self, coro: ET) -> ET:
 
         The coroutine must take only one argument the exception raised (except ``self`` in a class context).
 
-        By default this prints to :data:`sys.stderr` however it could be
+        By default this logs to the library logger however it could be
         overridden to have a different implementation.
 
         .. versionadded:: 1.4
 
+        .. versionchanged:: 2.0
+
+            Instead of writing to ``sys.stderr``, the library's logger is used.
+
         Parameters
         ------------
         coro: :ref:`coroutine <coroutine>`

docs/api.rst

@@ -195,7 +195,7 @@ overriding the specific events. For example: ::
 
 
 If an event handler raises an exception, :func:`on_error` will be called
-to handle it, which defaults to print a traceback and ignoring the exception.
+to handle it, which defaults to logging the traceback and ignoring the exception.
 
 .. warning::
 
@@ -350,19 +350,14 @@ Debug
 .. function:: on_error(event, *args, **kwargs)
 
     Usually when an event raises an uncaught exception, a traceback is
-    printed to stderr and the exception is ignored. If you want to
+    logged to stderr and the exception is ignored. If you want to
     change this behaviour and handle the exception for whatever reason
     yourself, this event can be overridden. Which, when done, will
     suppress the default action of printing the traceback.
 
     The information of the exception raised and the exception itself can
     be retrieved with a standard call to :func:`sys.exc_info`.
 
-    If you want exception to propagate out of the :class:`Client` class
-    you can define an ``on_error`` handler consisting of a single empty
-    :ref:`raise statement <py:raise>`. Exceptions raised by ``on_error`` will not be
-    handled in any way by :class:`Client`.
-
     .. note::
 
         ``on_error`` will only be dispatched to :meth:`Client.event`.
@@ -371,6 +366,10 @@ Debug
         :ref:`ext_commands_api_bot` listeners such as
         :meth:`~ext.commands.Bot.listen` or :meth:`~ext.commands.Cog.listener`.
 
+    .. versionchanged:: 2.0
+
+        The traceback is now logged rather than printed.
+
     :param event: The name of the event that raised the exception.
     :type event: :class:`str`
 

docs/migrating.rst

@@ -933,6 +933,17 @@ The following methods have been changed:
 - :meth:`Webhook.send`
 - :meth:`abc.GuildChannel.set_permissions`
 
+Logging Changes
+----------------
+
+The library now provides a default logging configuration if using :meth:`Client.run`. To disable it, pass ``None`` to the ``log_handler`` keyword parameter. Since the library now provides a default logging configuration, certain methods were changed to no longer print to :data:`sys.stderr` but use the logger instead:
+
+- :meth:`Client.on_error`
+- :meth:`discord.ext.tasks.Loop.error`
+- :meth:`discord.ext.commands.Bot.on_command_error`
+
+For more information, check :doc:`logging`.
+
 Removal of ``StoreChannel``
 -----------------------------