Print unhandled errors to stderr if logging is not configured

This should mitigate "the code doesn't work but there are no errors"
situations. Users not wanting this behaviour can configure logging
with a high-enough level that won't print anything, or set a filter.

telethon/client/updates.py

@@ -2,8 +2,11 @@ import asyncio
 import inspect
 import itertools
 import random
+import sys
 import time
+import traceback
 import typing
+import logging
 
 from .. import events, utils, errors
 from ..events.common import EventBuilder, EventCommon
@@ -281,6 +284,16 @@ class UpdateMethods:
 
     # region Private methods
 
+    def _log_exc(self: 'TelegramClient', msg, *args):
+        """
+        Log an exception, using `stderr` if `logging` is not configured.
+        """
+        if logging.root.hasHandlers():
+            self._log[__name__].exception(msg, *args)
+        else:
+            print('[ERROR/telethon]:', msg % args, file=sys.stderr)
+            traceback.print_exc()
+
     # It is important to not make _handle_update async because we rely on
     # the order that the updates arrive in to update the pts and date to
     # be always-increasing. There is also no need to make this async.
@@ -464,8 +477,7 @@ class UpdateMethods:
             except Exception as e:
                 if not isinstance(e, asyncio.CancelledError) or self.is_connected():
                     name = getattr(callback, '__name__', repr(callback))
-                    self._log[__name__].exception('Unhandled exception on %s',
-                                                  name)
+                    self._log_exc('Unhandled exception on %s', name)
 
     async def _dispatch_event(self: 'TelegramClient', event):
         """
@@ -506,8 +518,7 @@ class UpdateMethods:
             except Exception as e:
                 if not isinstance(e, asyncio.CancelledError) or self.is_connected():
                     name = getattr(callback, '__name__', repr(callback))
-                    self._log[__name__].exception('Unhandled exception on %s',
-                                                  name)
+                    self._log_exc('Unhandled exception on %s', name)
 
     async def _get_difference(self: 'TelegramClient', update, channel_id, pts_date):
         """
@@ -610,8 +621,7 @@ class UpdateMethods:
             self._log[__name__].warning('Failed to get missed updates after '
                                         'reconnect: %r', e)
         except Exception:
-            self._log[__name__].exception('Unhandled exception while getting '
-                                          'update difference after reconnect')
+            self._log_exc('Unhandled exception while getting update difference after reconnect')
 
     # endregion