Add check for asyncio event loop to remain the same

telethon/client/telegrambaseclient.py

@@ -399,6 +399,7 @@ class TelegramBaseClient(abc.ABC):
         self._borrowed_senders = {}
         self._borrow_sender_lock = asyncio.Lock()
 
+        self._loop = None  # only used as a sanity check
         self._updates_error = None
         self._updates_handle = None
         self._keepalive_handle = None
@@ -535,6 +536,11 @@ class TelegramBaseClient(abc.ABC):
         if self.session is None:
             raise ValueError('TelegramClient instance cannot be reused after logging out')
 
+        if self._loop is None:
+            self._loop = helpers.get_running_loop()
+        elif self._loop != helpers.get_running_loop():
+            raise RuntimeError('The asyncio event loop must not change after connection (see the FAQ for details)')
+
         if not await self._sender.connect(self._connection(
             self.session.server_address,
             self.session.port,

telethon/client/users.py

@@ -30,6 +30,10 @@ class UserMethods:
         return await self._call(self._sender, request, ordered=ordered)
 
     async def _call(self: 'TelegramClient', sender, request, ordered=False, flood_sleep_threshold=None):
+        if self._loop is not None and self._loop != helpers.get_running_loop():
+            raise RuntimeError('The asyncio event loop must not change after connection (see the FAQ for details)')
+        # if the loop is None it will fail with a connection error later on
+
         if flood_sleep_threshold is None:
             flood_sleep_threshold = self.flood_sleep_threshold
         requests = (request if utils.is_list_like(request) else (request,))