Fix custom.Message.buttons being None in channels (#870)

The implications behind this were bigger than expected. The sender
of the message is not actually needed for anything in the bot. The
bot itself may be needed, but only for KeyboardButtonSwitchInline.

telethon/tl/custom/message.py

@@ -146,6 +146,7 @@ class Message:
         if isinstance(self.original_message, types.MessageService):
             return self.original_message.action
 
+    # TODO Make a property for via_bot and via_input_bot, as well as get_*
     async def _reload_message(self):
         """
         Re-fetches this message to reload the sender and chat entities,
@@ -331,28 +332,56 @@ class Message:
         """
         return self._forward
 
-    def _set_buttons(self, sender, chat):
+    def _set_buttons(self, chat, bot):
         """
         Helper methods to set the buttons given the input sender and chat.
         """
         if isinstance(self.original_message.reply_markup, (
                 types.ReplyInlineMarkup, types.ReplyKeyboardMarkup)):
             self._buttons = [[
-                MessageButton(self._client, button, sender, chat,
+                MessageButton(self._client, button, chat, bot,
                               self.original_message.id)
                 for button in row.buttons
             ] for row in self.original_message.reply_markup.rows]
             self._buttons_flat = [x for row in self._buttons for x in row]
 
+    def _needed_markup_bot(self):
+        """
+        Returns the input peer of the bot that's needed for the reply markup.
+
+        This is necessary for :tl:`KeyboardButtonSwitchInline` since we need
+        to know what bot we want to start. Raises ``ValueError`` if the bot
+        cannot be found but is needed. Returns ``None`` if it's not needed.
+        """
+        for row in self.original_message.reply_markup.rows:
+            for button in row.buttons:
+                if isinstance(button, types.KeyboardButtonSwitchInline):
+                    if button.same_peer:
+                        bot = self.input_sender
+                        if not bot:
+                            raise ValueError('No input sender')
+                    else:
+                        return self._client.session.get_input_entity(
+                            self.original_message.via_bot_id)
+
     @property
     def buttons(self):
         """
         Returns a matrix (list of lists) containing all buttons of the message
         as `telethon.tl.custom.messagebutton.MessageButton` instances.
         """
+        if not isinstance(self.original_message, types.Message):
+            return  # MessageService and MessageEmpty have no markup
+
         if self._buttons is None and self.original_message.reply_markup:
-            if self.input_sender and self.input_chat:
-                self._set_buttons(self._input_sender, self._input_chat)
+            if not self.input_chat:
+                return
+            try:
+                bot = self._needed_markup_bot()
+            except ValueError:
+                return
+            else:
+                self._set_buttons(self._input_chat, bot)
 
         return self._buttons
 
@@ -361,11 +390,18 @@ class Message:
         Returns `buttons`, but will make an API call to find the
         input chat (needed for the buttons) unless it's already cached.
         """
-        if not self.buttons:
-            sender = await self.get_input_sender()
+        if not self.buttons and isinstance(
+                self.original_message, types.Message):
             chat = await self.get_input_chat()
-            if sender and chat:
-                self._set_buttons(sender, chat)
+            if not chat:
+                return
+            try:
+                bot = self._needed_markup_bot()
+            except ValueError:
+                await self._reload_message()
+                bot = self._needed_markup_bot()  # TODO use via_input_bot
+
+            self._set_buttons(chat, bot)
 
         return self._buttons
 

telethon/tl/custom/messagebutton.py

@@ -14,9 +14,9 @@ class MessageButton:
         button (:tl:`KeyboardButton`):
             The original :tl:`KeyboardButton` object.
     """
-    def __init__(self, client, original, from_user, chat, msg_id):
+    def __init__(self, client, original, chat, bot, msg_id):
         self.button = original
-        self._from = from_user
+        self._bot = bot
         self._chat = chat
         self._msg_id = msg_id
         self._client = client
@@ -82,7 +82,7 @@ class MessageButton:
                 return None
         elif isinstance(self.button, types.KeyboardButtonSwitchInline):
             return await self._client(functions.messages.StartBotRequest(
-                bot=self._from, peer=self._chat, start_param=self.button.query
+                bot=self._bot, peer=self._chat, start_param=self.button.query
             ))
         elif isinstance(self.button, types.KeyboardButtonUrl):
             return webbrowser.open(self.button.url)