Add the Chat.members_count dynamic attribute

This new attribute returns the number of members in a chat. This works
for all kinds of chats (private, groups/supergroups and channels).
This is possible thanks to a new API introduced in the Bot API 2.1
update.

Issue: GH-64

botogram/objects/chats.py

@@ -165,6 +165,20 @@ def creator(self):
 
         return self._cache_creator
 
+    @property
+    def members_count(self):
+        """Get the number of members of this chat"""
+        # This isn't *really* needed, but avoids an HTTP request
+        if self.type == "private":
+            return 1
+
+        # Be sure to cache the number of members
+        if not hasattr(self, "_cache_members_count"):
+            self._cache_members_count = self._api.call("getChatMembersCount",
+                                                       {"chat_id": self.id},
+                                                       expect=int)
+        return self._cache_members_count
+
     def leave(self):
         """Leave this chat"""
         if self.type not in ("group", "supergroup"):

docs/api/telegram.rst

@@ -434,6 +434,24 @@ about its business.
 
       .. versionadded:: 0.3
 
+   .. py:attribute:: members_count
+
+      Return the number of members of this chat. This works across all the
+      kinds of chat.
+
+      Please remember the content of this attribute is fetched from Telegram
+      the first time you access it (so it might be slow), but it's cached right
+      after, so the following accesses will involve no network communication.
+
+      .. code-block:: python
+
+         @bot.command("members")
+         def members_command(chat, message, args):
+             """Get the number of members in this group"""
+             chat.send(str(chat.members_count))
+
+      .. versionadded:: 0.3
+
    .. py:method:: leave()
 
       Kick the bot from this chat. This method is available only on groups and

docs/changelog.rst

@@ -49,6 +49,7 @@ New features
 * Added new attribute :py:attr:`botogram.Sticker.emoji`
 * Added new attribute :py:attr:`botogram.Chat.admins`
 * Added new attribute :py:attr:`botogram.Chat.creator`
+* Added new attribute :py:attr:`botogram.Chat.members_count`
 * Added new method :py:meth:`botogram.Chat.leave`
 * Every method which sends something to a chat now returns the sent
   :py:class:`~botogram.Message`