Add guild property to DMChannel, GroupChannel, and PartialMessageable

This improves generic duck typing programming by allowing narrowing
the types by just checking for a None guild attribute.

discord/channel.py

@@ -2348,6 +2348,16 @@ class DMChannel(discord.abc.Messageable, discord.abc.Connectable, Hashable):
         """:class:`datetime.datetime`: Returns the direct message channel's creation time in UTC."""
         return utils.snowflake_time(self.id)
 
+    @property
+    def guild(self) -> Optional[Guild]:
+        """Optional[:class:`Guild`]: The guild this DM channel belongs to. Always ``None``.
+
+        This is mainly provided for compatibility purposes in duck typing.
+
+        .. versionadded:: 2.0
+        """
+        return None
+
     @property
     def jump_url(self) -> str:
         """:class:`str`: Returns a URL that allows the client to jump to the channel.
@@ -2705,6 +2715,16 @@ class GroupChannel(discord.abc.Messageable, discord.abc.Connectable, Hashable):
         """:class:`ChannelType`: The channel's Discord type."""
         return ChannelType.group
 
+    @property
+    def guild(self) -> Optional[Guild]:
+        """Optional[:class:`Guild`]: The guild this group channel belongs to. Always ``None``.
+
+        This is mainly provided for compatibility purposes in duck typing.
+
+        .. versionadded:: 2.0
+        """
+        return None
+
     @property
     def icon(self) -> Optional[Asset]:
         """Optional[:class:`Asset`]: Returns the channel's icon asset if available."""
@@ -3050,6 +3070,11 @@ class PartialMessageable(discord.abc.Messageable, Hashable):
     async def _get_channel(self) -> PartialMessageable:
         return self
 
+    @property
+    def guild(self) -> Optional[Guild]:
+        """Optional[:class:`Guild`]: The guild this partial messageable is in."""
+        return self._state._get_guild(self.guild_id)
+
     @property
     def jump_url(self) -> str:
         """:class:`str`: Returns a URL that allows the client to jump to the channel."""

discord/client.py

@@ -1057,7 +1057,8 @@ class Client:
             The optional guild ID to create a partial messageable for.
 
             This is not required to actually send messages, but it does allow the
-            :meth:`PartialMessageable.jump_url` property to form a well formed URL.
+            :meth:`PartialMessageable.jump_url` and :attr:`PartialMessageable.guild`
+            properties to function properly.
         type: Optional[:class:`.ChannelType`]
             The underlying channel type for the partial messageable.