Update Member.joined_at on MESSAGE_CREATE and document it can be None.

Fixes #1638

discord/member.py

@@ -138,9 +138,9 @@ class Member(discord.abc.Messageable, _BaseUser):
 
     Attributes
     ----------
-    joined_at: `datetime.datetime`
+    joined_at: Optional[:class:`datetime.datetime`]
         A datetime object that specifies the date and time in UTC that the member joined the guild for
-        the first time.
+        the first time. In certain cases, this can be ``None``.
     activities: Tuple[Union[:class:`Game`, :class:`Streaming`, :class:`Spotify`, :class:`Activity`]]
         The activities that the user is currently doing.
     guild: :class:`Guild`
@@ -179,6 +179,16 @@ class Member(discord.abc.Messageable, _BaseUser):
     def __hash__(self):
         return hash(self._user)
 
+    @classmethod
+    def _from_message(cls, *, message, data):
+        author = message.author
+        data['user'] = {
+            attr: getattr(author, attr)
+            for attr in author.__slots__
+            if attr[0] != '_'
+        }
+        return cls(data=data, guild=message.guild, state=message._state)
+
     @classmethod
     def _copy(cls, member):
         self = cls.__new__(cls) # to bypass __init__

discord/message.py

@@ -35,6 +35,7 @@ from .calls import CallMessage
 from .enums import MessageType, try_enum
 from .errors import InvalidArgument, ClientException, HTTPException
 from .embeds import Embed
+from .member import Member
 
 class Attachment:
     """Represents an attachment from Discord.
@@ -277,7 +278,7 @@ class Message:
         self._try_patch(data, 'embeds', lambda x: list(map(Embed.from_data, x)))
         self._try_patch(data, 'nonce')
 
-        for handler in ('author', 'mentions', 'mention_roles', 'call'):
+        for handler in ('author', 'member', 'mentions', 'mention_roles', 'call'):
             try:
                 getattr(self, '_handle_%s' % handler)(data[handler])
             except KeyError:
@@ -298,6 +299,20 @@ class Message:
             if found is not None:
                 self.author = found
 
+    def _handle_member(self, member):
+        # The gateway now gives us full Member objects sometimes with the following keys
+        # deaf, mute, joined_at, roles
+        # For the sake of performance I'm going to assume that the only
+        # field that needs *updating* would be the joined_at field.
+        # If there is no Member object (for some strange reason), then we can upgrade
+        # ourselves to a more "partial" member object.
+        author = self.author
+        try:
+            if author.joined_at is None:
+                author.joined_at = utils.parse_time(member.get('joined_at'))
+        except AttributeError:
+            self.author = Member._from_message(message=self, data=member)
+
     def _handle_mentions(self, mentions):
         self.mentions = []
         if self.guild is None: