Add the message listener.

cogs/message_listener/__init__.py

@@ -0,0 +1,41 @@
+"""Defines cog for on message listener."""
+
+import nextcord
+from nextcord.ext import commands
+
+from main import birthday_list
+
+
+class SubmissionListener(commands.Cog, name="Message Listener"):
+    """Sets up message listener."""
+
+    def __init__(self, bot: commands.Bot) -> None:
+        self.bot = bot
+
+    @commands.Cog.listener()
+    async def on_message(self, message: nextcord.Message) -> None:
+        """Listen for message creation. Adds score and attachments to submission."""
+        # don't want to process any messages from bot.
+        if message.author.id == self.bot.user.id:  # type: ignore reportOptionalMemberAccess
+            return
+
+        await birthday_action(message)
+
+
+# This function will be called when this extension is loaded.
+# It is necessary to add these functions to the bot.
+def setup(bot: commands.Bot) -> None:
+    """Add function to bot."""
+    bot.add_cog(SubmissionListener(bot))
+
+
+async def birthday_action(message: nextcord.Message) -> None:
+    """
+    Check a message's author and mentions to see if they are in the birthday list.
+
+    Args:
+        message (nextcord.Message): The message to check for a birthday person.
+
+    """
+    if message.author.id in birthday_list or bool(set(message.raw_mentions) & set(birthday_list)):
+        await message.add_reaction("🎂")