switch to bot commands extension style bot setup

contest/bot.py

@@ -0,0 +1,44 @@
+import logging
+from typing import Optional
+
+import discord
+from discord.ext import commands
+
+from contest import constants
+from contest.db import ContestDatabase
+
+logger = logging.getLogger(__file__)
+logger.setLevel(constants.LOGGING_LEVEL)
+
+
+async def fetch_prefix(bot: 'ContestBot', message: discord.Message):
+    """Fetches the prefix used by the relevant guild."""
+    user_id = bot.user.id
+    base = [f'<@!{user_id}> ', f'<@{user_id}> ']
+
+    if message.guild:
+        if bot.db is not None:
+            base.append(await bot.db.get_prefix(message.guild.id))
+    return base
+
+
+class ContestBot(commands.Bot):
+    def __init__(self, **options):
+        self.db: Optional[ContestDatabase] = None
+        super().__init__(fetch_prefix, **options)
+
+    async def on_ready(self):
+        if self.db is None:
+            self.db = await ContestDatabase.create()
+        logger.info('Bot is now ready and connected to Discord.')
+        guild_count = len(self.guilds)
+        logger.info(
+            f'Connected as {self.user.name}#{self.user.discriminator} to {guild_count} guild{"s" if guild_count > 1 else ""}.')
+
+    async def on_guild_join(self, guild: discord.Guild) -> None:
+        logger.info(f'Added to new guild: {guild.name} ({guild.id})')
+        await self.db.setup_guild(guild.id)
+
+    async def on_guild_remove(self, guild: discord.Guild) -> None:
+        logger.info(f'Removed from guild: {guild.name} ({guild.id})')
+        await self.db.teardown_guild(guild.id)