Add a custom converter for extensions

The converter fully qualifies the extension's name and ensures the
extension exists.

* Make the extensions set a module constant instead of an instant
  attribute and make it a frozenset.
* Add a cog error handler to handle BadArgument locally and prevent the
  help command from showing for such errors.

1 files changed, 33 insertions, 8 deletions

diff --git a/bot/cogs/extensions.py b/bot/cogs/extensions.py
index 83048bb76..e50ef5553 100644
--- a/bot/cogs/extensions.py
+++ b/bot/cogs/extensions.py
@@ -5,7 +5,7 @@ from enum import Enum
 from pkgutil import iter_modules
 
 from discord import Colour, Embed
-from discord.ext.commands import Bot, Cog, Context, group
+from discord.ext.commands import BadArgument, Bot, Cog, Context, Converter, group
 
 from bot.constants import Emojis, MODERATION_ROLES, Roles, URLs
 from bot.pagination import LinePaginator
@@ -14,6 +14,7 @@ from bot.utils.checks import with_role_check
 log = logging.getLogger(__name__)
 
 KEEP_LOADED = ["bot.cogs.extensions", "bot.cogs.modlog"]
+EXTENSIONS = frozenset(ext for ext in iter_modules(("bot/cogs", "bot.cogs")) if ext.name[-1] != "_")
 
 
 class Action(Enum):
@@ -24,16 +25,36 @@ class Action(Enum):
     RELOAD = (Bot.unload_extension, Bot.load_extension)
 
 
+class Extension(Converter):
+    """
+    Fully qualify the name of an extension and ensure it exists.
+
+    The * and ** values bypass this when used with the reload command.
+    """
+
+    async def convert(self, ctx: Context, argument: str) -> str:
+        """Fully qualify the name of an extension and ensure it exists."""
+        # Special values to reload all extensions
+        if ctx.command.name == "reload" and (argument == "*" or argument == "**"):
+            return argument
+
+        argument = argument.lower()
+
+        if "." not in argument:
+            argument = f"bot.cogs.{argument}"
+
+        if argument in EXTENSIONS:
+            return argument
+        else:
+            raise BadArgument(f":x: Could not find the extension `{argument}`.")
+
+
 class Extensions(Cog):
     """Extension management commands."""
 
     def __init__(self, bot: Bot):
         self.bot = bot
 
-        log.info("Initialising extension names...")
-        modules = iter_modules(("bot/cogs", "bot.cogs"))
-        self.cogs = set(ext for ext in modules if ext.name[-1] != "_")
-
     @group(name='extensions', aliases=('c', 'ext', 'exts'), invoke_without_command=True)
     async def extensions_group(self, ctx: Context) -> None:
         """Load, unload, reload, and list active cogs."""
@@ -291,9 +312,6 @@ class Extensions(Cog):
         verb = action.name.lower()
         error_msg = None
 
-        if ext not in self.cogs:
-            return f":x: Extension {ext} does not exist.", None
-
         if (
             (action is Action.LOAD and ext not in self.bot.extensions)
             or (action is Action.UNLOAD and ext in self.bot.extensions)
@@ -321,6 +339,13 @@ class Extensions(Cog):
         """Only allow moderators and core developers to invoke the commands in this cog."""
         return with_role_check(ctx, *MODERATION_ROLES, Roles.core_developer)
 
+    # This cannot be static (must have a __func__ attribute).
+    async def cog_command_error(self, ctx: Context, error: Exception) -> None:
+        """Handle BadArgument errors locally to prevent the help command from showing."""
+        if isinstance(error, BadArgument):
+            await ctx.send(str(error))
+            error.handled = True
+
 
 def setup(bot: Bot) -> None:
     """Load the Extensions cog."""