Deseasonify: improve `in_month` command check

Raise a custom exception if the command fails. This is then handled
in the error handler, and the user will be informed of which months
allow the invoked command.

2 files changed, 15 insertions, 4 deletions

diff --git a/bot/decorators.py b/bot/decorators.py
index 8a1f00ee..f031d404 100644
--- a/bot/decorators.py
+++ b/bot/decorators.py
@@ -21,6 +21,12 @@ class InChannelCheckFailure(CheckFailure):
     pass
 
 
+class InMonthCheckFailure(CheckFailure):
+    """Check failure for when a command is invoked outside of its allowed month."""
+
+    pass
+
+
 def in_month(*allowed_months: Month) -> typing.Callable:
     """
     Check whether the command was invoked in one of `enabled_months`.
@@ -31,11 +37,16 @@ def in_month(*allowed_months: Month) -> typing.Callable:
         current_month = datetime.utcnow().month
         can_run = current_month in allowed_months
 
+        human_months = ", ".join(m.name for m in allowed_months)
         log.debug(
-            f"Command '{ctx.command}' is locked to months {allowed_months}. "
+            f"Command '{ctx.command}' is locked to months {human_months}. "
             f"Invoking it in month {current_month} is {'allowed' if can_run else 'disallowed'}."
         )
-        return can_run
+        if can_run:
+            return True
+        else:
+            raise InMonthCheckFailure(f"Command can only be used in {human_months}")
+
     return commands.check(predicate)
 
 
diff --git a/bot/seasons/evergreen/error_handler.py b/bot/seasons/evergreen/error_handler.py
index 0d8bb0bb..ba6ca5ec 100644
--- a/bot/seasons/evergreen/error_handler.py
+++ b/bot/seasons/evergreen/error_handler.py
@@ -7,7 +7,7 @@ from discord import Embed, Message
 from discord.ext import commands
 
 from bot.constants import Colours, ERROR_REPLIES, NEGATIVE_REPLIES
-from bot.decorators import InChannelCheckFailure
+from bot.decorators import InChannelCheckFailure, InMonthCheckFailure
 
 log = logging.getLogger(__name__)
 
@@ -55,7 +55,7 @@ class CommandErrorHandler(commands.Cog):
         if isinstance(error, commands.CommandNotFound):
             return
 
-        if isinstance(error, InChannelCheckFailure):
+        if isinstance(error, (InChannelCheckFailure, InMonthCheckFailure)):
             await ctx.send(embed=self.error_embed(str(error), NEGATIVE_REPLIES), delete_after=7.5)
             return