From f66a63501fe1ef8fb5390dfbe42ae9f95ea2bc28 Mon Sep 17 00:00:00 2001
From: Leon Sandøy <leon.sandoy@conmodo.com>
Date: Thu, 28 May 2020 01:29:34 +0200
Subject: Add custom exceptions for each error state.

The bot can get into trouble in three distinct ways:
- It has no Bot instance
- It has no namespace
- It has no parent instance.

These happen only if you're using it wrong. To make the test more
precise, and to add a little bit more readability (RuntimeError could be
anything!), we'll introduce some custom exceptions for these three
states.

This addresses a review comment by @aeros.
---
 bot/utils/redis_cache.py            | 22 +++++++++++++++++-----
 tests/bot/utils/test_redis_cache.py |  7 ++++---
 2 files changed, 21 insertions(+), 8 deletions(-)

diff --git a/bot/utils/redis_cache.py b/bot/utils/redis_cache.py
index 979ea5d47..6b3c68979 100644
--- a/bot/utils/redis_cache.py
+++ b/bot/utils/redis_cache.py
@@ -27,6 +27,18 @@ _KEY_PREFIXES = (
 )
 
 
+class NoBotInstanceError(RuntimeError):
+    """Raised when RedisCache is created without an available bot instance on the owner class."""
+
+
+class NoNamespaceError(RuntimeError):
+    """Raised when RedisCache has no namespace, for example if it is not assigned to a class attribute."""
+
+
+class NoParentInstanceError(RuntimeError):
+    """Raised when the parent instance is available, for example if called by accessing the parent class directly."""
+
+
 class RedisCache:
     """
     A simplified interface for a Redis connection.
@@ -149,7 +161,7 @@ class RedisCache:
                 "This object must be initialized as a class attribute."
             )
             log.error(error_message)
-            raise RuntimeError(error_message)
+            raise NoNamespaceError(error_message)
 
         if self.bot is None:
             error_message = (
@@ -159,7 +171,7 @@ class RedisCache:
                 "the RedisCache inside a class that has a Bot instance attribute."
             )
             log.error(error_message)
-            raise RuntimeError(error_message)
+            raise NoBotInstanceError(error_message)
 
         await self.bot.redis_ready.wait()
 
@@ -194,7 +206,7 @@ class RedisCache:
         if self._namespace is None:
             error_message = "RedisCache must be a class attribute."
             log.error(error_message)
-            raise RuntimeError(error_message)
+            raise NoNamespaceError(error_message)
 
         if instance is None:
             error_message = (
@@ -202,7 +214,7 @@ class RedisCache:
                 "before accessing it using the cog's class object."
             )
             log.error(error_message)
-            raise RuntimeError(error_message)
+            raise NoParentInstanceError(error_message)
 
         for attribute in vars(instance).values():
             if isinstance(attribute, Bot):
@@ -217,7 +229,7 @@ class RedisCache:
                 "the RedisCache inside a class that has a Bot instance attribute."
             )
             log.error(error_message)
-            raise RuntimeError(error_message)
+            raise NoBotInstanceError(error_message)
 
     def __repr__(self) -> str:
         """Return a beautiful representation of this object instance."""
diff --git a/tests/bot/utils/test_redis_cache.py b/tests/bot/utils/test_redis_cache.py
index 4f95dff03..8c1a40640 100644
--- a/tests/bot/utils/test_redis_cache.py
+++ b/tests/bot/utils/test_redis_cache.py
@@ -4,6 +4,7 @@ import unittest
 import fakeredis.aioredis
 
 from bot.utils import RedisCache
+from bot.utils.redis_cache import NoBotInstanceError, NoNamespaceError, NoParentInstanceError
 from tests import helpers
 
 
@@ -260,13 +261,13 @@ class RedisCacheTests(unittest.IsolatedAsyncioTestCase):
         cog = MyCog()
 
         # Raises "No Bot instance"
-        with self.assertRaises(RuntimeError):
+        with self.assertRaises(NoBotInstanceError):
             await cog.cache.get("john")
 
         # Raises "RedisCache has no namespace"
-        with self.assertRaises(RuntimeError):
+        with self.assertRaises(NoNamespaceError):
             await cog.other_cache.get("was")
 
         # Raises "You must access the RedisCache instance through the cog instance"
-        with self.assertRaises(RuntimeError):
+        with self.assertRaises(NoParentInstanceError):
             await MyCog.cache.get("afraid")
-- 
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