Additional comments and tests for slicing

2 files changed, 5 insertions, 2 deletions

diff --git a/bot/utils/message_cache.py b/bot/utils/message_cache.py
index 6d219c313..f5656cdeb 100644
--- a/bot/utils/message_cache.py
+++ b/bot/utils/message_cache.py
@@ -122,6 +122,9 @@ class MessageCache:
         Providing 0 will return the message at the position perceived by the user to be the beginning of the cache,
         meaning at `self._start`.
         """
+        # Keep in mind that for the modulo operator used throughout this function, Python modulo behaves similarly when
+        # the left operand is negative. E.g -1 % 5 == 4, because the closest number from the bottom that wholly divides
+        # by 5 is -5.
         if isinstance(item, int):
             if item >= len(self) or item < -len(self):
                 raise IndexError("cache index out of range")
diff --git a/tests/bot/utils/test_message_cache.py b/tests/bot/utils/test_message_cache.py
index 5e871cd19..04bfd28d1 100644
--- a/tests/bot/utils/test_message_cache.py
+++ b/tests/bot/utils/test_message_cache.py
@@ -168,7 +168,7 @@ class TestMessageCache(unittest.TestCase):
         slices = (
             slice(None), slice(2, None), slice(None, 2), slice(None, None, 2), slice(None, None, 3), slice(-1, 2),
             slice(-1, 3000), slice(-3, -1), slice(-10, 3), slice(-10, 4, 2), slice(None, None, -1), slice(None, 3, -2),
-            slice(None, None, -3)
+            slice(None, None, -3), slice(-1, -10, -2), slice(-3, -7, -1)
         )
 
         for size in sizes:
@@ -189,7 +189,7 @@ class TestMessageCache(unittest.TestCase):
         slices = (
             slice(None), slice(2, None), slice(None, 2), slice(None, None, 2), slice(None, None, 3), slice(-1, 2),
             slice(-1, 3000), slice(-3, -1), slice(-10, 3), slice(-10, 4, 2), slice(None, None, -1), slice(None, 3, -2),
-            slice(None, None, -3)
+            slice(None, None, -3), slice(-1, -10, -2), slice(-3, -7, -1)
         )
 
         for size in sizes: