From edadd8f2fd2cc07a489ac2375a6476b28c29dbcb Mon Sep 17 00:00:00 2001
From: Will <tekulvw@users.noreply.github.com>
Date: Fri, 22 Jun 2018 22:15:22 -0400
Subject: [PATCH] [V3 Config] Fix for unnecessary writes on context mgr exit
 (#1859)

* Fix for #1857

* And copy it so we don't have mutability issues

* Add a test
---
 redbot/core/config.py     |  6 +++++-
 tests/core/test_config.py | 11 +++++++++++
 2 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/redbot/core/config.py b/redbot/core/config.py
index 85a8f1df5..cf95ed640 100644
--- a/redbot/core/config.py
+++ b/redbot/core/config.py
@@ -29,12 +29,15 @@ class _ValueCtxManager:
     def __init__(self, value_obj, coro):
         self.value_obj = value_obj
         self.coro = coro
+        self.raw_value = None
+        self.__original_value = None
 
     def __await__(self):
         return self.coro.__await__()
 
     async def __aenter__(self):
         self.raw_value = await self
+        self.__original_value = deepcopy(self.raw_value)
         if not isinstance(self.raw_value, (list, dict)):
             raise TypeError(
                 "Type of retrieved value must be mutable (i.e. "
@@ -44,7 +47,8 @@ class _ValueCtxManager:
         return self.raw_value
 
     async def __aexit__(self, *exc_info):
-        await self.value_obj.set(self.raw_value)
+        if self.raw_value != self.__original_value:
+            await self.value_obj.set(self.raw_value)
 
 
 class Value:
diff --git a/tests/core/test_config.py b/tests/core/test_config.py
index b5f232961..1a4541647 100644
--- a/tests/core/test_config.py
+++ b/tests/core/test_config.py
@@ -1,3 +1,4 @@
+from unittest.mock import patch
 import pytest
 
 
@@ -399,6 +400,16 @@ async def test_ctxmgr_no_shared_default(config, member_factory):
     assert 1 not in await config.member(m2).foo()
 
 
+@pytest.mark.asyncio
+async def test_ctxmgr_no_unnecessary_write(config):
+    config.register_global(foo=[])
+    foo_value_obj = config.foo
+    with patch.object(foo_value_obj, "set") as set_method:
+        async with foo_value_obj() as foo:
+            pass
+        set_method.assert_not_called()
+
+
 @pytest.mark.asyncio
 async def test_get_then_mutate(config):
     """Tests that mutating an object after getting it as a value doesn't mutate the data store."""