diff --git a/nonebot/plugins/single_session.py b/nonebot/plugins/single_session.py
index 3bed8ea1..d0e5b4a6 100644
--- a/nonebot/plugins/single_session.py
+++ b/nonebot/plugins/single_session.py
@@ -1,10 +1,8 @@
-from typing import Generator, Dict
+from typing import Dict, Generator
+
 from nonebot.adapters import Event
-from nonebot.message import (
-    IgnoredException,
-    event_preprocessor
-)
 from nonebot.params import Depends
+from nonebot.message import IgnoredException, event_preprocessor
 
 _running_matcher: Dict[str, int] = {}
 
@@ -18,7 +16,7 @@ async def matcher_mutex(event: Event) -> Generator[bool, None, None]:
     else:
         current_event_id = id(event)
         event_id = _running_matcher.get(session_id, None)
-        if event_id :
+        if event_id:
             result = event_id != current_event_id
         else:
             _running_matcher[session_id] = current_event_id
@@ -26,8 +24,8 @@ async def matcher_mutex(event: Event) -> Generator[bool, None, None]:
         if result:
             del _running_matcher[session_id]
 
+
 @event_preprocessor
 async def preprocess(mutex: bool = Depends(matcher_mutex)):
     if mutex:
         raise IgnoredException("Another matcher running")
-