diff --git a/pdf2zh/cache.py b/pdf2zh/cache.py
index d186aa16..80d497be 100644
--- a/pdf2zh/cache.py
+++ b/pdf2zh/cache.py
@@ -45,7 +45,9 @@ def _sort_dict_recursively(obj):
         return obj
 
     def __init__(self, translate_engine: str, translate_engine_params: dict = None):
-        assert len(translate_engine) < 20, 'current cache require translate engine name less than 20 characters'
+        assert (
+            len(translate_engine) < 20
+        ), "current cache require translate engine name less than 20 characters"
         self.translate_engine = translate_engine
         self.replace_params(translate_engine_params)