you should actually implement the thing you claimed to

hypothesis-python/src/hypothesis/internal/escalation.py

@@ -32,20 +32,21 @@ def belongs_to(package: ModuleType) -> Callable[[str], bool]:
         return lambda filepath: False
 
     assert package.__file__ is not None
-    FILE_CACHE[package] = {}
+    FILE_CACHE[package].setdefault({})
+    cache = FILE_CACHE[package]
     root = Path(package.__file__).resolve().parent
 
     def accept(filepath: str) -> bool:
         try:
-            return FILE_CACHE[package][filepath]
+            return cache[filepath]
         except KeyError:
             pass
         try:
             Path(filepath).resolve().relative_to(root)
             result = True
         except Exception:
             result = False
-        FILE_CACHE[package][filepath] = result
+        cache[filepath] = result
         return result
 
     accept.__name__ = f"is_{package.__name__}_file"