avoids some del-related badness

Author: Martin Durant

fsspec/implementations/cached.py

@@ -56,7 +56,7 @@ def _open(self, path, mode='rb', **kwargs):
             return f
 
         def close_and_update(self, f, close):
-            if len(f.cache.blocks) * f.blocksize >= f.size:
+            if len(self.cache[f.path]['blocks']) * f.blocksize >= f.size:
                 self.cache[f.path]['blocks'] = True
             self.save_cache()
             close()