Update src/esmf_regrid/experimental/partition.py

Co-authored-by: Patrick Peglar <patrick.peglar@metoffice.gov.uk>

Author: stephenworsley

src/esmf_regrid/experimental/partition.py

@@ -184,8 +184,7 @@ def __repr__(self):
     @property
     def unsaved_files(self):
         """List of files not yet generated."""
-        files = set(self.file_names) - set(self.saved_files)
-        return [file for file in self.file_names if file in files]
+        return [file for file in self.file_names if file not in self.saved_files]
 
     def generate_files(self, files_to_generate=None):
         """Generate files with regridding information.