code: def is_file_modified(self, id_, signature):
          
  
          entry = self.__entries.get(id_)
          if entry is None:
              raise ValueError("Invalid filename id_ (%d)" % id_)
  
          # Is the signature already known?
          if entry.sig_valid:
              # use the cached signature
              filesig = entry.signature
          else:
              # compute the signature and store it
              filesig = self._get_signature(entry)
              entry.signature = filesig
              entry.sig_valid = True
  
          return filesig != signature
docs: Check if the file referred to by `id_` has been modified.