code: def new_checksum(self):
          
          if self.context.is_file(self.filename):
              friendly_name = format_path(self.filename)
              logger.debug("Calculating SHA1 of %s ..", friendly_name)
              context = hashlib.sha1()
              context.update(self.context.read_file(self.filename))
              checksum = context.hexdigest()
              logger.debug("The SHA1 digest of %s is %s.", friendly_name, checksum)
              return checksum
docs: Get the SHA1 digest of the contents of :attr:`filename` (a string).