code: def persist_checksum(self, requirement, cache_file):
          
          if not self.config.trust_mod_times:
              checksum_file = '%s.txt' % cache_file
              with AtomicReplace(checksum_file) as temporary_file:
                  with open(temporary_file, 'w') as handle:
                      handle.write('%s\n' % requirement.checksum)
docs: Persist the checksum of the input used to generate a binary distribution.
  
          :param requirement: A :class:`.Requirement` object.
          :param cache_file: The pathname of a cached binary distribution (a string).
  
          .. note:: The checksum is only calculated and persisted when
                    :attr:`~.Config.trust_mod_times` is :data:`False`.