code: def recall_checksum(self, cache_file):
          
          # EAFP instead of LBYL because of concurrency between pip-accel
          # processes (https://docs.python.org/2/glossary.html#term-lbyl).
          checksum_file = '%s.txt' % cache_file
          try:
              with open(checksum_file) as handle:
                  contents = handle.read()
              return contents.strip()
          except IOError as e:
              if e.errno == errno.ENOENT:
                  # Gracefully handle missing checksum files.
                  return None
              else:
                  # Don't swallow exceptions we don't expect!
                  raise
docs: Get the checksum of the input used to generate a binary distribution archive.
  
          :param cache_file: The pathname of the binary distribution archive (a string).
          :returns: The checksum (a string) or :data:`None` (when no checksum is available).