code: def no_cache_dir_callback(option, opt, value, parser):
      
      # The value argument will be None if --no-cache-dir is passed via the
      # command-line, since the option doesn't accept arguments.  However,
      # the value can be non-None if the option is triggered e.g. by an
      # environment variable, like PIP_NO_CACHE_DIR=true.
      if value is not None:
          # Then parse the string value to get argument error-checking.
          try:
              strtobool(value)
          except ValueError as exc:
              raise_option_error(parser, option=option, msg=str(exc))
  
      # Originally, setting PIP_NO_CACHE_DIR to a value that strtobool()
      # converted to 0 (like "false" or "no") caused cache_dir to be disabled
      # rather than enabled (logic would say the latter).  Thus, we disable
      # the cache directory not just on values that parse to True, but (for
      # backwards compatibility reasons) also on values that parse to False.
      # In other words, always set it to False if the option is provided in
      # some (valid) form.
      parser.values.cache_dir = False
docs: Process a value provided for the --no-cache-dir option.
  
      This is an optparse.Option callback for the --no-cache-dir option.