code: def _load_config_files(self):
          # type: () -> None
          
          config_files = dict(self._iter_config_files())
          if config_files[kinds.ENV][0:1] == [os.devnull]:
              logger.debug(
                  "Skipping loading configuration files due to "
                  "environment's PIP_CONFIG_FILE being os.devnull"
              )
              return
  
          for variant, files in config_files.items():
              for fname in files:
                  # If there's specific variant set in `load_only`, load only
                  # that variant, not the others.
                  if self.load_only is not None and variant != self.load_only:
                      logger.debug(
                          "Skipping file '%s' (variant: %s)", fname, variant
                      )
                      continue
  
                  parser = self._load_file(variant, fname)
  
                  # Keeping track of the parsers used
                  self._parsers[variant].append((fname, parser))
docs: Loads configuration from configuration files