code: def build_base_url(self):
          
          if self.base_url is None:
              return
          base_url = self.base_url
          if not (self.parent_url or self.base_ref or base_url.startswith("file:")):
              base_url = os.path.expanduser(base_url)
              if not is_absolute_path(base_url):
                  try:
                      base_url = os.getcwd()+"/"+base_url
                  except OSError as msg:
                      # occurs on stale remote filesystems (eg. NFS)
                      errmsg = _("Could not get current working directory: %(msg)s") % dict(msg=msg)
                      raise LinkCheckerError(errmsg)
                  if os.path.isdir(base_url):
                      base_url += "/"
              base_url = "file://"+base_url
          if os.name == "nt":
              base_url = base_url.replace("\\", "/")
              # transform c:/windows into /c|/windows
              base_url = re.sub("^file://(/?)([a-zA-Z]):", r"file:///\2|", base_url)
              # transform file://path into file:///path
              base_url = re.sub("^file://([^/])", r"file:///\1", base_url)
          self.base_url = unicode(base_url)
docs: The URL is normed according to the platform:
           - the base URL is made an absolute file:// URL
           - under Windows platform the drive specifier is normed