code: def mkdir(self, dir_name, mode=PERM_DEF, dir_fd=None):
          
          dir_name = self._path_with_dir_fd(dir_name, self.mkdir, dir_fd)
          try:
              self.filesystem.makedir(dir_name, mode)
          except IOError as e:
              if e.errno == errno.EACCES:
                  self.filesystem.raise_os_error(e.errno, dir_name)
              raise
docs: Create a leaf Fake directory.
  
          Args:
              dir_name: (str) Name of directory to create.
                  Relative paths are assumed to be relative to '/'.
              mode: (int) Mode to create directory with.  This argument defaults
                  to 0o777.  The umask is applied to this mode.
              dir_fd: If not `None`, the file descriptor of a directory,
                  with `dir_name` being relative to this directory.
                  New in Python 3.3.
  
          Raises:
              OSError: if the directory name is invalid or parent directory is
                  read only or as per FakeFilesystem.add_object.