def RegisterMountPoint(cls, mount_point, path_spec):
    """Registers a path specification mount point.

    Args:
      mount_point (str): mount point identifier.
      path_spec (PathSpec): path specification of the mount point.

    Raises:
      KeyError: if the corresponding mount point is already set.
    """
    if mount_point in cls._mount_points:
      raise KeyError('Mount point: {0:s} already set.'.format(mount_point))

    cls._mount_points[mount_point] = path_spec
Registers a path specification mount point.

    Args:
      mount_point (str): mount point identifier.
      path_spec (PathSpec): path specification of the mount point.

    Raises:
      KeyError: if the corresponding mount point is already set.