def join_scope(parent_scope, child_scope):
  """Joins a parent and child scope using `/`, checking for empty/none.

  Args:
    parent_scope: (string) parent/prefix scope.
    child_scope: (string) child/suffix scope.
  Returns:
    joined scope: (string) parent and child scopes joined by /.
  """
  if not parent_scope:
    return child_scope
  if not child_scope:
    return parent_scope
  return '/'.join([parent_scope, child_scope])