code: def _update_or_init_po_files(target, source, env):
    
    import SCons.Action
    from SCons.Tool.GettextCommon import _init_po_files
    for tgt in target:
      if tgt.rexists():
        action = SCons.Action.Action('$MSGMERGECOM', '$MSGMERGECOMSTR')
      else:
        action = _init_po_files
      status = action([tgt], source, env)
      if status : return status
    return 0
docs: Action function for `POUpdate` builder