def METHOD_NAME(target, source, env):
    """ Action function for `POUpdate` builder """
    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