def trim(s):

    if not isinstance(s,str) or len(s)==0:

        print("error!")

        return

    if s[:1]==' ':

        if s[-1:]==' ':

            s=s[1:-1]

        else:

            s=s[1:]

    elif s[-1:]==' ':

        s=s[:-1]

    else:

            pass

    return s