play for a fool
play (one) for a fool
To presume or believe that someone is very stupid or foolish. I'd advise you not to play her for a fool. She may play dumb, but she's very crafty.
play someone for a fool
Fig. to treat someone like a fool; to assume someone is naive or stupid. Don't play me for a fool. I won't have it. You are playing me for a fool! Stop it!