Most people think that when there is a singleton in dummy (in a suit contract), partner's card is automatically suit preference.
That is NOT the case.
Eddie Kantar suggests that partner's card should STILL be attitude. When the suit led is discouraged, partner must look at dummy; look at his/her hand; remember the bidding, and make the "obvious" or "logical" shift. (Big Hint: obvious shift is NOT dummy's longest suit; we don't help Declarer develop long suits. This is such an assumption that Eddie says if you DO want a shift to dummy's long suit, you have to play a "crazy" honor on partner's lead (with the singleton in dummy) in order to get the "crazy" shift.)
However, most of my partners and I have been playing that with 2 or fewer trumps in Dummy, the card is attitude and with three or more trumps in dummy, it is suit preference. There are good reasons to play attitude when dummy has two or fewer trumps. If I am sitting behind dummy with Kxx in their trump suit, I desperately want partner to continue the suit to force dummy to ruff, so Declarer canNOT finesse me out of my King. So, if we agree that ATTITUDE still applies when Dummy has two or fewer trumps, then these situations of protecting partner's vulnerable trump honor are covered. (Partner might also have Jxxx while you are void and if two trumps are left in dummy, Declarer can find out about bad break and then get back to dummy and finesse partner out of the jack. However, if dummy is forced to ruff, Declarer would need x-ray vision to finesse for the jack on the first round of the suit.—if Declarer DOES do that, hold your cards MUCH closer to your chest.)
However, another exception is when there is the potential of TRUMP PROMOTION if dummy is forced to ruff. Eddie has a cute example where a big hand doubles and then raises partner on AKQ in trump suit after partner has shown at least 6 in the suit. You are on lead with an AKxx and dummy has a singleton in that suit. Partner wants to ENCOURAGE that suit (NOT do suit preference), because if you force Dummy to ruff, you promote a trump trick for partner's Jxx.
Yet another exceptions is when the long trump hand is in dummy. (This could be on a transfer sequence; it could be after a 2-suited bid—such as Michaels or Unusual 2NT). Partner may WANT to force the long trump hand to ruff (so that eventually Declarer loses control of the hand). This exception requires a little thought. If partner COULD have three or more trumps, partner might want to force a dummy with a 5-card suit. So, an issue to discuss with various partners....