hearthstone
A Paladin has a Tirion Fordring in play and a Light's Justice weapon equipped.
A Hunter has a Knife Juggler in play and a Snake Trap secret up.
The Paladin starts attacking the Knife Juggler with Light's Justice triggering Snake Trap. The Knife Juggler starts juggling and deals fatal damage to Tirion Fordring.
What happens next?
Does the Paladin switch weapons to Ashbringer mid-attack and then hit the Knife Juggler with it? Does the attack cancel?
Yes, that counts as a weapon. Specifically the weapon is a card named "Blood Fury."
If the "weapon circle" is filled up, it counts as a weapon. Anything expressed in the format attack/durability is a weapon, the other effects that can give your hero an attack last a single turn and are just expressed as "x damage till end of turn"
Only those limited to one turn.
Mind Control and Mind Control Tech both take control of a Minion permanently. This is regarded by the game's rules engine as that Minion newly joining your army and thus follows the same attacking restrictions as a Minion you summon or play from you hand. That means, unless it has Charge, it can't attack that turn.
Shadow Madness takes control of a Minion for one turn only. This would simply not do much if the Minion didn't receive Charge implicitly. Actually, there has been quite a discussion about the decision not to write it on the card (supposedly to keep it simple), but it seems Shadow Madness is intended (and probably handled internally) to give Charge to the Minion.
From a logical point of view, this exception makes sense because the card would do very little without that mechanic (notably, it would work the same way it does now with Charge Minions, and might get a dangerous Taunt Minion out of the way). Seeing as this could theoretically be intentional behaviour, though, we could well see the card reworded in the future if and when cards are released that do exactly this. For the time being, it's safe to assume that cards which take control of a Minion only for one turn give that Minion Charge implicitly.
Best Answer
Here is a video proof : Youtube
The video shows a Paladin attacking with a weapon into a Hunter's Explosive Trap while Tirion Fordring is at 2 health. The secret triggers, kills Tirion, the Paladin equips Ashbringer, and only then does the Paladin hit the Hunter, now using Ashbringer instead of the original weapon.
Result : You can change weapon mid-attack.