The snub-nosed Python makes an appearance in the singleplayer mission "Payback," when Woods and Mason are forced to play Russian Roulette. Mason uses this revolver to kill the Viet Cong Bookie and takes his CZ75, but does not keep the Python, presumably because it only contained one round in the first place.
The snub nose is an attachment for the Python in multiplayer, costing 1000 CoD Points. It will get lower recoil, but the maximum damage is lowered as to be on par with the other semi-auto pistols, which have lower recoil, larger magazines, and faster reloads. The only advantage the Python has over the other pistols is a slightly longer three hit kill range, greater damage at a distance (which is negated by the small capacity and moderate visual recoil), and smaller hipfire spread.
Four snub-nosed Pythons appear in the opening cutscene for Mob of the Dead. Billy Handsome carries four, which he then hands out to the others. It does not appear in gameplay. Handsome states that these Pythons are from the warden's private collection.