The expression this_prize.choice is telling the interpreter that you want to access an attribute of this_prize with the name “choice”. But this attribute does not exist in this_prize.
What you actually want is to return the attribute of this_prize identified by the value of choice. So you just need to change your last line using the getattr() method…
from collections import namedtuple
import random
Prize = namedtuple("Prize", ["left", "right" ])
this_prize = Prize("FirstPrize", "SecondPrize")
if random.random() > .5:
choice = "left"
else:
choice = "right"
# retrieve the value of "left" or "right" depending on the choice
print "You won", getattr(this_prize, choice)