For a tiny function you only need once, often as a key, Python lets you write it inline with lambda:

people = [("Ada", 36), ("Grace", 85), ("Linus", 54)]
print(sorted(people, key=lambda p: p[1]))
print(max(people, key=lambda p: p[1])[0])

square = lambda n: n * n
print(square(7))

lambda p: p[1] means "a function that takes p and returns p[1]". A lambda can only contain a single expression, with no return keyword. If it needs more than that, write a normal def. (Storing a lambda in a variable, like square above, works but is considered poor style: use def for anything with a name.)