What is the purpose of using w in lines 4 to 6 of the code?

def median(x,y,z):
    print(x,y,z)
    if x>y:
       w = x
       x = y
       y=w
    print(x,y,z)
    if z<x:
         return x
    if z<y:
       return z
    returny
assert median(3,1,2) == 2