Pls. explain why this syntax answer is 8

Question

class asdf:
def __init__(self,x):
self.x = x

def __setitem__(self,y,z):
print(y + z)

x = asdf([1,2,3,4])
x[4]=4

Answer 1

class asdf:
def __init__(self,x):
self.x = x

def __setitem__(self,y,z):
print(y + z) #4 + 4

x = asdf([1,2,3,4])
x[4]=4 #x[y] = z --> print(y + z) --> print(4 + 4) --> print(8)