How does class creation work?

from __future__ import print_function

class S():
    _ = print("executing class S")
    x = 1
    y = x

print("S.y", S.y)

try:
    class A(S):
        _ = print("executing class A")
        y = x
except NameError:
    pass

try:
    A
except NameError:
    print("A is not defined; as expected")
else:
    assert not "A is defined"

class B(S):
    _ = print("executing class B")

print("B.y", B.y)