class desc:
    def __set_name__(self, cls, name):
        print("set name", name, "in", cls.__name__)
        cls.foo = 1  # This line is important. It modifies the cls __dict__


class Class:
    a = desc()
    b = desc()
    c = desc()
    d = desc()
    e = desc()

# Expected output:
# set name a in Class
# set name b in Class
# set name c in Class
# set name d in Class
# set name e in Class

# Actual output might depend on various factors.
# But you can see that some "set name" lines are not printed, because the
#  corresponding __set_name__ is not called.
# You might have to comment out some of the "x = desc()" lines to see the bug.
# You might even have to add more lines to see the bug.