How do i prevent users from inserting something other than Yes No

Question

def ex1():
while 1==1:
print("Yes/No please")

def yes():
while 1==1:
print ("WHY")

def no():
print("You have chosen the right path")
def check_input(user_input):
if user_input.lower() == "Yes":
yes()
elif user_input.lower() == "No":
no()
user_input = input("whatever? (Yes/No): ")
check_input(user_input)

input()

Answer 1

𝐊𝐢𝐢𝐛𝐨 𝐆𝐡𝐚𝐲𝐚𝐥 i have the while 1==1 to spam, and yes i have ex1 to tell them to enter yes/no but i dont know how to put it in, i want the programme to close if they enter something that is not yes/no