basic FACTORIAL algorithm on PYTHON with "while" loop

Question

For the first time wrote "Do you want rety ? (y or n)" I always get an this output "wrong ! try again .". After that its working good but it first time gives error.

def factorial(n):
    r=1
    if n==0:
        return 
    else:
        for i in range(2,n+1):
            r=r*i
        return r

n=int(input("Give a number : "))
b=factorial(n)
print (n,"!", "= ",b)

a=str(input("Do you want rety ? (y or n) \n"))

while (a != "y" or a != "n" or a!="Y" or a!="N"):
    print("wrong ! try again .")
    a=str(input("Do you want rety ? (y or n) \n")) 
    if a=="y" or a=="n" or a=="Y" or a=="N":
        break

while a=="y" or a=="Y":
    n=int(input("Give a number : "))
    b=factorial(n)
    print (n,"!", "= ",b)
    a=str(input("Do you want rety ? (y or n) \n")) 
if (a=="n" or a=="N"):
    print("thanks for using")

Answer 1

Change your condition in first while loop from a != "y" or a != "n" or a!="Y" or a!="N" to

while a not in ['y', 'Y', 'n', 'N']

Explanation:

Let say a = "Y", then a won't be equal to "n". Hence your condition with or will always be True.

If you are interested in or logic, write it as (but not in list is cleaner):

while not (a == "y" or a == "n" or a=="Y" or a=="N")