Шифр Виженера

def ok(x):
if 65 <= x <= 90:
x = x - 65
return(x)
elif 97<= x <=122:
x = x - 97
return(x)
i = 0
sh = '\0'
while True:
print(' шифратор-1\n дешифратор-2')
q=int(input('commad'))
if q==1:
text = input('text')
key = input('key')
while i < len(text):
a = ord(text[i])
k = ord(key[i%len(key)])
if 65 <= a <= 90:
a = a - 65 + ok(k)
if a>25:
a = a - 26
a = a + 65
elif 97<= a <=122:
a = a - 97 + ok(k)
if a>25:
a = a - 26
a = a + 97
sh = sh + chr(a)
i+=1
print ('\n \n' + sh)
elif q==2:
text = input('text')
key = input('key')
while i < len(text):
a = ord(text[i])
k = ord(key[i%len(key)])
if 65 <= a <= 90:
a = a - 65 - ok(k)
if 0>a:
a = a + 26
a = a + 65
elif 97<= a <=122:
a = a - 97 - ok(k)
if 0>a:
a = a + 26
a = a + 97
sh = sh + chr(a)
i+=1
print ('\n \n' + sh)
else:
print('this command not exist')