Combining string an integers for the Vigenere cipher in python?

Question

I am trying to write a vigenere cipher encrypter in python. I am getting another error...

def vigenere(string,key):
for i in range(len(key)):
    if key[i].isupper():
        tempList = list(key)
        tempList[i] = chr(ord(key[i])-65)
        key = "".join(tempList)
    elif key[i].islower():
        tempList = list(key)
        tempList[i] = chr(ord(key[i])-97)
        key = "".join(tempList)
k = 0
newstring = ''
for i in string:
    if i.isupper():
        newstring = newstring + ((ord(i)-65)+(key[k % len(key)]))%26 + 65
    elif i.islower():
        newstring = newstring + ((ord(i)-97)+(key[k % len(key)]))%26 + 97
    k = k + 1
return newstring

"unsupported operand type(s) for +: 'int' and 'str'" -- any help?

Answer 1

First, you need to change:

key[i] + ord(key[i])-97

To:

key[i] = ord(key[i])-97

It seems that is a mistyping.

Second, the ord(...) function returns an int. You want to convert it back to a char using chr(...):

key[i] = chr(ord(key[i])-97)

Finally, in Python, strings are immutable. This means you can't change the individual char of a string. This is an easy way to do it:

if key[i].isupper():
    tempList = list(key)
    tempList[i] = chr(ord(key[i])-65)
    key = "".join(tempList)
elif key[i].islower():
    tempList = list(key)
    tempList[i] = chr(ord(key[i])-97)
    key = "".join(tempList)