Python caesar cipher how to decode a specific sentence?

Question

I am trying to decode enciphered message 'lmnopqrstuvwxyzabcdefghijk'. I got that it shifted by 11 and I must decipher 'I wtvp olel decfnefcpd lyo lwrzctesxd'.

Here's what I wrote so far:

#enciphered message = 'I wtvp olel decfnefcpd lyo lwrzctenter code hereesxd'

plain = 'abcdefghijklmnopqrstuvwxyz'
#plain Index= 0123456789........25 

cipher = 'lmnopqrstuvwxxyzabcdefghijk'
#11 12 13 14 15 16 17 18 19 20 21 22 23 24 25...12345678910

cipher_text = input('Enter enciphered message: ')
clean_text ='I '
for i in cipher_text:
    if cipher_text.index(i) != " ":
        clean_text = clean_text + plain.index(cipher[(ord(i)-ord('a'))])
    else:
        clean_text = clean_text + " "

print(clean_text)

When I ran it:

Enter enciphered message: I wtvp olel decfnefcpd lyo lwrzctesxd
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-97-ac338a9d79fc> in <module>
     18 for i in cipher_text:
     19     if cipher_text.index(i) != " ":
---> 20         clean_text = clean_text + plain.index(cipher[(ord(i)-ord('a'))])
     21         #chr((cipher.index(cipher_text[i]))+ ord('a')) - 11)
     22     else:

TypeError: can only concatenate str (not "int") to str

I am very new to Python, and I have no idea how to solve it.

add commend: The enciphered message that I want to decode has capital "I" as well as space between words, so I want to know how to decode capital and lowercase at the same time

Answer 1

list.index() returns the index of that item's first occurence in the list. So the expression plain.index(cipher[(ord(i)-ord('a'))]) is an integer, which Python won't allow adding to a string.

What you probably want for the clean_text is the element in plain which has that index. So can use the standard list[index] notation to get the element: plain[plain.index(...)]

That line would now be:

clean_text = clean_text + plain[plain.index(cipher[(ord(i)-ord('a'))])]

>>> print(clean_text)
I wxxyzabcdefghhijlmnopqrstuv

---

Other changes/improvements:

1. For the line if cipher_text.index(i) != " ":, the expression will never be true since list.index() returns an integer, or a ValueError if not found, which will never match as equal to a space " ". You also already have the character you're checking in the loop variable i. So that could be:

   if i != " ":
   

2. There's a possible typo in cipher and cipher_text, the letter x appears twice.

3. Wrt your comment:

   It also has capital "I" at the beginning of the sentence as well as space between words. :(

   That's from the line clean_text ='I ' just before the loop. Change that to clean_text =''.

4. Don't concatenate strings with x = x + 'something new'. Keep appending each element to a list and then use str.join():

   # before the loop:
   clean_text = []  # empty list
   # inside the loop:
   clean_text.append(plain[plain.index(cipher[(ord(i)-ord('a'))])])
   # later, after the loop, put:
   ''.join(clean_text)