I want all characters in a string to increase, except for certain ones

I am working on a small script to encrypt a string that the user provides. The user can also choose how much each character in the string should increment by (on a scale from 1 to 25).

But, I want certain characters to not increment at all (commas, periods, question marks, and spaces) and I have these in a list. How exactly is the possible?

Below is the Python Script.

user_str = str(input("Enter String: "))
user_inc = int(input("How much do you want to increase by: "))

if user_inc > 25:
    print("You cannot increase by a number larger than 25.")

elif user_inc <= 0:
    print("You cannot increase by a number lower than 1.")

else: 
    final_str = ""
    for letter in user_str:
        new_str = ord(letter)
        enc_str = new_str + user_inc
        if enc_str > 122:
            enc_str -= 26      
        
# Here is where I need help.

        if letter in [',', '.', ' ', '?']:            
            letter + enc_str
        
        final_str += chr(enc_str)
    print(final_str)

I’ve tried a few different things like trying to add or subtract nothing from the letter, trying to add letter to the final_str, and so far nothing has worked. How exactly can this be done?

>Solution :

You use an if statement to determine whether to add the original value, or the new value to the ciphertext.

[...]
else: 
    final_str = ""
    for letter in user_str:
        new_str = ord(letter)
        enc_str = new_str + user_inc
        if enc_str > 122:
            enc_str -= 26      
        
# Here is one idea

        if letter in [',', '.', ' ', '?']:            
            final_str += letter
        else:
            final_str += chr(enc_str)               
        
        
    print(final_str)