How to change random letter using number in Python

Question

I am working on a program in Python that sort-of encrypts a message. I have converted the string (which will vary based on user input) to a list and have a number. I want to automatically change each letter in the list with the number. For example, if the number is three, every A changes into a D. I have a dictionary with the vales for letters, such as {"a" : 1}, {"b" : 2}, etc.

I can not seem to get how I could change the letters (without knowing what they are) and possibly in accordance with my dictionary.

What I have so far (dictionary is somewhere else):

def numtolist(n):
    seedstring = str(n)
    numlist = []
    for digit in seedstring:
        numlist.append(int(digit))
    return numlist

currentnumber = seed^2
newmessage = str()
for letter in messageList:
    numtolist(currentnumber)
    num1 = numlist[0]

Answer 1

If the transform is as simple as an alphabetic shift you can simply do it by:

if original_character.isupper():  # Determine whether it is upper/lower case
    base_a = 'A'                  # use 'A' as a base for uppercase
else:                             #  
    base_a = 'a'                  # use 'a' as a base for lowercase


orig_char_ascii = ord(original_character)  # Get the original ascii value

orig_char_alpha_index = orig_char_ascii - ord(base_a) # Get the `0-25` alphabetic
                                                      # index of the character

shift_number = 3  # Set the amount to shift by

new_char_alpha_index = orig_char_alpha_index + shift_number # Add the shift value

new_char_alpha_index = new_char_alpha_index % 26  # Take the modulus to impose
                                                  # periodic boundary conditions
                                                  # i.e. if you had 'z' + 3
                                                  #      'z' + 3 -> 25 + 3 -> 28
                                                  #      28 % 26 = 2,  2 -> 'c'

new_char_ascii_index = ord(base_a) + new_char_alpha_index  # scale back to the ascii
                                                           # value

new_char = chr(new_char_ascii_index)

The basic idea is that each character corresponds to an ascii number which can be gotten by ord (i.e. ord('a') = 97). The chr method reverses this: chr(97) = 'a'.

The method is briefly: you get the ascii value, scale to a 0-25 alphabetic range, add your shift, wrap values that overflow the alphabetic range, scale back to ascii, and get a character back via chr.

You can compact this method a lot, I was verbose in the name of education:

def shift_char(ch,shift):
    if not ch.isalpha():
        return ch   # return non-alphabetic characters as is
    is_upper = 'A' <= original_character <= 'Z'
    if is_upper:
        base_a = 'A'
    else:
        base_a = 'a'
    return chr(ord(base_a) + (ord(ch)-ord(base_a)+shift)%26)

If you wanted to use the same random shift for a whole string you could split this into a couple methods:

def shift_char(ch,shift):
    if not ch.isalpha():
        return ch   # return non-alphabetic characters as is
    if ch.isupper():
        base_a = 'A'
    else:
        base_a = 'a'
    return chr(ord(base_a) + (ord(ch)-ord(base_a)+shift)%26)

def shift_string(s,shift):
    return ''.join(shift_char(i,r) for i in s)

If you want to be able to decode it later you need a reversal method and to store the shift value:

def shift_char(ch,shift):
    if not ch.isalpha():
        return ch   # return non-alphabetic characters as is
    if ch.isupper():
        base_a = 'A'
    else:
        base_a = 'a'
    return chr(ord(base_a) + (ord(ch)-ord(base_a)+shift)%26)

def encode_string(s,shift):
    return ''.join(shift_char(i,r) for i in s), shift  # NOTE EXTRA return value

def decode_string(s,shift):
    return ''.join(shift_char(i,-shift) for i in s)

# usage
s = 'Hello, StackOverflow!'
enc_s,shift = encode_string(s)
dec_s = decode_string(enc_s,shift)

If your shift is random you can just pass that as the argument:

import random

def shift_char(ch,shift):
    if not ch.isalpha():
        return ch   # return non-alphabetic characters as is
    if ch.isupper():
        base_a = 'A'
    else:
        base_a = 'a'
    return chr(ord(base_a) + (ord(ch)-ord(base_a)+shift)%26)

def encode_string(s,shift):
    return ''.join(shift_char(i,shift) for i in s)

def decode_string(s,shift):
    return ''.join(shift_char(i,-shift) for i in s)

# usage
s = 'Hello, StackOverflow!'
shift = random.randint(1,25)
enc_s = encode_string(s,shift)
dec_s = decode_string(enc_s,shift)

or rearrange the methods

import random

def shift_char(ch,shift):
    if not ch.isalpha():
        return ch   # return non-alphabetic characters as is
    if ch.isupper():
        base_a = 'A'
    else:
        base_a = 'a'
    return chr(ord(base_a) + (ord(ch)-ord(base_a)+shift)%26)

def encode_string(s):
    shift = random.randint(1,25)
    return ''.join(shift_char(i,shift) for i in s), shift

def decode_string(s,shift):
    return ''.join(shift_char(i,-shift) for i in s)

# usage
s = 'Hello, StackOverflow!'
enc_s, shift = encode_string(s)
dec_s = decode_string(enc_s,shift)