Python string concatation for Caesar cipher, comparable with Java code

Question

I've finished writing the encryption code for Caesar cipher in Java and my plan is to rewrite the program using the same logic in Python as well. Although it can be done in many ways, is there any way I can use something equivalent to StringBuilder in Python? If I have a hold variable in Python that stores a string value then how can I append new characters or strings in a way similar done in Java below?

    static String encrypt(String s, int shift)
    {
        shift = shift % 26 + 26;
        StringBuilder hold = new StringBuilder();
        for (char i : s.toCharArray()) 
        {
            if (Character.isLetter(i)) 
            {
                if (Character.isUpperCase(i))
                    hold.append((char)('A' + (i - 'A' + shift) % 26 ));
                else
                    hold.append((char)('a' + (i - 'a' + shift) % 26 ));
            } 
            else
                hold.append(i);
        }
        return hold.toString();
    }

Answer 1

You can use direct list conversion of a string, and then use use the ord()/chr() functions to mess with an individual character's ASCII value.

Strings are immutable, but lists aren't. You can concatenate strings just fine in Python, and strings can be changed via list conversion and reconversion with split() and join().

Python strings also have isupper(), islower(), isdigit(), and isalpha() methods that give a boolean value.

I can't test this right now, because I don't fully understand your cipher, but I think it's close to replicating your JAVA code. You might want to tweak the cipher bits somewhat.

def encrypt(encryptString, shift):
    shift = shift % 26 + 26
    stringList = list(encryptString)
    for eachCharacter in stringList:
        if eachCharacter.isalpha() == True:
            if eachCharacter.isupper() == True:
                eachCharacter = chr( ( ord('A') + ord(eachCharacter) - ord('A') + shift) % 26)
            else:
                eachCharacter = chr( ( ord('a') + ord(eachCharacter) - ord('a') + shift) % 26)

    encryptString = ''.join(stringList)
    return encryptString

Answer 2

I would suggest using a list of characters i.e.

>>> x = ['a', 'b', 'c']
>>> x
['a', 'b', 'c']
>>> x.append('d')
>>> x += ['e', 'f']
>>> x
['a', 'b', 'c', 'd', 'e', 'f']
>>> "".join(x)
'abcdef'