adding string to a string

I have a problem adding a string to another string. I am new to Python.

The string cannot remember the previous values I added.

Anyone who can help me? The following is the code snippet in Python.

My problem is in the while loop of encrypt().

THANKS IN ADVANCE.

class Cipher:


    def __init__(self):
        self.alphabet = "abcdefghijklmnopqrstuvwxyz1234567890 "
        self.mixedalpha = ""
        self.finmix = ""

    def encrypt(self, plaintext, pw):
        keyLength = len(pw)
        alphabetLength = len(self.alphabet)
        ciphertext = ""

        if len(self.mixedalpha) != len(self.alphabet):
            #print 'in while loop'

            x = 0
            **while x < len(self.alphabet):
                mixed = self.mixedalpha.__add__(pw)
                if mixed.__contains__(self.alph开发者_如何学JAVAabet[x]):
                    print 'already in mixedalpha'
                else:
                    add = mixed.__add__(str(self.alphabet[x]))
                    lastIndex = len(add)-1
                    fin = add[lastIndex]
                    print 'fin: ', fin
                    self.finmix.__add__(fin)
                    print 'self.finmix: ', self.finmix
                x+=1**


        print 'self.finmix: ', self.finmix
        print 'self.mixedalpha: ', self.mixedalpha
        for pi in range(len(plaintext)):
            #looks for the letter of plaintext that matches the alphabet, ex: n is 13
            a  = self.alphabet.index(plaintext[pi])
            #print 'a: ',a
            b = pi % keyLength
            #print 'b: ',b
            #looks for the letter of pw that matches the alphabet, ex: e is 4
            c = self.alphabet.index(pw[b])
            #print 'c: ',c
            d = (a+c) % alphabetLength
            #print 'd: ',d
            ciphertext += self.alphabet[d]
            #print 'self.alphabet[d]: ', self.alphabet[d]
        return ciphertext

Python strings are immutable, so you should reassign the variable name to a new string.

Functions with "__" in them are not normally what you really want to use. Let the interpreter make that call for you by using the built in operators/functions (in this case the "+" operator).

So, instead of:

self.finmix.__add__(fin)

I suggest you try:

self.finmix = self.finmix + fin

or the equivalent and terser:

self.finmix += fin

If you make this sort of change throughout, your problem will probably go away.

I don't have a solution to your problem, but I have a couple of more general suggestions.

• The private methods .__add__ and .__contains__ aren't meant to be used directly. You should use the the + and in operators directly instead.

• Instead of going through the indexes of self.alphabet with a while loop...

  while x < len(self.alphabet):
      print self.alphabet[x]
      x += 1
  

  you could just iterate over the letters

  for letter in self.alphabet:
      print letter
  

• class Cipher: triggers a sort of backwards-compatibility mode that doesn't work with some newer features. Specifying class Cipher(object): would be better.

I'm guessing but the following:

self.finmix.__add__(fin)

#should be
self.finmix = self.finmix.__add__(fin)