TypeError: _transform() takes 2 positional arguments but 3 were given

Question

I learned the CaesarCipher:

In [90]: !cat caesar_cipher.py                                                                                                                        
class CaesarCipher:
    """Construct Caesar cipher using given integer shift for rotation."""
    def __init__(self, shift):
        encoder = [None] * 26
        decoder = [None] * 26
        for k in range(26):
            encoder[k] = chr((k + shift)%26 + ord('A'))
            decoder[k] = chr((k - shift)%26 + ord('A'))  #find the number of Letters
        self.encoder = "".join(encoder)
        self.decoder = "".join(decoder)

    def encrypt(self, message):
        print(self.encoder)
        return self._transform(message, self.encoder)

    def decrypt(self, message):
        return self._transform(message, self.decoder)

    def _transform(original, code):
        msg = list(original)
        for k in range(len(msg)):
            j = ord(msg[k]) - ord('A')
            msg[k] = code[j]
        return "".join(msg)

if __name__ == "__main__":
    cipher = CaesarCipher(3)
    message = "THE EAGLE IS IN PLAY; MEET AT JOE'S."
    coded = cipher.encrypt(message)
    print("Secret: ", coded)
    answer = cipher.decrypt(coded)
    print("Message: ", answer)

It report error on _trasform

In [91]: !python caesar_cipher.py                                                                                                                     
DEFGHIJKLMNOPQRSTUVWXYZABC
Traceback (most recent call last):
  File "caesar_cipher.py", line 29, in <module>
    coded = cipher.encrypt(message)
  File "caesar_cipher.py", line 14, in encrypt
    return self._transform(message, self.encoder)
TypeError: _transform() takes 2 positional arguments but 3 were given

"_transform() takes 2 positional arguments" and I did give 2 arguments

Why it report 3 were given?

Answer 1

You need to define it as

def _transform(self, original, code)

Answer 2

You have to add self argument at first, like

def _transform(self, original, code)...

Or you can also make it as a staticmethod.

@staticmethod
def _transform(original, code)...

Answer 3

There is no need to have _transform() method in the CaesarCipher class, because it is a stateless one (as it don't use instance variables) and it is has no particular relation to Ceasar Cipher - it is a generic character substitution cipher.

So you may define it as a regular function (outside of any class) and instead of commands

return self._transform(message, self.encoder)
return self._transform(message, self.decoder)

(in your encrypt() and decrypt() methods) use

return transform(message, self.encoder)
return transform(message, self.decoder)