Multi tap encoding and decoding

multitap.py

from copy import deepcopy

class InvalidMultiTap(Exception):
    """Input is invalid"""

class MultiTap:

    mapping = {1: ['1'],
               2: ['a', 'b', 'c', '2', 'A', 'B', 'C'],
               3: ['d', 'e', 'f', '3', 'D', 'E', 'F'],
               4: ['g', 'h', 'i', '4', 'G', 'H', 'I'],
               5: ['j', 'k', 'l', '5', 'J', 'K', 'L'],
               6: ['m', 'n', 'o', '6', 'M', 'N', 'O'],
               7: ['p', 'q', 'r', 's', '7', 'P', 'Q', 'R', 'S'],
               8: ['t', 'u', 'v', '8', 'T', 'U', 'V'],
               9: ['w', 'x', 'y', 'z', '9', 'W', 'X', 'Y', 'Z'],
               0: [' ']}
    # for the extended characters there does not seem to be a standard, so i came up with mine 
    # and it follows the keyboard of my Mac pro line by line.
    extended = ['`', '~', '!', '@', '#', '$', '%', '^', '&', '*', '(', ')', '_', '-', '+', '=', 
                '{', '}', '[', ']', '|', '\\',
                ':', ';', '"', "'",
                '<', ',', '>', '.', '?', '/']

    def __init__(self, extended=False):
        self._dialpad = deepcopy(self.mapping)
        if extended:
            self._dialpad[1].extend(self.extended) 

    @property
    def dialpad(self):
        return self._dialpad

    def encode(self, text):
        result = []
        for character in list(text):
            for index, character_set in self.dialpad.items():
                if character in character_set:
                    result.append(str(index) * (character_set.index(character) + 1))
                    continue
        return ' '.join(result)

    def decode(self, text):
        result = []
        for character_group in text.split():
            if len(set(character_group)) > 1:
                raise InvalidMultiTap(f'{character_group} does not seem to be valid multi-tap encoding.')
            try:
                index = int(character_group[0])
                result.append(self.dialpad[index][len(character_group) - 1])
            except (ValueError, IndexError):
                raise InvalidMultiTap(f'{character_group} does not seem to be valid multi-tap encoding.')
        return ''.join(result)