An unbounded integer stored in a string.

string_integer.py

#!/usr/bin/env python3
import unittest
import random

try:
    # Python3, cool!
    from itertools import zip_longest
except ImportError:
    # I am on python2
    from itertools import izip_longest as zip_longest

class StringIntegerException(Exception):
    pass

class StringInteger(str):
    '''
    A integer stored as strings. Currently only supports addition.

    Usage:
    > integer_f = StringInteger("45345")
    > integer_s = StringInteger("4953")
    > integer_f + integer_s
    '50298' # A new StringInteger object.

    '''


    def __init__(self, string):
        '''
        Iniitializer. Acts as a validator - Rejects non numeric characters as input.
        '''
        for char in string.upper():
            if not (48 <= ord(char) <= 70):
                raise StringIntegerException("Invalid character %c input for '%s'"
                                             %(char, self.__class__.__name__))

        super(StringInteger, self).__init__()

    @property
    def as_iterable(self):
        '''
        Return an iterable object over the digits as integers.
        '''

        return map(int, self)

    @property
    def as_list(self):
        '''
        Return a list of digits as integers.
        '''
        return list(self.as_iterable)

    @classmethod
    def from_int(cls, integer):
        '''
        Generate a StringInteger from a integer as a input
        '''

        return cls(str(integer))

    @classmethod
    def from_list(cls, integer_list):
        '''
        Generate a StringInteger from a list of integer digits as a input
        '''

        return cls("".join(map(str, integer_list)))

    def __add__(self, other_sint):
        '''
        Override add, to behave like arithmetic addition of two numbers.
        Returns a new StringInteger instance
        '''

        first = self.as_list
        second = other_sint.as_list

        first.reverse()
        second.reverse()

        carry = 0
        result = []

        for fst, snd in zip_longest(first, second, fillvalue=0):
            digit_sum = str(fst + snd + carry)
            result_digit = int(digit_sum[-1])
            carry = int(digit_sum[:-1] or 0)

            result.insert(0, result_digit)

        if carry > 0:
            # Prepend any remaining carrys
            result = list(map(int, str(carry))) + result

        return self.__class__.from_list(result)

SI = StringInteger.from_int

class StringIntegerTest(unittest.TestCase):

    def setUp(self):
        # Set a custom initial random seed, for comparibility across tests.
        random.seed(1)

    def _rand_int(self):
        return random.randint(0, 10 ** 10)

    def _test_add(self, first, second):
        self.assertEqual(str(first + second),
                         SI(first) + SI(second))


    def test_multiple_additions(self):
        for i in range(5000):
            first = self._rand_int()
            second = self._rand_int()

            self._test_add(first, second)

if __name__ == '__main__':
    unittest.main()