Simple class used to generate "shortened" URLs

Shortener.py

import math

class Shortener:
    """
    Simple class to shorten URLs, or rather, numerical IDs.

    Each URL must be saved to a database, with the data about source,
    category, country, publisher, etc... The database will give back
    a numerical ID. We're using that ID to convert it to a base 62
    number and using a symbol table, converting it to a sequence
    of alphanumeric chars.

    Our base sequence of 62 symbols will be arranged like this:

        'A' .. 'Z'            0 .. 25
        'a' .. 'z'           26 .. 51
        '0' .. '9'           52 .. 61

    For example, the number 62 in base 62 is 10, so the resulting
    string will be "BA", where "B" -> 1, "A" -> 0.
    """

    def shorturl_char(self, i):
        if i < 26:
            return chr(65 + i)   # 'A' .. 'Z'
        elif 26 <= i < 52:
            return chr(71 + i)   # 'a' .. 'z'
        elif 52 <= i < 62:
            return chr(i - 4)    # '0' .. '9'
        else:
            raise ValueError("Can't convert int %d to char" % i)

    def char_to_number(self, char):
        n = ord(char)

        if 65 <= n <= 90:
            return n - 65
        elif 97 <= n <= 122:
            return n - 71
        elif 48 <= n < 58:
            return n + 4
        else:
            raise ValueError("Can't convert char %s to int" % char)

    def token_to_number(self, token):
        n = 0
        reverse_token = token[::-1]
        pos = 0
        for c in reverse_token:
            n += self.char_to_number(c) * int(math.pow(62, pos))
            pos += 1

        return n

    def convert_number(self, n):
        token = ''

        if n == 0:
            return 'A'

        while n > 0:
            rmd = n % 62
            token = self.shorturl_char(rmd) + token
            n = n / 62

        return token