Algorithms in Python: Base Expansion

I felt it would be helpful to folks interested in Python and studying algorithms, to review some commonly studied algorithms in Computer Science by providing a small description and a Python implementation of each algorithm.

This week, we’ll cover an introductory algorithm for converting from one numeral base to another. Here is the python code:

"""Simple implementation of a base expansion algorithm"""
 
import math
 
def base_expand(base, val):
    """This simple function performs a base-expansion from decimal
    using moduli and a translation table. The translation table is
    a clear limitation here, in that it implies the maximum base
    is 36."""
 
    if (base < 2) or (base > 36):
        raise BaseOutOfBoundsError(base)
 
    trans_table = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ'
    res = ''
 
    while val != 0:
        res += trans_table[(int(val % base))]
        val = math.floor(val / base)
    return res[::-1]
 
class BaseOutOfBoundsError(Exception):
    """Base must be between 2 and 36"""
    def __init__(self, val):
        self.val = val
    def __str__(self):
        print "\nInvalid base: %s. Base must be (x | x > 1; x < 37)" % \
            self.val

Here is a unit-test for base_expand.py:

"""Unit tests for base_expansion.py"""
 
import unittest
from base_expansion import base_expand, BaseOutOfBoundsError
 
# Unit tests for base_expansion.py
class TestVals(unittest.TestCase):
    """Test suite"""
 
    def test_known_values(self):
        """Testing against known values"""
 
        vals = [{'val': 8, 'base': 2, 'expect': '1000'},
            {'val': 915652, 'base': 16, 'expect': 'DF8C4'},
            {'val': 256, 'base': 10, 'expect': '256'},
            {'val': 88189, 'base': 8, 'expect': '254175'}]
 
        for val in vals:
            self.assertEqual(val['expect'], \
                base_expand(val['base'], val['val']))
 
    def test_invalid_base(self):
        """ Testing invalid bases """
 
        bases = [1, 37, 50, 100]
        for base in bases:
            self.assertRaises(BaseOutOfBoundsError, base_expand, \
                base, 1)
 
 
 
if '__main__' == __name__:
    unittest.main()