Dive Into Python

Example 14.9. roman4.py

This file is available in py/roman/stage4/ in the examples directory.

If you have not already done so, you can download this and other examples used in this book.

"""Convert to and from Roman numerals"""

#Define exceptions
class RomanError(Exception): pass
class OutOfRangeError(RomanError): pass
class NotIntegerError(RomanError): pass
class InvalidRomanNumeralError(RomanError): pass

#Define digit mapping
romanNumeralMap = (('M',  1000),
                   ('CM', 900),
                   ('D',  500),
                   ('CD', 400),
                   ('C',  100),
                   ('XC', 90),
                   ('L',  50),
                   ('XL', 40),
                   ('X',  10),
                   ('IX', 9),
                   ('V',  5),
                   ('IV', 4),
                   ('I',  1))

# toRoman function omitted for clarity (it hasn't changed)

def fromRoman(s):
    """convert Roman numeral to integer"""
    result = 0
    index = 0
    for numeral, integer in romanNumeralMap:
        while s[index:index+len(numeral)] == numeral: 
            result += integer
            index += len(numeral)
    return result

The pattern here is the same as toRoman. You iterate through your Roman numeral data structure (a tuple of tuples), and instead of matching the highest integer values as often as possible, you match the “highest” Roman numeral character strings as often as possible.

Example 14.10. How fromRoman works

If you're not clear how fromRoman works, add a print statement to the end of the while loop:

        while s[index:index+len(numeral)] == numeral:
            result += integer
            index += len(numeral)
            print 'found', numeral, 'of length', len(numeral), ', adding', integer

>>> import roman4
>>> roman4.fromRoman('MCMLXXII')
found M , of length 1, adding 1000
found CM , of length 2, adding 900
found L , of length 1, adding 50
found X , of length 1, adding 10
found X , of length 1, adding 10
found I , of length 1, adding 1
found I , of length 1, adding 1
1972

Example 14.11. Output of romantest4.py against roman4.py

fromRoman should only accept uppercase input ... FAIL
toRoman should always return uppercase ... ok
fromRoman should fail with malformed antecedents ... FAIL
fromRoman should fail with repeated pairs of numerals ... FAIL
fromRoman should fail with too many repeated numerals ... FAIL
fromRoman should give known result with known input ... ok 
toRoman should give known result with known input ... ok
fromRoman(toRoman(n))==n for all n ... ok                  
toRoman should fail with non-integer input ... ok
toRoman should fail with negative input ... ok
toRoman should fail with large input ... ok
toRoman should fail with 0 input ... ok

Two pieces of exciting news here. The first is that fromRoman works for good input, at least for all the known values you test.

The second is that the sanity check also passed. Combined with the known values tests, you can be reasonably sure that both toRoman and fromRoman work properly for all possible good values. (This is not guaranteed; it is theoretically possible that toRoman has a bug that produces the wrong Roman numeral for some particular set of inputs, and that fromRoman has a reciprocal bug that produces the same wrong integer values for exactly that set of Roman numerals that toRoman generated incorrectly. Depending on your application and your requirements, this possibility may bother you; if so, write more comprehensive test cases until it doesn't bother you.)

======================================================================
FAIL: fromRoman should only accept uppercase input
----------------------------------------------------------------------
Traceback (most recent call last):
  File "C:\docbook\dip\py\roman\stage4\romantest4.py", line 156, in testFromRomanCase
    roman4.fromRoman, numeral.lower())
  File "c:\python21\lib\unittest.py", line 266, in failUnlessRaises
    raise self.failureException, excName
AssertionError: InvalidRomanNumeralError
======================================================================
FAIL: fromRoman should fail with malformed antecedents
----------------------------------------------------------------------
Traceback (most recent call last):
  File "C:\docbook\dip\py\roman\stage4\romantest4.py", line 133, in testMalformedAntecedent
    self.assertRaises(roman4.InvalidRomanNumeralError, roman4.fromRoman, s)
  File "c:\python21\lib\unittest.py", line 266, in failUnlessRaises
    raise self.failureException, excName
AssertionError: InvalidRomanNumeralError
======================================================================
FAIL: fromRoman should fail with repeated pairs of numerals
----------------------------------------------------------------------
Traceback (most recent call last):
  File "C:\docbook\dip\py\roman\stage4\romantest4.py", line 127, in testRepeatedPairs
    self.assertRaises(roman4.InvalidRomanNumeralError, roman4.fromRoman, s)
  File "c:\python21\lib\unittest.py", line 266, in failUnlessRaises
    raise self.failureException, excName
AssertionError: InvalidRomanNumeralError
======================================================================
FAIL: fromRoman should fail with too many repeated numerals
----------------------------------------------------------------------
Traceback (most recent call last):
  File "C:\docbook\dip\py\roman\stage4\romantest4.py", line 122, in testTooManyRepeatedNumerals
    self.assertRaises(roman4.InvalidRomanNumeralError, roman4.fromRoman, s)
  File "c:\python21\lib\unittest.py", line 266, in failUnlessRaises
    raise self.failureException, excName
AssertionError: InvalidRomanNumeralError
----------------------------------------------------------------------
Ran 12 tests in 1.222s

FAILED (failures=4)