深入 Python

例 14.9. roman4.py

此文件位于示例目录的 py/roman/stage4/ 中。

如果您还没有这样做，可以 下载本书中使用的这个和其他示例。

"""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

这里的模式与 toRoman 相同。您遍历罗马数字数据结构（元组的元组），而不是尽可能频繁地匹配最高的整数值，而是尽可能频繁地匹配“最高”的罗马数字字符字符串。

例 14.10. fromRoman 如何工作

如果您不清楚 fromRoman 如何工作，请在 while 循环的末尾添加一个 print 语句

        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

例 14.11. romantest4.py 针对 roman4.py 的输出

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

	这里有两个令人兴奋的消息。首先是 fromRoman 对于良好的输入有效，至少对于您测试的所有 已知值 都是如此。
	其次是 健全性检查 也通过了。结合已知值测试，您可以相当确定 toRoman 和 fromRoman 对于所有可能的良好值都能正常工作。（这不是绝对保证；理论上，toRoman 可能存在一个错误，对于某些特定的输入集产生错误的罗马数字，并且 fromRoman 存在一个互惠错误，对于 toRoman 错误生成的罗马数字集产生相同的错误整数值。根据您的应用程序和您的要求，这种可能性可能会困扰您；如果是这样，请编写更全面的测试用例，直到它不再困扰您。）

======================================================================
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)