现在 toRoman 对于有效的输入（1 到 3999 整数）已能正确工作，是正确处理那些无效输入（任何其他输入）的时候了。

例 14.6. roman3.py

这个文件可以在例子目录下的 py/roman/stage3/ 目录中找到。

如果您还没有下载本书附带的例子程序, 可以 下载本程序和其他例子程序。

"""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))

def toRoman(n):
    """convert integer to Roman numeral"""
    if not (0 < n < 4000):                                             
        raise OutOfRangeError, "number out of range (must be 1..3999)" 
    if int(n) <> n:                                                    
        raise NotIntegerError, "non-integers can not be converted"

    result = ""                                                        
    for numeral, integer in romanNumeralMap:
        while n >= integer:
            result += numeral
            n -= integer
    return result

def fromRoman(s):
    """convert Roman numeral to integer"""
    pass

	这个写法很 Pythonic：一次进行多个比较。 这等价于if not ((0 < n) and (n < 4000))， 但是更容易让人理解。这是在进行范围检查，可以将过大的数，负数和零查出来。
	你使用 raise 语句引发自己的异常。 你可以引发任何内建异常或者已定义的自定义异常。第二个参数是可选的，如果给定，则在异常未被处理时显示于追踪信息（trackback）之中。
	这是一个非整数检查。非整数无法转化为罗马数字表示。
	函数的其他部分未被更改。

例 14.7. 观察 toRoman 如何处理无效输入

>>> import roman3
>>> roman3.toRoman(4000)
Traceback (most recent call last):
  File "<interactive input>", line 1, in ?
  File "roman3.py", line 27, in toRoman
    raise OutOfRangeError, "number out of range (must be 1..3999)"
OutOfRangeError: number out of range (must be 1..3999)
>>> roman3.toRoman(1.5)
Traceback (most recent call last):
  File "<interactive input>", line 1, in ?
  File "roman3.py", line 29, in toRoman
    raise NotIntegerError, "non-integers can not be converted"
NotIntegerError: non-integers can not be converted

例 14.8. 用 romantest3.py 测试 roman3.py 的结果

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 ... FAIL
toRoman should give known result with known input ... ok 
fromRoman(toRoman(n))==n for all n ... FAIL
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

	toRoman 仍然能通过 已知值测试，这很令人鼓舞。 所有 第 2 阶段 通过的测试仍然能通过，这说明新的代码没有对原有代码构成任何负面影响。
	更令人振奋的是所有的 无效输入测试 现在都通过了。 testNonInteger 这个测试能够通过是因为有了 int(n) <> n 检查。 当一个非整数传递给 toRoman， int(n) <> n 检查出问题并引发 NotIntegerError 异常，这正是 testNonInteger 所期待的。
	testNegative 这个测试能够通过是因为 not (0 < n < 4000) 检查引发了 testNegative 期待的 OutOfRangeError 异常。

======================================================================
FAIL: fromRoman should only accept uppercase input
----------------------------------------------------------------------
Traceback (most recent call last):
  File "C:\docbook\dip\py\roman\stage3\romantest3.py", line 156, in testFromRomanCase
    roman3.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\stage3\romantest3.py", line 133, in testMalformedAntecedent
    self.assertRaises(roman3.InvalidRomanNumeralError, roman3.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\stage3\romantest3.py", line 127, in testRepeatedPairs
    self.assertRaises(roman3.InvalidRomanNumeralError, roman3.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\stage3\romantest3.py", line 122, in testTooManyRepeatedNumerals
    self.assertRaises(roman3.InvalidRomanNumeralError, roman3.fromRoman, s)
  File "c:\python21\lib\unittest.py", line 266, in failUnlessRaises
    raise self.failureException, excName
AssertionError: InvalidRomanNumeralError
======================================================================
FAIL: fromRoman should give known result with known input
----------------------------------------------------------------------
Traceback (most recent call last):
  File "C:\docbook\dip\py\roman\stage3\romantest3.py", line 99, in testFromRomanKnownValues
    self.assertEqual(integer, result)
  File "c:\python21\lib\unittest.py", line 273, in failUnlessEqual
    raise self.failureException, (msg or '%s != %s' % (first, second))
AssertionError: 1 != None
======================================================================
FAIL: fromRoman(toRoman(n))==n for all n
----------------------------------------------------------------------
Traceback (most recent call last):
  File "C:\docbook\dip\py\roman\stage3\romantest3.py", line 141, in testSanity
    self.assertEqual(integer, result)
  File "c:\python21\lib\unittest.py", line 273, in failUnlessEqual
    raise self.failureException, (msg or '%s != %s' % (first, second))
AssertionError: 1 != None
----------------------------------------------------------------------
Ran 12 tests in 0.401s

FAILED (failures=6) 

你已将失败降至6个，而且他们都是关于 fromRoman 的：已知值测试，三个独立的无效输入测试，大小写检查和完备性检查。这意味着 toRoman 通过了所有可以独立通过的测试（完备性测试也测试它，但需要 fromRoman 编写后一起测试）。 这就是说，你应该停止对 toRoman 的代码编写。不必再推敲，不必再做额外的检查 “恰到好处”。 停下来吧！ 现在，别再敲键盘了。