尽管你很努力地编写全面的单元测试，但是 bug 还是会出现。 我所说的 “bug” 是什么呢？ Bug 是你还没有编写的测试用例。

例 15.1. 关于 Bug

>>> import roman5
>>> roman5.fromRoman("") 
0

在前面的 章节中 你注意到一个空字符串会匹配那个检查罗马数字有效性的正则表达式了吗？ 对于最终版本中的正则表达式这一点仍然没有改变。这就是一个 Bug ，你希望空字符串能够像其他无效的罗马数字表示一样引发 InvalidRomanNumeralError 异常。

例 15.2. 测试 bug （romantest61.py）

class FromRomanBadInput(unittest.TestCase):                                      

    # previous test cases omitted for clarity (they haven't changed)

    def testBlank(self):
        """fromRoman should fail with blank string"""
        self.assertRaises(roman.InvalidRomanNumeralError, roman.fromRoman, "") 

这里很简单。以空字符串调用 fromRoman 并确保它会引发一个 InvalidRomanNumeralError 异常。 难点在于找出 Bug，既然你已经知道它了，测试就简单了。

例 15.3. 用 romantest61.py 测试 roman61.py 的结果

fromRoman should only accept uppercase input ... ok
toRoman should always return uppercase ... ok
fromRoman should fail with blank string ... FAIL
fromRoman should fail with malformed antecedents ... ok
fromRoman should fail with repeated pairs of numerals ... ok
fromRoman should fail with too many repeated numerals ... ok
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

======================================================================
FAIL: fromRoman should fail with blank string
----------------------------------------------------------------------
Traceback (most recent call last):
  File "C:\docbook\dip\py\roman\stage6\romantest61.py", line 137, in testBlank
    self.assertRaises(roman61.InvalidRomanNumeralError, roman61.fromRoman, "")
  File "c:\python21\lib\unittest.py", line 266, in failUnlessRaises
    raise self.failureException, excName
AssertionError: InvalidRomanNumeralError
----------------------------------------------------------------------
Ran 13 tests in 2.864s

FAILED (failures=1)

例 15.4. 修改 Bug （roman62.py）

这个文件可以在例子目录下的 py/roman/stage6/ 目录中找到。

def fromRoman(s):
    """convert Roman numeral to integer"""
    if not s: 
        raise InvalidRomanNumeralError, 'Input can not be blank'
    if not re.search(romanNumeralPattern, s):
        raise InvalidRomanNumeralError, 'Invalid Roman numeral: %s' % s

    result = 0
    index = 0
    for numeral, integer in romanNumeralMap:
        while s[index:index+len(numeral)] == numeral:
            result += integer
            index += len(numeral)
    return result

只需要两行代码：一行直接检查空字符串和一行 raise 语句。

例 15.5. 用 romantest62.py 测试 roman62.py 的结果

fromRoman should only accept uppercase input ... ok
toRoman should always return uppercase ... ok
fromRoman should fail with blank string ... ok 
fromRoman should fail with malformed antecedents ... ok
fromRoman should fail with repeated pairs of numerals ... ok
fromRoman should fail with too many repeated numerals ... ok
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

----------------------------------------------------------------------
Ran 13 tests in 2.834s

OK 

	空字符串测试用例现在通过了，说明 Bug 被改正了。
	所有其他测试用例依然通过，证明这个 Bug 修正没有影响到其他部分。 不需要再编程了。