深入 Python :Dive Into Python 中文版

例 15.1. 關于 Bug

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

在前面的章節中你注意到一個空字符串會匹配上那個檢查羅馬數字有效性的正則表達式了嗎？對于最終版本中的正則表達式這一點仍然沒有改變。這就是一個 Bug ，你希望空字符串能夠像其他無效的羅馬數字表示一樣引發 InvalidRomanNumeralError 異常。

例 15.2. 測試 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, "") 

這里很簡單。以空字符串調用 fromRoman 并確保它會引發一個 InvalidRomanNumeralError 異常。難點在于找出 Bug，既然你已經知道它了，測試就簡單了。

例 15.3. 用 romantest61.py 測試 roman61.py 的結果

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)

這個文件可以在例子目錄下的 py/roman/stage6/ 目錄中找到。

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

只需要兩行代碼：一行直接檢查空字符串和一行 raise 語句。

例 15.5. 用 romantest62.py 測試 roman62.py 的結果

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 

	空字符串測試用例現在通過了，說明 Bug 被修正了。
	所有其他測試用例依然通過，證明這個 Bug 修正沒有影響到其他部分。不需要再編程了。