14.2. roman.py, 第 2 阶段
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## 14.2. `roman.py`, 第 2 阶段

现在你有了 `roman` 模块的大概框架，到了开始写代码以通过测试的时候了。

### 例 14.3. `roman2.py`

这个文件可以从 `py/roman/stage2/` 目录中找到。

如果您还没有下载本书附带的样例程序, 可以 下载本程序和其他样例程序。

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

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

| `romanNumeralMap` 是一个用来定义三个内容的元组的元组： 代表大部分罗马数字的字符。注意不只是单字符的罗马数字，你同样在这里定义诸如 `CM` (“比一千少一百，即 900”) 的双字符，这可以让稍后编写的 `toRoman` 简单一些。 罗马数字的顺序。它们是以降序排列的，从`M` 一路到 `I`。 每个罗马数字所对应的数值。每个内部的元组都是一个 `(numeral，value)` 数值对。
| 这里便显示出你丰富的数据结构带来的优势，你不需要什么特定的逻辑处理减法规则。你只需要通过搜寻 `romanNumeralMap` 寻找不大于输入数值的最大对应整数即可。只要找到，就在结果的结尾把这个整数对应的罗马字符添加到输出结果的末尾，从输入值中减去这个整数，一遍遍这样继续下去。

### 例 14.4. `toRoman` 如何工作

如果你不明了 `toRoman` 如何工作，在 `while` 循环的结尾添加一个 `print` 语句：

            while n >= integer:
                result += numeral
                n -= integer
                print 'subtracting', integer, 'from input, adding', numeral, 'to output'

    >>> import roman2
    >>> roman2.toRoman(1424)
    subtracting 1000 from input, adding M to output
    subtracting 400 from input, adding CD to output
    subtracting 10 from input, adding X to output
    subtracting 10 from input, adding X to output
    subtracting 4 from input, adding IV to output
    'MCDXXIV'

看来 `toRoman` 可以运转了，至少手工测试可以。但能通过单元测试吗？啊哈，不，不完全可以。

### 例 14.5. 以 `romantest2.py` 测试 `roman2.py` 的输出

要记得用 `-v` 命令行选项运行 `romantest2.py` 开启详细信息模式。

    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 ... FAIL
    toRoman should fail with negative input ... FAIL
    toRoman should fail with large input ... FAIL
    toRoman should fail with 0 input ... FAIL

| 事实上，`toRoman` 的返回值总是大写的，因为 `romanNumeralMap` 定义的罗马字符都是以大写字母表示的。因此这个测试已经通过了。
| 好消息来了：这个版本的 `toRoman` 函数能够通过已知值测试。记住，这并不能证明完全没问题，但至少通过测试多种有效输入考验了这个函数：包括每个单一字符的罗马数字，可能的最大输入 (`3999`)，以及可能的最长的罗马数字 (对应于 `3888`)。从这点来看，你有理由相信这个函数对于任何有效输入都不会出问题。
| 但是，函数还没办法处理无效输入，每个无效输入测试都失败了。这很好理解，因为你还没有对无效输入进行检查，测试用例希望捕捉到特定的异常 (通过 `assertRaises`)，而你根本没有让这些异常引发。这是你下一阶段的工作。

下面是单元测试结果的剩余部分，列出了所有失败的详细信息，你已经让它降到了 10 个。

    ======================================================================
    FAIL: fromRoman should only accept uppercase input
    ----------------------------------------------------------------------
    Traceback (most recent call last):
      File "C:\docbook\dip\py\roman\stage2\romantest2.py", line 156, in testFromRomanCase
        roman2.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\stage2\romantest2.py", line 133, in testMalformedAntecedent
        self.assertRaises(roman2.InvalidRomanNumeralError, roman2.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\stage2\romantest2.py", line 127, in testRepeatedPairs
        self.assertRaises(roman2.InvalidRomanNumeralError, roman2.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\stage2\romantest2.py", line 122, in testTooManyRepeatedNumerals
        self.assertRaises(roman2.InvalidRomanNumeralError, roman2.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\stage2\romantest2.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\stage2\romantest2.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
    ======================================================================
    FAIL: toRoman should fail with non-integer input
    ----------------------------------------------------------------------
    Traceback (most recent call last):
      File "C:\docbook\dip\py\roman\stage2\romantest2.py", line 116, in testNonInteger
        self.assertRaises(roman2.NotIntegerError, roman2.toRoman, 0.5)
      File "c:\python21\lib\unittest.py", line 266, in failUnlessRaises
        raise self.failureException, excName
    AssertionError: NotIntegerError
    ======================================================================
    FAIL: toRoman should fail with negative input
    ----------------------------------------------------------------------
    Traceback (most recent call last):
      File "C:\docbook\dip\py\roman\stage2\romantest2.py", line 112, in testNegative
        self.assertRaises(roman2.OutOfRangeError, roman2.toRoman, -1)
      File "c:\python21\lib\unittest.py", line 266, in failUnlessRaises
        raise self.failureException, excName
    AssertionError: OutOfRangeError
    ======================================================================
    FAIL: toRoman should fail with large input
    ----------------------------------------------------------------------
    Traceback (most recent call last):
      File "C:\docbook\dip\py\roman\stage2\romantest2.py", line 104, in testTooLarge
        self.assertRaises(roman2.OutOfRangeError, roman2.toRoman, 4000)
      File "c:\python21\lib\unittest.py", line 266, in failUnlessRaises
        raise self.failureException, excName
    AssertionError: OutOfRangeError
    ======================================================================
    FAIL: toRoman should fail with 0 input
    ----------------------------------------------------------------------
    Traceback (most recent call last):
      File "C:\docbook\dip\py\roman\stage2\romantest2.py", line 108, in testZero
        self.assertRaises(roman2.OutOfRangeError, roman2.toRoman, 0)
      File "c:\python21\lib\unittest.py", line 266, in failUnlessRaises
        raise self.failureException, excName
    AssertionError: OutOfRangeError
    ----------------------------------------------------------------------
    Ran 12 tests in 0.320s

    FAILED (failures=10)