Now that toRoman behaves correctly with good input (integers from 1 to 3999), it's time to make it behave correctly with bad input (everything else).
If you have not already done so, you can download this and other examples used in this book.
"""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, "decimals 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
| | This is a nice Pythonic shortcut: multiple comparisons at once. This is equivalent to if not ((0 < n) and (n < 4000)), but it's much easier to read. This is our range check, and it should catch inputs that are too large, negative, or zero. |
| | You raise exceptions yourself with the raise statement. You can raise any of the built-in exceptions, or you can raise any of your custom exceptions that you've defined. The second parameter, the error message, is optional; if given, it is displayed in the traceback that is printed if the exception is never handled. |
| | This is our decimal check. Decimals can not be converted to Roman numerals. |
| | The rest of the function is unchanged. |
Example 6.13. Watching toRoman handle bad input
>>> 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, "decimals can not be converted" NotIntegerError: decimals can not be converted
Example 6.14. Output of romantest3.py against roman3.py
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
| | toRoman still passes the known values test, which is comforting. All the tests that passed in stage 2 still pass, so our latest code hasn't broken anything. |
| | More exciting is the fact that all of our bad input tests now pass. This test, testDecimal, passes because of the int(n) <> n check. When a decimal is passed to toRoman, the int(n) <> n check notices it and raises the NotIntegerError exception, which is what testDecimal is looking for. |
| | This test, testNegative, passes because of the not (0 < n < 4000) check, which raises an OutOfRangeError exception, which is what testNegative is looking for. |
====================================================================== 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)
| | We're down to 6 failures, and all of them involve fromRoman: the known values test, the three separate bad input tests, the case check, and the sanity check. That means that toRoman has passed all the tests it can pass by itself. (It's involved in the sanity check, but that also requires that fromRoman be written, which it isn't yet.) Which means that we must stop coding toRoman now. No tweaking, no twiddling, no extra checks “just in case”. Stop. Now. Back away from the keyboard. |
 | |
| The most important thing that comprehensive unit testing can tell you is when to stop coding. When all the unit tests for a function pass, stop coding the function. When all the unit tests for an entire module pass, stop coding the module. | |