三类用例一起跑

（每道题开头都有同一段：被测模块 c_to_f / length，和一个静音跑测试的 run()，它交回「跑了几条/挂了几条/出错几条」。）

正常、边界、异常各一条。运行程序：

import io, unittest

LEN = {"m": 1.0, "km": 1000.0, "cm": 0.01}

def c_to_f(c):
    return c * 9 / 5 + 32

def length(v, src, to):
    for u in (src, to):
        if u not in LEN:
            raise ValueError("不认识的单位：" + u)
    return v * LEN[src] / LEN[to]

def run(*cases):
    suite = unittest.TestSuite()
    for c in cases:
        suite.addTests(unittest.defaultTestLoader.loadTestsFromTestCase(c))
    r = unittest.TextTestRunner(stream=io.StringIO()).run(suite)
    return str(r.testsRun) + "/" + str(len(r.failures)) + "/" + str(len(r.errors))

class T(unittest.TestCase):
    def test_normal(self):
        self.assertEqual(length(3, "km", "m"), 3000)
    def test_zero(self):
        self.assertEqual(length(0, "m", "km"), 0)
    def test_unknown(self):
        with self.assertRaises(ValueError):
            length(1, "li", "m")

print(run(T))