Make new code more idiomatic

src/allmydata/test/common_util.py

@@ -73,7 +73,7 @@ def flip_bit(good, which):
 def flip_one_bit(s, offset=0, size=None):
     """ flip one random bit of the string s, in a byte greater than or equal to offset and less
     than offset+size. """
-    assert isinstance(s, binary_type)
+    precondition(isinstance(s, binary_type))
     if size is None:
         size=len(s)-offset
     i = randrange(offset, offset+size)

src/allmydata/test/test_common_util.py

@@ -22,9 +22,9 @@ class TestFlipOneBit(unittest.TestCase):
     def setUp(self):
         random.seed(42)  # I tried using version=1 on PY3 to avoid the if below, to no avail.
 
-    def test_bytestring(self):
+    def test_accepts_byte_string(self):
         actual = flip_one_bit(b'foo')
         self.assertEqual(actual, b'fno' if PY2 else b'fom')
 
-    def test_unicode(self):
+    def test_rejects_unicode_string(self):
         self.assertRaises(AssertionError, flip_one_bit, u'foo')