Also add tests for TextIOWrapper.writelines() (issue #15744).

Lib/test/test_io.py

@@ -2287,6 +2287,28 @@ class TextIOWrapperTest(unittest.TestCase):
             reads += c
         self.assertEqual(reads, "A"*127+"\nB")
 
+    def test_writelines(self):
+        l = ['ab', 'cd', 'ef']
+        buf = self.BytesIO()
+        txt = self.TextIOWrapper(buf)
+        txt.writelines(l)
+        txt.flush()
+        self.assertEqual(buf.getvalue(), b'abcdef')
+
+    def test_writelines_userlist(self):
+        l = UserList(['ab', 'cd', 'ef'])
+        buf = self.BytesIO()
+        txt = self.TextIOWrapper(buf)
+        txt.writelines(l)
+        txt.flush()
+        self.assertEqual(buf.getvalue(), b'abcdef')
+
+    def test_writelines_error(self):
+        txt = self.TextIOWrapper(self.BytesIO())
+        self.assertRaises(TypeError, txt.writelines, [1, 2, 3])
+        self.assertRaises(TypeError, txt.writelines, None)
+        self.assertRaises(TypeError, txt.writelines, b'abc')
+
     def test_issue1395_1(self):
         txt = self.TextIOWrapper(self.BytesIO(self.testdata), encoding="ascii")