[2.7] bpo-31271: Fix an assertion failure in io.TextIOWrapper.write. (GH-3201) (#3951)

Author: orenmn

Lib/test/test_io.py

@@ -2666,6 +2666,22 @@ class NonbytesStream(self.StringIO):
  t = self.TextIOWrapper(NonbytesStream('a'))
  self.assertEqual(t.read(), u'a')
 
+ def test_illegal_encoder(self):
+ # bpo-31271: A TypeError should be raised in case the return value of
+ # encoder's encode() is invalid.
+ class BadEncoder:
+ def encode(self, dummy):
+ return u'spam'
+ def get_bad_encoder(dummy):
+ return BadEncoder()
+ rot13 = codecs.lookup("rot13")
+ with support.swap_attr(rot13, '_is_text_encoding', True), \
+ support.swap_attr(rot13, 'incrementalencoder', get_bad_encoder):
+ t = io.TextIOWrapper(io.BytesIO(b'foo'), encoding="rot13")
+ with self.assertRaises(TypeError):
+ t.write('bar')
+ t.flush()
+
  def test_illegal_decoder(self):
  # Issue #17106
  # Bypass the early encoding check added in issue 20404