diff --git a/Lib/test/test_io.py b/Lib/test/test_io.py
index 3dc6e9ffcac157..dd102c42e231bb 100644
--- a/Lib/test/test_io.py
+++ b/Lib/test/test_io.py
@@ -2666,6 +2666,14 @@ class NonbytesStream(self.StringIO):
t = self.TextIOWrapper(NonbytesStream('a'))
self.assertEqual(t.read(), u'a')
+ def test_illegal_encoder(self):
+ # Issue 31271: Calling write() while the return value of encoder's
+ # encode() is invalid shouldn't cause an assertion failure.
+ rot13 = codecs.lookup("rot13")
+ with support.swap_attr(rot13, '_is_text_encoding', True):
+ t = io.TextIOWrapper(io.BytesIO(b'foo'), encoding="rot13")
+ self.assertRaises(TypeError, t.write, 'bar')
+
def test_illegal_decoder(self):
# Issue #17106
# Bypass the early encoding check added in issue 20404
diff --git a/Misc/NEWS.d/next/Core and Builtins/2017-08-25-20-43-22.bpo-31271.YMduKF.rst b/Misc/NEWS.d/next/Core and Builtins/2017-08-25-20-43-22.bpo-31271.YMduKF.rst
new file mode 100644
index 00000000000000..7bb7880105781e
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2017-08-25-20-43-22.bpo-31271.YMduKF.rst
@@ -0,0 +1,2 @@
+Fix an assertion failure in the write() method of `io.TextIOWrapper`, when
+the encoder doesn't return a bytes object. Patch by Oren Milman.
diff --git a/Modules/_io/textio.c b/Modules/_io/textio.c
index d9e036db386d3b..6d24755085a479 100644
--- a/Modules/_io/textio.c
+++ b/Modules/_io/textio.c
@@ -1301,6 +1301,13 @@ textiowrapper_write(textio *self, PyObject *args)
Py_DECREF(text);
if (b == NULL)
return NULL;
+ if (!PyBytes_Check(b)) {
+ PyErr_Format(PyExc_TypeError,
+ "encoder should return a bytes object, not '%.200s'",
+ Py_TYPE(b)->tp_name);
+ Py_DECREF(b);
+ return NULL;
+ }
if (self->pending_bytes == NULL) {
self->pending_bytes = PyList_New(0);
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