From 9bcbc6cba385a83cac8f1ff430cad7617184d2bc Mon Sep 17 00:00:00 2001
From: Oren Milman <orenmn@gmail.com>
Date: Sat, 26 Aug 2017 20:29:40 +0300
Subject: [PATCH] [3.6] bpo-31271: Fix an assertion failure in
 io.TextIOWrapper.write. (GH-3201) (#3209)

(cherry picked from commit a5b4ea15b61e3f3985f4f0748a18f8b888a63532)
---
 Lib/test/test_io.py                                       | 8 ++++++++
 .../2017-08-25-20-43-22.bpo-31271.YMduKF.rst              | 2 ++
 Modules/_io/textio.c                                      | 7 +++++++
 3 files changed, 17 insertions(+)
 create mode 100644 Misc/NEWS.d/next/Core and Builtins/2017-08-25-20-43-22.bpo-31271.YMduKF.rst

diff --git a/Lib/test/test_io.py b/Lib/test/test_io.py
index 6a2c7a9fca4..7057c927c3f 100644
--- a/Lib/test/test_io.py
+++ b/Lib/test/test_io.py
@@ -3163,6 +3163,14 @@ class TextIOWrapperTest(unittest.TestCase):
         t = self.TextIOWrapper(self.StringIO('a'))
         self.assertRaises(TypeError, t.read)
 
+    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 00000000000..7bb78801057
--- /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 61adcf60508..801bb1745bb 100644
--- a/Modules/_io/textio.c
+++ b/Modules/_io/textio.c
@@ -1331,6 +1331,13 @@ _io_TextIOWrapper_write_impl(textio *self, PyObject *text)
     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);