commit 0464de0f9a226cfa32b803e0326c12b2432aba26
author Zackery Spytz <zspytz@gmail.com> Fri Jun 29 14:07:13 2018 -0600
committer Serhiy Storchaka <storchaka@gmail.com> Fri Jun 29 23:07:13 2018 +0300
tree d4c456790417ad6aeed6f5fd732546d3d3dbc001
parent 11ba050986b96701f72f1356d22adde8c7dbf211

[2.7] bpo-25862: Fix assertion failures in io.TextIOWrapper.tell(). (GH-3918). (GH-8013)

(cherry picked from commit 23db935bcf258657682e66464bf8512def8af830)

Co-authored-by: Zackery Spytz <zspytz@gmail.com>

Lib/_pyio.py

diff --git a/Lib/_pyio.py b/Lib/_pyio.py
index f022a4e..98c2d58 100644
--- a/Lib/_pyio.py
+++ b/Lib/_pyio.py
@@ -1619,6 +1619,7 @@
         self.buffer.write(b)
         if self._line_buffering and (haslf or "\r" in s):
             self.flush()
+        self._set_decoded_chars('')
         self._snapshot = None
         if self._decoder:
             self._decoder.reset()

Lib/test/test_io.py

diff --git a/Lib/test/test_io.py b/Lib/test/test_io.py
index a725711..ebaf796 100644
--- a/Lib/test/test_io.py
+++ b/Lib/test/test_io.py
@@ -2741,6 +2741,17 @@
         with self.maybeRaises(TypeError):
             t.read(42)
 
+    def test_issue25862(self):
+        # Assertion failures occurred in tell() after read() and write().
+        t = self.TextIOWrapper(self.BytesIO(b'test'), encoding='ascii')
+        t.read(1)
+        t.read()
+        t.tell()
+        t = self.TextIOWrapper(self.BytesIO(b'test'), encoding='ascii')
+        t.read(1)
+        t.write('x')
+        t.tell()
+
 
 class CTextIOWrapperTest(TextIOWrapperTest):