commit eabebbb54c2604fd8d08e8019ea1be634aed8c2f
author Miss Islington (bot) <31488909+miss-islington@users.noreply.github.com> Fri Jun 29 03:34:34 2018 -0700
committer GitHub <noreply@github.com> Fri Jun 29 03:34:34 2018 -0700
tree 753a4129f0ff70097c60b461f916f230352d747b
parent d0b442803ad4ba66b0bdb3cf183179d4c9a56f06

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

(cherry picked from commit 23db935bcf258657682e66464bf8512def8af830)

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

Lib/test/test_io.py

diff --git a/Lib/test/test_io.py b/Lib/test/test_io.py
index 286ae76..a03a7f7 100644
--- a/Lib/test/test_io.py
+++ b/Lib/test/test_io.py
@@ -3549,6 +3549,17 @@
         expected = 'linesep' + os.linesep + 'LF\nLF\nCR\rCRLF\r\n'
         self.assertEqual(txt.detach().getvalue().decode('ascii'), expected)
 
+    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 MemviewBytesIO(io.BytesIO):
     '''A BytesIO object whose read method returns memoryviews