[3.6] bpo-25862: Fix assertion failures in io.TextIOWrapper.tell(). (GH-3918). (GH-8012)

(cherry picked from commit 23db935bcf258657682e66464bf8512def8af830)

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

1 files changed, 11 insertions, 0 deletions

diff --git a/Lib/test/test_io.py b/Lib/test/test_io.py
index 0b5b033..88fd6ce 100644
--- a/Lib/test/test_io.py
+++ b/Lib/test/test_io.py
@@ -3348,6 +3348,17 @@ class TextIOWrapperTest(unittest.TestCase):
         F.tell = lambda x: 0
         t = self.TextIOWrapper(F(), encoding='utf-8')
 
+    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