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

2 files changed, 12 insertions, 0 deletions

diff --git a/Lib/_pyio.py b/Lib/_pyio.py
index c91a647..f0d4f4e 100644
--- a/Lib/_pyio.py
+++ b/Lib/_pyio.py
@@ -2149,6 +2149,7 @@ class TextIOWrapper(TextIOBase):
         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()
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 @@ class TextIOWrapperTest(unittest.TestCase):
         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