fix #4862 in _pyio: reset the decoder on seek(0)

diff --git a/Lib/_pyio.py b/Lib/_pyio.py
index 5f2e40000579c571e480e62bcccf022c97d7e8d5..a2ee9525f5cc2d26f2a16b19484cdf3567c274a2 100644 (file)
--- a/Lib/_pyio.py
+++ b/Lib/_pyio.py
@@ -1667,7 +1667,9 @@ class TextIOWrapper(TextIOBase):
         self._snapshot = None
 
         # Restore the decoder to its state from the safe start point.
-        if self._decoder or dec_flags or chars_to_skip:
+        if cookie == 0 and self._decoder:
+            self._decoder.reset()
+        elif self._decoder or dec_flags or chars_to_skip:
             self._decoder = self._decoder or self._get_decoder()
             self._decoder.setstate((b'', dec_flags))
             self._snapshot = (dec_flags, b'')

diff --git a/Lib/test/test_io.py b/Lib/test/test_io.py
index 58f33a2933a7ca25de5c55acf764d9e29c7a6ef4..1c7d1f14ff257d2024ff1f18b880698d90344515 100644 (file)
--- a/Lib/test/test_io.py
+++ b/Lib/test/test_io.py
@@ -1706,6 +1706,8 @@ class TextIOWrapperTest(unittest.TestCase):
             f.write(data)
             f.seek(0)
             self.assertEquals(f.read(), data * 2)
+            f.seek(0)
+            self.assertEquals(f.read(), data * 2)
             self.assertEquals(buf.getvalue(), (data * 2).encode(encoding))
 
     def test_read_one_by_one(self):