bpo-33361: Fix bug with seeking in StreamRecoders (GH-8278)

1 files changed, 25 insertions, 0 deletions

diff --git a/Lib/test/test_codecs.py b/Lib/test/test_codecs.py
index f665febfc9..47df88ceda 100644
--- a/Lib/test/test_codecs.py
+++ b/Lib/test/test_codecs.py
@@ -3166,6 +3166,31 @@ class StreamRecoderTest(unittest.TestCase):
         sr.write(text.encode('latin1'))
         self.assertEqual(bio.getvalue(), text.encode('utf-8'))
 
+    def test_seeking_read(self):
+        bio = io.BytesIO('line1\nline2\nline3\n'.encode('utf-16-le'))
+        sr = codecs.EncodedFile(bio, 'utf-8', 'utf-16-le')
+
+        self.assertEqual(sr.readline(), b'line1\n')
+        sr.seek(0)
+        self.assertEqual(sr.readline(), b'line1\n')
+        self.assertEqual(sr.readline(), b'line2\n')
+        self.assertEqual(sr.readline(), b'line3\n')
+        self.assertEqual(sr.readline(), b'')
+
+    def test_seeking_write(self):
+        bio = io.BytesIO('123456789\n'.encode('utf-16-le'))
+        sr = codecs.EncodedFile(bio, 'utf-8', 'utf-16-le')
+
+        # Test that seek() only resets its internal buffer when offset
+        # and whence are zero.
+        sr.seek(2)
+        sr.write(b'\nabc\n')
+        self.assertEqual(sr.readline(), b'789\n')
+        sr.seek(0)
+        self.assertEqual(sr.readline(), b'1\n')
+        self.assertEqual(sr.readline(), b'abc\n')
+        self.assertEqual(sr.readline(), b'789\n')
+
 
 @unittest.skipIf(_testcapi is None, 'need _testcapi module')
 class LocaleCodecTest(unittest.TestCase):