gh-160071: Fix the gzip write-mode seek return value

Lib/gzip.py

@@ -456,12 +456,11 @@ def seek(self, offset, whence=io.SEEK_SET):
             for i in range(count // self._buffer_size):
                 self.write(chunk)
             self.write(b'\0' * (count % self._buffer_size))
+            return offset
         elif self.mode == READ:
             self._check_not_closed()
             return self._buffer.seek(offset, whence)
 
-        return self.offset
-
     def readline(self, size=-1):
         self._check_not_closed()
         return self._buffer.readline(size)

Lib/test/test_gzip.py

@@ -317,9 +317,24 @@ def test_seek_write(self):
         # Try seek, write test
         with gzip.GzipFile(self.filename, 'w') as f:
             for pos in range(0, 256, 16):
-                f.seek(pos)
+                self.assertEqual(f.seek(pos), pos)
                 f.write(b'GZ\n')
 
+    def test_seek_write_return_value(self):
+        for whence in (io.SEEK_SET, io.SEEK_CUR):
+            for padding in (16, gzip._WRITE_BUFFER_SIZE + 16):
+                with self.subTest(whence=whence, padding=padding):
+                    data = io.BytesIO()
+                    with gzip.GzipFile(fileobj=data, mode='wb') as f:
+                        f.write(b'header')
+                        position = len(b'header') + padding
+                        offset = position if whence == io.SEEK_SET else padding
+                        self.assertEqual(f.seek(offset, whence), position)
+                        f.write(b'record')
+                        self.assertEqual(f.tell(), position + len(b'record'))
+                    self.assertEqual(gzip.decompress(data.getvalue()),
+                                     b'header' + b'\0' * padding + b'record')
+
     def test_mode(self):
         self.test_write()
         with gzip.GzipFile(self.filename, 'r') as f:

Misc/NEWS.d/next/Library/2026-10-21-19-29-18.gh-issue-160071.nG4pTs.rst

@@ -0,0 +1,2 @@
+Fix :class:`gzip.GzipFile` returning a stale position from ``seek()`` when
+forward seeking in write mode leaves zero padding in the write buffer.