bpo-40287: Fix SpooledTemporaryFile.seek() return value (GH-19540)

It has not returned the file position after the seek.
(cherry picked from commit 485e715cb1)

Co-authored-by: Inada Naoki <songofacandy@gmail.com>

Lib/tempfile.py

@@ -735,7 +735,7 @@ class SpooledTemporaryFile:
         return self._file.readlines(*args)
 
     def seek(self, *args):
-        self._file.seek(*args)
+        return self._file.seek(*args)
 
     @property
     def softspace(self):

Lib/test/test_tempfile.py

@@ -1025,7 +1025,8 @@ class TestSpooledTemporaryFile(BaseTestCase):
         # Verify writelines with a SpooledTemporaryFile
         f = self.do_create()
         f.writelines((b'x', b'y', b'z'))
-        f.seek(0)
+        pos = f.seek(0)
+        self.assertEqual(pos, 0)
         buf = f.read()
         self.assertEqual(buf, b'xyz')
 
@@ -1043,7 +1044,8 @@ class TestSpooledTemporaryFile(BaseTestCase):
         # when that occurs
         f = self.do_create(max_size=30)
         self.assertFalse(f._rolled)
-        f.seek(100, 0)
+        pos = f.seek(100, 0)
+        self.assertEqual(pos, 100)
         self.assertFalse(f._rolled)
         f.write(b'x')
         self.assertTrue(f._rolled)

Misc/NEWS.d/next/Library/2020-04-15-17-21-48.bpo-40287.-mkEJH.rst

@@ -0,0 +1 @@
+Fixed ``SpooledTemporaryFile.seek()`` to return the position.