gh-159677: fix FileIO.seekable incorrectly returning false

Lib/test/test_io/test_fileio.py

@@ -50,6 +50,9 @@ def testWeakRefs(self):
 
     def testSeekTell(self):
         self.f.write(bytes(range(20)))
+        # An invalid argument must not mark the file as non-seekable.
+        self.assertRaises(OSError, self.f.seek, -1)
+        self.assertTrue(self.f.seekable())
         self.assertEqual(self.f.tell(), 20)
         self.f.seek(0)
         self.assertEqual(self.f.tell(), 0)
@@ -61,6 +64,7 @@ def testSeekTell(self):
         self.assertEqual(self.f.tell(), 10)
         self.f.seek(-5, 2)
         self.assertEqual(self.f.tell(), 15)
+        self.assertTrue(self.f.seekable())
 
     def testAttributes(self):
         # verify expected attributes exist
@@ -553,6 +557,13 @@ def testAbles(self):
             f.close()
 
             if sys.platform != "win32":
+                # On Windows, lseek somehow succeeds on pipes.
+                r, w = os.pipe()
+                os.close(w)
+                with self.FileIO(r, "r") as f:
+                    self.assertRaises(OSError, f.seek, 0)
+                    self.assertEqual(f.seekable(), False)
+
                 try:
                     f = self.FileIO("/dev/tty", "a")
                 except OSError:

Misc/NEWS.d/next/Library/2026-10-18-17-26-00.gh-issue-159677.fIoSk1.rst

@@ -0,0 +1,2 @@
+Fix :class:`io.FileIO` ``seekable`` being cached as false because of an invalid
+argument to :meth:`!io.FileIO.seek`

Modules/_io/fileio.c

@@ -1013,7 +1013,13 @@ portable_lseek(fileio *self, PyObject *posobj, int whence, bool suppress_pipe_er
     _Py_END_SUPPRESS_IPH
     Py_END_ALLOW_THREADS
 
-    if (self->seekable < 0) {
+    if (self->seekable < 0
+        && !(res < 0 && (errno == EINVAL
+#ifdef EOVERFLOW
+                         || errno == EOVERFLOW
+#endif
+                         || errno == ENXIO)))
+    {
         self->seekable = (res >= 0);
     }