gh-159677: fix FileIO.seekable incorrectly returning false

Lib/test/test_io/test_fileio.py

@@ -62,6 +62,10 @@ def testSeekTell(self):
         self.f.seek(-5, 2)
         self.assertEqual(self.f.tell(), 15)
 
+    def testSeekableAfterFailedSeek(self) -> None:
+        self.assertRaises(OSError, self.f.seek, -1)
+        self.assertTrue(self.f.seekable())
+
     def testAttributes(self):
         # verify expected attributes exist
         f = self.f

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);
     }