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gh-159677: fix FileIO.seekable incorrectly returning false #159678

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gh-159677: fix FileIO.seekable incorrectly returning false #159678

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4d02c20
fix: FileIO.seekable incorrectly returning false
KowalskiThomas Oct 18, 2026
875e5fb
chore: add release note
KowalskiThomas Oct 18, 2026
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4 changes: 4 additions & 0 deletions Lib/test/test_io/test_fileio.py
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Original file line number Diff line number Diff line change
Expand Up @@ -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
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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Library/2026-10-18-17-26-00.gh-issue-159677.fIoSk1.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
Fix :meth:`io.FileIO.seekable` always returning ``False`` for a seekable file

@cmaloney cmaloney Oct 18, 2026 •
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seekable and seek are only defined on IOBase in the docs

cpython/Doc/library/io.rst

Lines 422 to 425 in 7dbc797

.. method:: seekable()
Return ``True`` if the stream supports random access. If ``False``,
:meth:`seek`, :meth:`tell` and :meth:`truncate` will raise :exc:`OSError`.

I would probably do something like:

Fix :class:`io.FileIO` ``seekable`` being cached as false because of an invalid argument to :meth:`!io.FileIO.seek`

when the first call to :meth:`~io.FileIO.seek` failed.
2 changes: 1 addition & 1 deletion Modules/_io/fileio.c
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Expand Up @@ -1013,7 +1013,7 @@ 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 == ESPIPE || posobj == NULL)) {

@cmaloney cmaloney Oct 18, 2026

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Not sure across the set of OS + kernel + version which Python runs on this is correct... Can work on evaluating that thoroughly but my leaning would be just making it so specific returns we know about that indicate "bad argument" pass through.

For instance EBADF should probably still mark seekable as False / I could see code relying on that. Something like: EINVAL, ENXIO, EOVERFLOW for Linux; EBADF and EINVAL for OpenBSD.

self->seekable = (res >= 0);
}

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