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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
06cf12d
chore: update 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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@@ -0,0 +1,2 @@
Fix :class:`io.FileIO` ``seekable`` being cached as false because of an invalid
argument to :meth:`!io.FileIO.seek`
2 changes: 1 addition & 1 deletion Modules/_io/fileio.c
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Original file line number Diff line number Diff line change
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.

@KowalskiThomas KowalskiThomas Oct 18, 2026

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The idea here was that only in the following cases we're 100% sure we know for sure what seekable should be:

  • res >= 0 i.e. it succeeded
  • errno == ESPIPE i.e. Illegal seek
  • posobj == NULL i.e. offset == 0 in which case a failure would indicate a seek for SEEK_CUR/SEEK_END failed (so the problem is the file itself) => it's OK to cache the result of seeking directly

If we don't set the cache, "someone else" hopefully will later, but at least we're sure we never store a false value here.

Does that sound reasonable?

@cmaloney cmaloney Oct 19, 2026

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(Thinking, will get back later this week)

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

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