id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
36056	Fix ignored exceptions in OutputWrapper.flush()	Adam Johnson	Adam Johnson	"Running pytest on two different projects with Python 3.13, I’ve seen this error logged after tests pass:

{{{
$ pytest
...
==== 1 passed in 0.01s ====
Exception ignored in: <django.core.management.base.OutputWrapper object at 0x173326230>
Traceback (most recent call last):
  File ""/.../.venv/lib/python3.13/site-packages/django/core/management/base.py"", line 171, in flush
    self._out.flush()
ValueError: I/O operation on closed file.
...
}}}

It can appear multiple times, depending on how many management commands are tested.

This message is from Python’s [https://docs.python.org/3.12/library/sys.html#sys.unraisablehook unraisable exception hook], logging the minimal available information for an exception raised during a destructor or garbage collection.

I minimized the test case to find that it requires a test that captures the exception from a failing management command, minimized to:

{{{#!python
import pytest
from django.core.management import call_command
from django.core.management.base import CommandError
from django.test import SimpleTestCase


class ExampleTests(SimpleTestCase):
    def test_it(self):
        with pytest.raises(CommandError) as excinfo:
            call_command(""check"", ""--non"")
}}}

This test runs the `check` management command with a non-existent option, triggering an error.
It can be run with `pytest` and `pytest-django` installed like:

{{{
$ pytest --ds=test_example test_example.py
========================= test session starts =========================
platform darwin -- Python 3.13.1, pytest-8.3.4, pluggy-1.5.0
django: version: 5.1.4, settings: test_example (from option)
rootdir: /.../example
configfile: pyproject.toml
plugins: cov-6.0.0, django-4.9.0
collected 1 item

test_example.py .                                               [100%]

========================== 1 passed in 0.01s ==========================
Exception ignored in: <django.core.management.base.OutputWrapper object at 0x10533b760>
Traceback (most recent call last):
  File ""/.../.venv/lib/python3.13/site-packages/django/core/management/base.py"", line 171, in flush
    self._out.flush()
ValueError: I/O operation on closed file.
Exception ignored in: <django.core.management.base.OutputWrapper object at 0x10549b760>
Traceback (most recent call last):
  File ""/.../.venv/lib/python3.13/site-packages/django/core/management/base.py"", line 171, in flush
    self._out.flush()
ValueError: I/O operation on closed file.
}}}

(`-ds=test_example` specifies the Django settings module as the test module, so you don’t need a whole project set up.)

After some drilling down, I determined the cause is `OutputWrapper` instances that are created during `BaseCommand.__init__` with references to `sys.stdout` and `sys.stderr`. It seems that when they’re deleted, they interact poorly if the underlying file objects have already been closed. pytest’s output caputring installs a mock `sys.stdout` and closes it after tests are done, but the captured traceback retains a reference to the `OutputWrapper` until tests finish. When the `OutputWrapper` is finally garbage collected, the error is logged. Disabling output capturing with `pytest -s` makes the error go away.

I tried to replicate within Django’s test framework with its output capturing option, `--buffer`, but it didn’t work. I think it has some differences that mean the error doesn’t appear.

But I did manage to minimize down to this test script, free of pytest and management commands:

{{{#!python
import io

from django.core.management.base import OutputWrapper

out = io.TextIOWrapper(io.BytesIO())
wrapper = OutputWrapper(out)
out.close()
}}}

Running it shows:

{{{
$ python example_simplest.py
Exception ignored in: <django.core.management.base.OutputWrapper object at 0x1049cbb20>
Traceback (most recent call last):
  File ""/.../.venv/lib/python3.13/site-packages/django/core/management/base.py"", line 172, in flush
ValueError: I/O operation on closed file.
}}}

I also found that the error is reproducible on Python < 3.13 when enabling development mode with `python -X dev`. It turns out the logging was special-cased to development mode within the io module, but this special-casing was removed in Python 3.13 in [https://github.com/python/cpython/commit/58a2e0981642dcddf49daa776ff68a43d3498cee commit 58a2e0981642dcddf49daa776ff68a43d3498cee]. So the error has been there all along, but now it’s more visible.

I think the fix is to stop `OutputWrapper` from inheriting from `TextIOBase`, as was added in dc8834cad41aa407f402dc54788df3cd37ab3e22. That commit was focused on removing `force_str()` calls and it’s not clear why the inheritance was added. But it seems that the path [https://github.com/python/cpython/blob/e1baa778f602ede66831eb34b9ef17f21e4d4347/Lib/_pyio.py#L397 from `IOBase.__del__()`] to the custom `flush()` is what’s causing the error.
"	Cleanup/optimization	closed	Core (Management commands)	dev	Normal	fixed			Ready for checkin	1	0	0	0	0	0