Invoking subprocess.check_output makes pytest fail under certain circumstances

• a detailed description of the bug or problem you are having

Invoking subprocess.check_output passing in sys.stderr as an argument to stderr within a test makes pytest fail if run with the --capture=sys or --capture=tee-sys options.

• output of pip list from the virtual environment you are using

Package         Version
--------------- -------                                                                                                                                                                                                                                                                   attrs           22.2.0
exceptiongroup  1.1.0
iniconfig       2.0.0                                                                                                                                                                                                                                                                     packaging       23.0
pip             21.3.1                                                                                                                                                                                                                                                                    pluggy          1.0.0
py              1.11.0
pytest          7.2.0                                                                                                                                                                                                                                                                     pytest-html     3.2.0
pytest-metadata 2.0.4
pytest-timeout  2.1.0
setuptools      59.6.0
tomli           2.0.1

• pytest and operating system versions

pytest 7.2.0
Fedora Workstation 36

• minimal example if possible

import subprocess
import sys
def test_something():
    print("This gets printed")
    subprocess.check_output(["echo", "hello"], stderr=sys.stderr)
    print("This does not")

When running the file with pytest:

pytest test_itdoesntreallymatter.py --capture=tee-sys --html somepaththatiwant.html --self-contained-html

This generates an UnsupportedOperation exception.

FAILED test_itdoesntreallymatter.py::test_something - io.UnsupportedOperation: fileno

Start by running the minimal test with pytest 7.2.0 using --capture=sys or --capture=tee-sys and observe the io.UnsupportedOperation: fileno failure. Trace the capture behavior when subprocess.check_output receives sys.stderr. Done means the test completes without the exception and the later print remains handled correctly.