output capture broken when sys.stdout passed to print()

[Laharah]

##Environ:
python 3.6, OS X 10.12.2, pytest 3.0.5
##pip list:

pip (9.0.1)
py (1.4.32)
pytest (3.0.5)
setuptools (32.3.1)
wheel (0.29.0)

When the print function is passed file=sys.stdout (which is listed as the default argument in the python docs). Output capture with capsys and capfd fails. capfd works only if the -s flag is passed to py.test

##Example:

# func.py
import sys

def log(msg, file=sys.stdout):
    print('log msg: {}'.format(msg), file=file, flush=True)
###
# test_func.py
import pytest
from func import log

def test_log(capfd):
    log('test')
    out, err = capfd.readouterr()
    assert out == 'log msg: test\n'

I came across it testing a logging decorator that had the option to write to a file. I tried every variation of capfd/capsys.disable() as well to no avail. I couldn't find this issue anywhere else, and a quick test on py2.7 shows the same behavior.

[RonnyPfannschmidt]

Your module level default prevents the test level capture Form applying, usw None as default and re-obtain stdout in each use

[Laharah]

thanks for the heads up, that makes sense.