capsys does not follow the OS newline rules for print

[gaborbernat]

Given the following code (test_a.py):

def main():
    print()
    print()

import os
def test(capsys):
    main()
    out, err = capsys.readouterr()
    expected = os.linesep.join(['', '', ''])
    assert out == expected

λ python -m pytest
============================= test session starts =============================
platform win32 -- Python 3.6.1, pytest-3.4.0, py-1.5.2, pluggy-0.6.0
rootdir: D:\work\test, inifile:
plugins: devpi-server-4.4.0
collected 1 item

test_main.py F                                                           [100%]

================================== FAILURES ===================================
____________________________________ test _____________________________________

capsys = <_pytest.capture.CaptureFixture object at 0x00000000044D45C0>

    def test(capsys):
        main()
        out, err = capsys.readouterr()
        expected = os.linesep.join(['', '', ''])
>       assert out == expected
E       AssertionError: assert '\n\n' == '\r\n\r\n'
test_main.py:11: AssertionError
========================== 1 failed in 0.13 seconds ===========================
λ python -c "import sys; print(sys.platform)"
win32

λ python -c "import sys; print(sys.version)"
3.6.1 |Anaconda 4.4.0 (64-bit)| (default, May 11 2017, 13:25:24) [MSC v.1900 64 bit (AMD64)]

Surely capsys should use the os.linesep exactly as that's the case with the print function:

python -c "import subprocess; import sys; print(repr(subprocess.check_output([sys.executable, '-c', 'print(); print()'])))"
b'\r\n\r\n'

[aklajnert]

That's not a bug - that's the default behavior of the print() function. As you can see in the documentation, the print() function has default argument end='\n', which is responsible for the end of line. You can change your print() to print(end=os.linesep) to make your test pass.

The subprocess.check_output() output has system-specific line ends which will be captured by capsys correctly.