Capturing of stdout fails

[mrlegohead0x45]

I am trying to write tests for a function that eventually writes to sys.stdout (without explicitly flushing it).

The test I have written is thus:

from cppstream import cout

def test_cout(capsys):
    # with capsys.disabled():
    cout << "Hello World!" # eventually calls sys.stdout.write

    # cout.stream.flush() # explicitly flush
    cap = capsys.readouterr()
    assert cap.out == "Hello World!"

However, it is failing unexpectedly, saying that it captured the printing to stdout in the main output, but appears to have failed to make this captured stdout available in the test.

Output on a GitHub Actions machine can be found here, as well as the source code that I am testing.

Adding the explicit call to sys.stdout.flush() does not change anything locally.

Pytest full local output

$ pytest
=========================================== test session starts ===========================================
platform win32 -- Python 3.10.4, pytest-7.1.2, pluggy-1.0.0
rootdir: C:\Users\<me>\programming\python\cppstream
collected 1 item

tests\test_cout.py F                                                                                 [100%]

================================================ FAILURES =================================================
________________________________________________ test_cout ________________________________________________

capsys = <_pytest.capture.CaptureFixture object at 0x000001CCDB905F30>

    def test_cout(capsys):
        # with capsys.disabled():
        cout << "Hello World!"

        # cout.stream.flush()
        # print("Hello World!")
        cap = capsys.readouterr()
>       assert cap.out == "Hello World!"
E       AssertionError: assert '' == 'Hello World!'
E         - Hello World!

tests\test_cout.py:10: AssertionError
------------------------------------------ Captured stdout call -------------------------------------------
Hello World!
========================================= short test summary info =========================================
FAILED tests/test_cout.py::test_cout - AssertionError: assert '' == 'Hello World!'
============================================ 1 failed in 0.22s ============================================

[RonnyPfannschmidt]

while not familiar with the cppstream library, its highly unlikely its using sys.stdout, but rather a lower level, please use fd capture and ensure the c++ streams flush sanely

[RonnyPfannschmidt]

after reading the related code, i determined that this is a stream caching issue

the sys.stdout thats stored in the stream is a different object than the sys.stdout the test will see (sys capture requires replacing the attributes on sys)

[mrlegohead0x45]

Using capfd fails as well, https://github.com/mrlegohead0x45/cppstream/runs/6335570330?check_suite_focus=true
Replacing the cout._stream in the test file to sys.stdout and using capsys works though, although it fails when trying to use a fixture to do this: https://github.com/mrlegohead0x45/cppstream/runs/6335640068?check_suite_focus=true

[RonnyPfannschmidt]

i commented in https://github.com/mrlegohead0x45/cppstream/pull/2/files#r867378611

i recommend dynamically fetching the stream for that handler dynamically to account for stdio redirection

[mrlegohead0x45]

That works, thanks