FakeProcess stdout and stderr doesn't work with selectors

[blcdb]

Hello,

In order to have a the logs of a time lasting command printed at the user while the command is still running i did this code :

def exec_command(
    command: str, log_stdout: bool = True
) -> int:
    try:
        process = subprocess.Popen(
            shlex.split(command),
            stdout=subprocess.PIPE,
            stderr=subprocess.PIPE,
        )
        sel = selectors.DefaultSelector()
        sel.register(process.stdout, selectors.EVENT_READ)
        sel.register(process.stderr, selectors.EVENT_READ)
        while True:
            should_break = False
            for key, _ in sel.select():
                data = key.fileobj.read1().decode()
                if not data:
                    should_break = True
                    break
                if key.fileobj is process.stdout:
                    if log_stdout:
                        logger.info(data)
                else:
                    logger.error(data)
            if should_break:
                break
        process.poll()
        return process.returncode

    except Exception as e:
        logger.error("An error occured during the run of the command : " + command)
        raise e

It's a solution that avoid to have thread to be able to read both stdout and stderr while the command is still running.

I tried to test my code whith pytest-subprocess with a test similar to this :

def test_exec_comande(fp: FakeProcess):
    fp.register(
        ["test"], stdout="mocked", stderr="mocked err", returncode=1
    )
    exec_command("test")

but the test failed because selectors.register need an object like an io.BufferedReader (that is returned by the subprocess.Popen() upward.) However fp.register is unable to take an io.BufferedReader for stdout and stderr.

Do you think it is possible to allow this stdout and stderr mocking ?

[aklajnert]

Thank you for reporting. I have never used selectors, so I didn't incorporate them into a plugin. Feel free to submit a PR to fix this issue. Otherwise, I'll try to work on it in the future, but I can't tell you when it will be.

[aklajnert]

The core issue is architectural: pytest-subprocess stores fake stdout/stderr as in-memory BytesIO/StringIO buffers, which have no real OS file descriptor. selectors.DefaultSelector.register() requires a real file descriptor (via fileno()), so the two are fundamentally incompatible.

Making them compatible would require replacing the entire internal buffer model with real OS pipes for every stdout=PIPE/stderr=PIPE call — a full refactor with a high risk of regressions. The maintenance cost vs. added value doesn't justify it, especially since there's a simpler workaround.

Recommended approach: mock selectors itself in the test rather than the subprocess:

from unittest.mock import MagicMock, patch

def test_exec_command(fp):
    fp.register(["test"], stdout=b"mocked\n", stderr=b"mocked err\n", returncode=1)

    mock_key_stdout = MagicMock()
    mock_key_stdout.fileobj.read1.return_value = b"mocked\n"

    mock_key_stderr = MagicMock()
    mock_key_stderr.fileobj.read1.return_value = b""  # triggers break

    with patch("selectors.DefaultSelector") as mock_sel_cls:
        mock_sel = mock_sel_cls.return_value.__enter__.return_value
        mock_sel.select.return_value = [
            (mock_key_stdout, selectors.EVENT_READ),
            (mock_key_stderr, selectors.EVENT_READ),
        ]
        result = exec_command("test")

    assert result == 1

Closing as won't fix.