Print warning early in pytest output

[JP-Ellis]

I have a repository whereby the majority of the tests are independent and using python-xdist works great; however, I also have a specific directory containing a set of tests which must be executed in a specific order and therefore is incompatible with having multiple xdist workers.

For that directory containing these order-dependent tests, I want to raise a warning if multiple workers are detected. For example, something like adding this to the directory's conftest.py:

@pytest.fixture(scope="session", autouse=True)
def check_xdist() -> None:
    if int(os.getenv("PYTEST_XDIST_WORKER_COUNT", "1")):
        warnings.warn(
            "Running the examples with multiple workers may cause issues. "
            "Consider running the examples with a single worker."
        )

The problem with this solution is that when the test inevitably fails, the warning is buried below all the error messages that (most likely) inevitably are generated.

I have considered replacing warnings.warn with pytest.fail, but this would prevent the test from running at all which is overkill and would prevent individual tests from running.

So my question is:

Can I ensure that the warnings.warn call (or equivalent) is printed early? E.g., to produce an output akin to:

===================================== test session starts =====================================
platform darwin -- Python 3.13.1, pytest-8.3.4, pluggy-1.5.0
rootdir: /Users/joshua.ellis/src/pact-foundation/pact-python
configfile: pyproject.toml
plugins: asyncio-0.24.0, cov-6.0.0,nyio-4.7.0, bdd-8.1.0, xdist-3.6.1
asyncio: mode=Mode.STRICT, default_loop_scope=session
10 workers [18 items]     
WARNING: Detected multiple xdist workers. Examples will most likely fail.   <-- THIS
...EEEF...FF.F

EDIT: to be clear, I do not want solutions which require addition command line arguments, as that would imply that the person calling pytest is already aware of the warning. I want the warning to be displayed when people are naively running pytest.

[dongfangtianyu]

the warning is buried below all the error messages that (most likely) inevitably are generated.

Implemented by the built-in plugin warnings, you can use the -p to disable the plugin and use the -s to display warning content

@pytest.fixture(scope='session', autouse=True)
def hello():
    warnings.warn('hello')

pytest -p no:warnings -s -n3
 ======================== test session starts ========================
platform win32 -- Python 3.12.0, pytest-8.3.4, pluggy-1.5.0
rootdir: C:\pytest834
configfile: pytest.ini
plugins: xdist-3.6.1
3 workers [3 items]     
C:\pytest834\conftest.py:8: UserWarning: hello
  warnings.warn('hello')
C:\pytest834\conftest.py:8: UserWarning: hello
  warnings.warn('hello')
C:\pytest834\conftest.py:8: UserWarning: hello
  warnings.warn('hello')
...
 ======================== 3 passed in 0.70s ========================

[JP-Ellis]

The problem with that solution is that it requires prior knowledge of the warning to know to add -p no:warnings and -s. By that stage, they already have seen the warning and should not be adding -n3.

Imagine instead of scenario where PYTEST_ADDOPTS has been set, and pytest is executed as part of an IDE where one does not see/control the arguments as easily. I want the warning to appear then.

[dongfangtianyu]

Don't need to know anything in advance, just use command parameters, configuration files, or PYTEST ADDOPTS, and use -p and - s to take effect. If everything is uncontrollable, then the outcome will not change

[JP-Ellis]

Are you trying to say that without adding -s and -p no:warnings, what I'm asking is impossible?

I guess an alternative question would be: how can I adjust/augment the information that is printed at the start of a pytest run? Is it possible to hook in to pytest's print-to-terminal functionality (e.g., the way it displays information at the start, or progress during runs)?

[dongfangtianyu]

Three ideas:

1. Use the method provided by pytest (through configuration or parameters)
2. Change pytest (through hook or modify source code)
3. Execute custom python code before pytest

Depends on what you can control. If you can't control parameters, configuration files, or environment variables, but can modify conftest.py, or you can try hook

[JP-Ellis]

The solution ultimately was to use a hook. I can't speak for all hooks, but it would appear that PyTest's stdout and stderr capturing are not yet enabled, thereby making warnings like I have above print to the console.

In my specific case of pytest-xdist, they conveniently expose the hook pytest_xdist_setupnodes which I can inspect to verify whether multiple workers are being used:

conftest.py

def pytest_xdist_setupnodes(
    config: pytest.Config,
    specs: Sequence[execnet.XSpec],
) -> None:
    if len(specs) > 1:
        sys.stderr.write("\n")
        warnings.warn(
            "Running the examples with multiple workers may cause issues. "
            "Consider running the examples with a single worker by setting "
            "`--numprocesses=1` or using `hatch run example`.",
            stacklevel=1,
        )