No stdout/stderr is captured with invoke during tests in combination with structlog

[ivasic]

There seems to be an issue with capturing stdout/stderr when using structlog with stdlib Logger in tests.
I've raised an issue with structlog but they are not convinced the problem originates there since the logs are simply passed to the stdlib Logger.

I've traced this down to the click's isolation context manager which seems to replace sys.stdout/sys.stderr with mocks during testing and my guess would be that this could have something to do with the problem but I'm hoping you may know better.

I've tried numerous things like explicitly setting a StreamHandler(sys.stderr) to the stdlib basicConfig, experimented with mix_stderr etc. nothing helped. Any ideas how to fix or work around this issue would be most welcome, thank you.

An example

# app.py
import structlog
import click

logger = structlog.get_logger()
structlog.configure(logger_factory=structlog.stdlib.LoggerFactory())


@click.command()
def hello() -> None:
    logger.info("Hello!")

if __name__ == '__main__':
    hello()

A test case:

# test_app.py
from click.testing import CliRunner
from app import hello

def test_log() -> None:
    runner = CliRunner()
    result = runner.invoke(hello)
    print(result.stdout)
    assert "Hello" in result.stdout

Expected
Hello! (to be printed)
assertion to pass

Actual
nothing printed (result.stdout is empty)
assertion failure:

>       assert "Hello" in result.stdout
E       AssertionError: assert 'Hello' in ''
E        +  where '' = <Result okay>.stdout

test_app.py:8: AssertionError```

Environment: macOS

- Python version: 3.12.7
- Click version: 8.1.7