Non-deterministic logging.INFO capture in pytest depending on test scope (monorepo vs single package)

[VincentMalara]

• a detailed description of the bug or problem you are having
• output of pip list from the virtual environment you are using
• pytest and operating system versions
• minimal example if possible

---

Detailed description

pytest captures logging.info() calls differently depending on whether you run the full monorepo test suite or a single package. The same logging bug can be visible in one context and completely invisible in the other.

# Full monorepo — logging.info() formatting errors surface → tests FAIL
uv run pytest

# Single package — logging.info() formatting errors are silently swallowed → tests PASS
uv run pytest components/my-package/

Root cause

No log_level is set in [tool.pytest.ini_options]. The root logger therefore starts at Python's default level (WARNING). When logger.info() is called:

• effective level = WARNING → isEnabledFor(INFO) returns False → call is a no-op, no record is created, no formatting is attempted → bug invisible
• effective level = INFO → record is created, reaches pytest's handler, formatting is attempted → bug visible

When running the full monorepo, a fixture in another component sets the root logger to INFO. This state leaks into the rest of the test session. The my-package tests then run with INFO active and a malformed logger.info("message", value) call (missing %s placeholder) raises:

TypeError: not all arguments converted during string formatting

When running only my-package, no component ever sets the root logger to INFO, so it stays at WARNING and the bug is invisible.

---

Minimal example

# src/mymodule.py
import logging
logger = logging.getLogger(__name__)

def buggy():
    logger.info("value is", 42)  # ← missing %s placeholder

# tests/test_mymodule.py
from mymodule import buggy

def test_buggy():
    buggy()  # PASS if root logger is WARNING, FAIL if INFO

Running pytest tests/ alone → passes.
Running alongside a test suite that configures INFO logging → fails.

---

Fix

Add log_level = "INFO" to [tool.pytest.ini_options] in pyproject.toml so pytest always sets the root logger to INFO at session startup, making behavior deterministic regardless of test scope or execution order:

[tool.pytest.ini_options]
log_level = "INFO"

---

pytest and OS versions

• pytest 8.4.2
• OS: Windows 10 (win32 10.0.26100)

---

pip list

Full output

Package                        Version     Editable project location
------------------------------ ----------- -----------------------------------------------------------------------
altair                         5.5.0
annotated-types                0.7.0
anyio                          4.12.1
networkx                       3.3
numpy                          1.26.4
packaging                      25.0
pandas                         2.2.3
pluggy                         1.6.0
polyfactory                    3.2.0
pydantic                       2.12.5
pydantic-core                  2.41.5
pyhamcrest                     2.1.0
pytest                         8.4.2
pytest-custom-exit-code        0.3.0
pytest-mock                    3.15.1
pytest-timeout                 2.4.0
python-dotenv                  1.2.1
ruff                           0.5.7
scikit-learn                   1.3.2
scikit-learn-extra             0.3.0
scipy                          1.14.1
my-package             0.1.0       components/my-package
umap-learn                     0.5.7

[The-Compiler]

...and where is the pytest bug? All I see is an AI generated text that describes an issue (and fix!) for your testsuite.