Python Log Levels vs Warnings: Capture Two Different Diagnostics

Start with a small, explicit task

A fictional label function records an informational event and warns when its old option is used. Capture the log in memory and the warning in a local context. Then lower the logger’s verbosity and show that the compatibility warning is still emitted. The exercise does not modify global warning filters permanently.

Run the complete example

This original example was checked with Python 3.12.14. Save it as python_logging_vs_warnings.py and run it with Python 3.12. It uses only the standard library. The assertions check the stated results.

import io
import logging
import warnings
stream = io.StringIO()
logger = logging.getLogger("label-example")
logger.handlers.clear()
logger.propagate = False
logger.setLevel(logging.INFO)
handler = logging.StreamHandler(stream)
handler.setFormatter(logging.Formatter("%(levelname)s:%(message)s"))
logger.addHandler(handler)
def label(old_option=False):
    logger.info("building label")
    if old_option:
        warnings.warn("old_option is deprecated", DeprecationWarning,
                      stacklevel=2)
    return "label"
with warnings.catch_warnings(record=True) as captured:
    warnings.simplefilter("always", DeprecationWarning)
    assert label(True) == "label"
    logger.setLevel(logging.ERROR)
    assert label(True) == "label"
assert stream.getvalue() == "INFO:building label\n"
assert len(captured) == 2
assert all(item.category is DeprecationWarning for item in captured)
print(stream.getvalue().strip())
print("captured warnings:", len(captured))
handler.close()
logger.removeHandler(handler)

Explain the result

The first call passes the INFO threshold and writes one line to the memory stream. The second call runs with an ERROR logger threshold, so its INFO event is not written. Both calls still produce a DeprecationWarning because the local warning filter requests always. The two warning objects and the single log line are checked separately.

The warning’s category describes a compatibility concern, not an operational log level. stacklevel points the warning toward the caller instead of the helper’s internal line. The example uses an explicit local filter because default visibility can vary with execution context; it does not promise that every deprecation warning appears on every console.

Avoid these mistakes

Do not hide all warnings just to reduce log noise. Choose the correct logger and warning policy for the audience. The example also disables propagation for its own logger so the same record is not repeated by a parent handler; that is separate from warning filtering.