Testing with stogger

How to test log output in projects that use stogger.

Setup

Add pytest-structlog as a dev dependency:

uv add --dev pytest-structlog

No configuration needed — pytest-structlog auto-discovers structlog and provides a log fixture that captures all structlog events.

Unit Tests

Use the log fixture (type: StructuredLogCapture) to assert on captured events:

from pytest_structlog import StructuredLogCapture

import stogger
import structlog

stogger.init_logging()
log_module = structlog.get_logger()


def process_order(order_id: int):
    log_module.info("order-processed", _replace_msg="Order {order_id} done", order_id=order_id)
    log_module.debug("order-details", items=3, total_cents=9900)


def test_process_order(log: StructuredLogCapture):
    process_order(42)

    # Assert specific event exists
    assert log.has("order-processed", order_id=42, level="info")

    # Assert debug context
    assert log.has("order-details", items=3)

    # Assert full event list
    assert log.events == [
        log.info("order-processed", order_id=42),
        log.debug("order-details", items=3, total_cents=9900),
    ]

    # Count occurrences
    assert log.count("order-processed") == 1

Assertion Patterns

  • log.has(event_name, **context) — check a single event with optional context

  • log.events — full list of captured event dicts for exact matching

  • log.count(event_name) — number of times an event was logged

  • log.events >= [expected] — ordered subset match (checks subset, preserves order)

# Subset match: only check the events you care about
assert log.events >= [
    {"event": "order-processed", "level": "info"},
    {"event": "order-shipped", "level": "info"},
]

# Negation: event was NOT logged with this context
assert not log.has("order-processed", order_id=999)

CLI / Integration Tests with Typer CliRunner

The log fixture captures events directly from the structlog pipeline. This works regardless of where stogger renders output (stderr, file, journal). No special configuration needed.

from typer.testing import CliRunner
from pytest_structlog import StructuredLogCapture

from myapp.cli import app

runner = CliRunner()


def test_deploy_command(log: StructuredLogCapture):
    result = runner.invoke(app, ["deploy", "myapp", "--env", "staging"])

    assert result.exit_code == 0
    assert log.has("deployment-started", package="myapp", env="staging")
    assert log.has("deployment-completed")

Two kinds of assertions, two tools:

  • Structured assertions: log.has("event-id") — verify the right events were emitted with the right context

  • User-visible output: result.output — check rendered text that the user would see (CliRunner captures stderr too)

_replace_msg Assertions

_replace_msg is a processor concern — it transforms the event dict for display. In tests, assert on the raw structured data, not the rendered string:

def test_user_greeting(log: StructuredLogCapture):
    log_module.info("welcome", _replace_msg="Welcome {username}!", username="alice")

    # Assert on the event name and structured data
    assert log.has("welcome", username="alice")

    # _replace_msg appears in the event dict if you need it
    assert log.has("welcome", _replace_msg="Welcome {username}!", username="alice")

When to Use log_to_stdlib

The log_to_stdlib processor bridges structlog events to Python’s logging module. This is useful only for:

  • Legacy code that reads from stdlib logging handlers

  • Ad-hoc debugging with logging.basicConfig()

Do not add log_to_stdlib to your pipeline just to use pytest caplog. Instead, use pytest-structlog which captures events directly — no bridge needed, no duplicate output.

Note

log_to_stdlib is documented in the API reference for stogger.core. It is intentionally not part of the default pipeline. Adding it causes every event to appear twice.

Anti-Patterns

  • Don’t reconfigure structlog in test conftests. Replacing stogger’s pipeline with a custom processor defeats the purpose of a shared logging library. The log fixture captures events without touching the pipeline.

  • Don’t import PartialFormatter or other internals from stogger.core. If you need _replace_msg formatting in tests, assert on the raw event data with log.has() instead of re-rendering templates yourself.

  • Don’t build custom capture processors. pytest-structlog already provides event capture. Adding a second capture mechanism creates maintenance burden and diverges from the project’s logging conventions.

  • Don’t use caplog or log_to_stdlib — pytest-structlog is the right tool for structlog-based projects.