Python Test Debugging Cheat Sheet

pytest and unittest debugging patterns for isolating failures, flaky tests, fixtures, and CI-only issues.

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## Selecting What to Run

Run tests matching a keyword

pytest -k "user and not slow"

# Filter tests by substring expression.

Run tests by marker

pytest -m integration

# Target a marked test group.

Stop after N failures

pytest --maxfail=2

# Limit noise during triage.

## Fixtures and Output

Show fixture setup order

pytest --setup-show

# Trace fixture behavior during a run.

Show fixture setup plan

pytest --setup-plan

# Print fixture plan without full execution noise.

Disable all capture

pytest --capture=no

# See stdout, stderr, and interactive output directly.

## CI and Flaky Tests

Re-run a failing test repeatedly

while pytest tests/test_jobs.py::test_sync_worker; do :; done

# Loop a single test to reproduce flakiness locally.

Print key environment values in a test

import os

def test_env_debug():
    print({k: os.environ.get(k) for k in ['CI', 'APP_ENV', 'DATABASE_URL']})

# Reveal CI-only configuration mismatches.

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