Systematic Debugging

Systematic Debugging for Django

Core Principle

NO FIXES WITHOUT ROOT CAUSE FIRST

Never apply patches that mask underlying problems. Understand WHY something fails before attempting to fix it.

Four-Phase Framework

Phase 1: Reproduce and Investigate

Before touching any code:

1. Write a failing test - Captures the bug behavior
2. Read error messages thoroughly - Every word matters
3. Examine recent changes - git diff, git log
4. Trace data flow - Follow the call chain to find where bad values originate

# Write a failing test first
@pytest.mark.django_db
def test_bug_reproduction():
    """Reproduces issue #123."""
    user = UserFactory()
    response = Client().post("/profile/", {"bio": "New"})
    assert response.status_code == 200  # Currently failing

Phase 2: Isolate

Narrow down the problem:

# Add strategic logging
import logging
logger = logging.getLogger(__name__)

def problematic_view(request):
    logger.debug(f"Method: {request.method}")
    logger.debug(f"POST: {request.POST}")
    logger.debug(f"User: {request.user}")

    form = MyForm(request.POST)
    logger.debug(f"Valid: {form.is_valid()}")
    logger.debug(f"Errors: {form.errors}")

Phase 3: Identify Root Cause

• Read the full stack trace
• Use debugger to inspect state
• Check what assumptions are violated

Phase 4: Fix and Verify

1. Implement fix at the root cause
2. Run reproduction test (should pass)
3. Run full test suite
4. Verify manually if needed

Django Debug Tools

Django Debug Toolbar

# settings/dev.py
INSTALLED_APPS += ["debug_toolbar"]
MIDDLEWARE += ["debug_toolbar.middleware.DebugToolbarMiddleware"]
INTERNAL_IPS = ["127.0.0.1"]

Check SQL panel for N+1 queries, slow queries > 10ms.

Python Debugger

def problematic_view(request):
    breakpoint()  # Execution stops here

    # Commands: n(ext), s(tep), c(ontinue), p var, q(uit)

# Drop into debugger on test failure
uv run pytest --pdb -x

Query Debugging

# Log all SQL queries
LOGGING = {
    "loggers": {
        "django.db.backends": {"level": "DEBUG", "handlers": ["console"]},
    },
}

# Count queries in tests
from django.test.utils import CaptureQueriesContext
from django.db import connection

def test_no_n_plus_one():
    with CaptureQueriesContext(connection) as ctx:
        list(Post.objects.select_related("author"))
    assert len(ctx) <= 2

Common Django Issues

N+1 Queries

# Problem
for post in Post.objects.all():
    print(post.author.email)  # Query per post!

# Fix
for post in Post.objects.select_related("author"):
    print(post.author.email)  # Single query

Form Not Saving

# Check these:
# 1. form.is_valid() returns True?
# 2. form.save() called?
# 3. If commit=False, did you call .save() on instance?

def debug_form(request):
    form = MyForm(request.POST)
    print(f"Valid: {form.is_valid()}")
    print(f"Errors: {form.errors}")

CSRF 403 Errors

<!-- Check: csrf_token in form -->
<form method="post">
    {% csrf_token %}
</form>

Migration Issues

uv run python manage.py showmigrations
uv run python manage.py migrate app_name 0001 --fake

Debugging Celery

# Run synchronously for debugging
my_task(arg)  # Direct call, not .delay()

# Or set in settings
CELERY_TASK_ALWAYS_EAGER = True

Debugging HTMX

<script>htmx.logAll();</script>

def view(request):
    print(f"HTMX: {request.headers.get('HX-Request')}")

Checklist

Before claiming fixed:

• Root cause identified
• Reproduction test passes
• Full test suite passes (uv run pytest)
• No type errors (uv run pyright)
• No lint errors (uv run ruff check .)

Red Flags

Stop if you're thinking:

• "Quick fix now, investigate later"
• "One more attempt" (after 3+ failures)
• "This should work" (without understanding why)

Three consecutive failed fixes = architectural problem. Stop and discuss.

Integration with Other Skills

• pytest-django-patterns: Write reproduction tests
• django-models: Debug QuerySet issues
• celery-patterns: Debug async task failures
• htmx-alpine-patterns: Debug HTMX requests
• django-extensions: Use show_urls, list_model_info, and shell_plus for project introspection
• skill-creator: Create debugging-specific skills for recurring issues