Debug:django

Django Debugging Guide

Overview

Django debugging requires understanding the framework's layered architecture: settings, URL routing, views, templates, models, and middleware. Effective debugging follows a systematic approach - reproducing the bug, isolating the problem, gathering evidence, and implementing verified fixes.

Key principles:

• Reproduce first: Repeat the exact steps that cause the issue
• Isolate systematically: Identify the exact module, function, or code path
• Gather evidence: Collect error messages, stack traces, logs, and database state
• Test your fix: Verify the solution with automated tests

Common Error Patterns

TemplateDoesNotExist

Cause: Django cannot find the specified template file.

Investigation steps:

1. Check template exists in the correct directory
2. Verify TEMPLATES['DIRS'] setting includes your template directories
3. Check for typos in template path
4. Ensure app is in INSTALLED_APPS if using app-specific templates

# Check template configuration
python manage.py shell -c "from django.conf import settings; print(settings.TEMPLATES)"

# Verify template directories exist
python manage.py shell -c "import os; from django.conf import settings; print([d for d in settings.TEMPLATES[0]['DIRS'] if os.path.exists(d)])"

ImproperlyConfigured

Cause: Django configuration error in settings.py or environment.

Common causes:

• Missing or incorrect DJANGO_SETTINGS_MODULE
• Invalid database configuration
• Missing required settings
• Incorrect app configuration

# Check settings module
echo $DJANGO_SETTINGS_MODULE

# Validate configuration
python manage.py check

# Compare with defaults
python manage.py diffsettings

IntegrityError / Database Errors

Cause: Database constraint violations or missing migrations.

OperationalError (missing column/table):

# Check migration status
python manage.py showmigrations

# Create missing migrations
python manage.py makemigrations

# Apply migrations
python manage.py migrate

# Check SQL that would be executed
python manage.py sqlmigrate <app_name> <migration_number>

IntegrityError (constraint violation):

# Check for duplicates before insert
from django.db import IntegrityError

try:
    obj.save()
except IntegrityError as e:
    # Log the error, check constraint name
    print(f"Constraint violation: {e}")

N+1 Query Problems

Cause: Inefficient database queries in loops.

Detection:

# In settings.py (development only)
LOGGING = {
    'version': 1,
    'handlers': {'console': {'class': 'logging.StreamHandler'}},
    'loggers': {
        'django.db.backends': {
            'level': 'DEBUG',
            'handlers': ['console'],
        }
    }
}

Solution - use select_related/prefetch_related:

# Bad: N+1 queries
for order in Order.objects.all():
    print(order.customer.name)  # Extra query per order

# Good: Single query with join
for order in Order.objects.select_related('customer'):
    print(order.customer.name)

# For many-to-many or reverse foreign keys
orders = Order.objects.prefetch_related('items')

Migration Conflicts

Cause: Multiple developers creating migrations on same app.

# View migration graph
python manage.py showmigrations --plan

# Merge conflicting migrations
python manage.py makemigrations --merge

# Reset migrations (development only - DANGEROUS)
python manage.py migrate <app_name> zero
python manage.py makemigrations <app_name>
python manage.py migrate <app_name>

CSRF Verification Failures

Cause: Missing CSRF token or misconfigured trusted origins.

Checklist:

1. Include {% csrf_token %} in forms
2. Check CSRF_TRUSTED_ORIGINS for HTTPS sites
3. Verify middleware includes CsrfViewMiddleware
4. For AJAX, include CSRF token in headers

# settings.py - for cross-origin requests
CSRF_TRUSTED_ORIGINS = [
    'https://your-domain.com',
]

Import Errors / Circular Imports

Cause: Circular dependencies between modules.

Detection:

# Check for import errors
python -c "import your_app"

# Verbose import tracing
python -v -c "import your_app" 2>&1 | grep "import"

Solutions:

• Move imports inside functions (lazy import)
• Restructure modules to break cycles
• Use string references for model relationships

# Avoid circular import with lazy import
def my_function():
    from other_module import SomeClass  # Import inside function
    return SomeClass()

# Use string reference in ForeignKey
class Order(models.Model):
    customer = models.ForeignKey('customers.Customer',

DoesNotExist

Cause: Querying for an object that does not exist in the database.

# Bad: Raises DoesNotExist
user = User.objects.get(id=999)

# Good: Handle missing objects
from django.shortcuts import get_object_or_404
user = get_object_or_404(User, id=999)

# Or use get_or_create
user, created = User.objects.get_or_create(
    username='john',
    defaults={'email': 'john@example.com'}
)

# Or wrap in try-except
try:
    user = User.objects.get(id=999)
except User.DoesNotExist:
    user = None

DisallowedHost

Cause: Request host not in ALLOWED_HOSTS setting.

# settings.py
ALLOWED_HOSTS = [
    'localhost',
    '127.0.0.1',
    'your-domain.com',
    '.your-domain.com',  # Wildcard for subdomains
]

NoReverseMatch

Cause: URL reverse lookup failed - URL name not found or wrong arguments.

# List all URL patterns
python manage.py show_urls  # requires django-extensions

# Or manually inspect
python manage.py shell -c "from django.urls import get_resolver; print([p.name for p in get_resolver().url_patterns])"

# Check URL name matches exactly
from django.urls import reverse
url = reverse('app_name:view_name', args=[object_id])
url = reverse('app_name:view_name', kwargs={'pk': object_id})

Debugging Tools

Django Debug Toolbar

The most powerful visual debugging tool for Django development.

Installation:

pip install django-debug-toolbar

Configuration:

# settings.py (development only)
INSTALLED_APPS = [
    # ...
    'debug_toolbar',
]

MIDDLEWARE = [
    'debug_toolbar.middleware.DebugToolbarMiddleware',
    # ... other middleware
]

INTERNAL_IPS = ['127.0.0.1']

# For Docker
DEBUG_TOOLBAR_CONFIG = {
    'SHOW_TOOLBAR_CALLBACK': 'debug_toolbar.middleware.show_toolbar_with_docker',
}

# urls.py
from debug_toolbar.toolbar import debug_toolbar_urls

urlpatterns = [
    # ... your URLs
] + debug_toolbar_urls()

Key panels:

• SQL: All database queries with timing and EXPLAIN
• Templates: Rendered templates and context variables
• Cache: Cache hits/misses
• Request: Headers, cookies, session data
• Signals: Django signals fired

Python Debugger (pdb/ipdb)

# Insert breakpoint in code
breakpoint()  # Python 3.7+ (uses PYTHONBREAKPOINT env var)

# Or explicitly
import pdb; pdb.set_trace()

# For better experience
import ipdb; ipdb.set_trace()

Common pdb commands:

• n (next): Execute next line
• s (step): Step into function
• c (continue): Continue execution
• p variable: Print variable value
• pp variable: Pretty print
• l (list): Show current code
• w (where): Show stack trace
• q (quit): Exit debugger

django-extensions shell_plus

pip install django-extensions

# Add to INSTALLED_APPS
INSTALLED_APPS = ['django_extensions', ...]

# Use enhanced shell with auto-imports
python manage.py shell_plus

# With IPython
python manage.py shell_plus --ipython

# Print SQL queries
python manage.py shell_plus --print-sql

Logging Configuration

# settings.py
LOGGING = {
    'version': 1,
    'disable_existing_loggers': False,
    'formatters': {
        'verbose': {
            'format': '{levelname} {asctime} {module} {message}',
            'style': '{',
        },
    },
    'handlers': {
        'console': {
            'class': 'logging.StreamHandler',
            'formatter': 'verbose',
        },
        'file': {
            'class': 'logging.FileHandler',
            'filename': 'debug.log',
            'formatter': 'verbose',
        },
    },
    'loggers': {
        'django': {
            'handlers': ['console'],
            'level': 'INFO',
        },
        'django.db.backends': {
            'handlers': ['console'],
            'level': 'DEBUG',  # Shows all SQL queries
        },
        'myapp': {
            'handlers': ['console', 'file'],
            'level': 'DEBUG',
        },
    },
}

Usage in code:

import logging
logger = logging.getLogger(__name__)

logger.debug('Detailed information')
logger.info('General information')
logger.warning('Warning message')
logger.error('Error occurred', exc_info=True)  # Include traceback
logger.exception('Exception occurred')  # Auto-includes traceback

SQL Query Logging

# In Django shell
from django.db import connection
from django.db import reset_queries

reset_queries()
# ... execute your queries ...
print(connection.queries)

# Or use QuerySet.explain()
queryset = User.objects.filter(is_active=True)
print(queryset.explain())

The Four Phases (Django-Specific)

Phase 1: Root Cause Investigation

Gather evidence systematically:

# 1. Check Django configuration
python manage.py check
python manage.py check --deploy  # Production security checks

# 2. Review migration state
python manage.py showmigrations
python manage.py showmigrations --plan  # Execution order

# 3. Inspect settings
python manage.py diffsettings  # Compare with Django defaults

# 4. Check logs
tail -f debug.log
docker compose logs -f api  # If using Docker

Examine the error traceback:

• Note the exception type (determines error category)
• Find the last line of YOUR code (not Django internals)
• Check variable values in the frame

Database state inspection:

# In shell
python manage.py shell

from myapp.models import MyModel
MyModel.objects.count()
MyModel.objects.filter(status='error').values()

Phase 2: Pattern Analysis

Compare with working patterns:

1. Check similar working code: Find views/functions that work and compare
2. Review Django documentation: Verify correct usage of APIs
3. Check third-party package versions: Ensure compatibility

# Check installed versions
pip freeze | grep -i django
pip show django-rest-framework

Common pattern mismatches:

• Missing return in view functions
• Wrong queryset method order
• Incorrect URL pattern syntax
• Missing model field attributes

Phase 3: Hypothesis and Testing

Write a failing test first:

# tests/test_debug.py
import pytest
from django.test import TestCase, Client

@pytest.mark.django_db
class TestBugFix(TestCase):
    def test_reproduces_bug(self):
        """This test should fail until bug is fixed."""
        client = Client()
        response = client.get('/problematic-url/')
        self.assertEqual(response.status_code, 200)
        # Add assertions that currently fail

    def test_edge_case(self):
        """Test the specific input that causes the bug."""
        from myapp.services import problematic_function
        result = problematic_function(edge_case_input)
        self.assertEqual(result, expected_output)

Run tests in isolation:

# Run single test
pytest tests/test_debug.py::TestBugFix::test_reproduces_bug -v

# Run with verbose SQL output
pytest tests/test_debug.py -v --capture=no

# Run with debugger on failure
pytest tests/test_debug.py --pdb

# Run with coverage
pytest tests/test_debug.py --cov=myapp --cov-report=term-missing

Phase 4: Implementation

Implement the fix:

1. Make minimal changes to fix the issue
2. Ensure existing tests still pass
3. Add new tests for the specific bug
4. Document the fix in commit message

Verify the fix:

# Run all tests
pytest -v

# Run specific test file
pytest tests/test_affected_module.py -v

# Check for regressions
pytest tests/ -v --tb=short

# Verify in development server
python manage.py runserver

Post-fix checklist:

• All tests pass
• No new warnings introduced
• python manage.py check passes
• Related functionality manually tested
• Code reviewed

Quick Reference Commands

Django Management Commands

# System checks
python manage.py check                    # Run system checks
python manage.py check --deploy           # Production deployment checks
python manage.py check --tag security     # Security-specific checks

# Migrations
python manage.py showmigrations           # List all migrations and status
python manage.py showmigrations --plan    # Show execution order
python manage.py sqlmigrate app 0001      # Show SQL for migration
python manage.py makemigrations --dry-run # Preview migration creation
python manage.py migrate --plan           # Show migration plan

# Settings
python manage.py diffsettings             # Show non-default settings
python manage.py diffsettings --all       # Show all settings

# Database
python manage.py dbshell                  # Database shell
python manage.py inspectdb                # Generate models from database

# Shell
python manage.py shell                    # Django shell
python manage.py shell_plus               # Enhanced shell (django-extensions)
python manage.py shell_plus --print-sql   # Shell with SQL logging

# URLs
python manage.py show_urls                # List all URLs (django-extensions)

# Cleanup
python manage.py clearsessions            # Clear expired sessions
python manage.py flush                    # Clear database (DANGEROUS)

Quick Debugging Snippets

# Print all SQL queries in view
from django.db import connection
def my_view(request):
    # ... view logic ...
    for query in connection.queries:
        print(query['sql'])

# Check if request is AJAX
request.headers.get('X-Requested-With') == 'XMLHttpRequest'

# Inspect request data
print(request.GET.dict())
print(request.POST.dict())
print(request.body)
print(dict(request.headers))

# Debug template context
# In template:
{% debug %}

# Or in view:
from django.template import engines
engine = engines['django']
template = engine.from_string("{{ debug }}")

Environment Debugging

# Check Python environment
which python
python --version
pip list

# Check Django version
python -c "import django; print(django.VERSION)"

# Check database connection
python manage.py dbshell -c "SELECT 1;"

# Check Redis connection (if using)
redis-cli ping

# Check Celery workers
celery -A config inspect active
celery -A config inspect stats

Additional Resources

• Django Debug Toolbar Documentation
• Django Logging Documentation
• Django Exceptions Reference
• Common Django Errors and Solutions
• Django Error Handling Patterns
• Debugging Tips and Techniques by Matt Layman