python-django

Python & Django for Saleor

Before writing code

Fetch live docs:

1. Web-search site:docs.djangoproject.com topics models querysets for current Django ORM documentation
2. Web-search site:docs.djangoproject.com topics signals for Django signals reference
3. Web-search site:docs.celeryq.dev userguide tasks for Celery task patterns and configuration
4. Web-search site:docs.saleor.io developer for Saleor-specific Django conventions
5. Fetch https://docs.python.org/3/library/typing.html for Python type hints reference

Django ORM Patterns for Saleor

QuerySet Operations

| Operation | Method | Example Use | |-----------|--------|-------------| | Filter | .filter(kwargs) | Filter products by type | | Exclude | .exclude(kwargs) | Exclude draft orders | | Annotate | .annotate(expr) | Add computed fields (totals, counts) | | Aggregate | .aggregate(expr) | Compute sum, avg across queryset | | Select related | .select_related("fk") | Join foreign keys (avoid N+1) | | Prefetch related | .prefetch_related("m2m") | Batch-load many-to-many (avoid N+1) | | Values | .values("field") | Return dictionaries instead of objects | | Order by | .order_by("field") | Sort results |

F and Q Objects

| Object | Purpose | Example Use | |--------|---------|-------------| | F("field") | Reference model field in expressions | Update stock: F("quantity") - 1 | | Q(condition) | Complex lookups with OR/AND/NOT | Q(status="active") | Q(featured=True) |

• Use F() for atomic field updates without race conditions
• Combine Q() objects with | (OR), & (AND), ~ (NOT) for complex filters

Model Relationships in Saleor

| Relationship | Field Type | Example | |-------------|-----------|---------| | One-to-Many | ForeignKey | Order -> User, OrderLine -> Order | | Many-to-Many | ManyToManyField | Product -> Category (via ProductCategory) | | One-to-One | OneToOneField | User -> UserProfile | | Generic | GenericForeignKey | Attribute values on multiple entity types |

• Always set on_delete explicitly (CASCADE, PROTECT, SET_NULL)
• Use related_name for reverse lookups
• Index foreign keys used in frequent queries

Django Signals

| Signal | Fires When | Common Use | |--------|-----------|------------| | pre_save | Before model.save() | Validate or transform data | | post_save | After model.save() | Trigger side effects, send notifications | | pre_delete | Before model.delete() | Clean up related resources | | post_delete | After model.delete() | Remove cached data | | m2m_changed | Many-to-many field modified | Update denormalized counters |

• Saleor uses webhooks as the primary event mechanism, not Django signals
• Use signals sparingly for internal side effects only
• Never perform expensive I/O in signal handlers (use Celery tasks instead)
• Connect signals in AppConfig.ready() to avoid import issues

Celery Task Patterns

Defining Tasks

| Pattern | Decorator | Use Case | |---------|-----------|----------| | Shared task | @shared_task | Framework-agnostic, recommended | | App task | @app.task | Tied to specific Celery app | | Bound task | @shared_task(bind=True) | Access self for retries |

Retry and Error Handling

| Parameter | Description | Example | |-----------|-------------|---------| | max_retries | Maximum retry attempts | 3 | | default_retry_delay | Seconds between retries | 60 | | retry_backoff | Enable exponential backoff | True | | autoretry_for | Exception classes to auto-retry | (ConnectionError,) | | acks_late | Acknowledge after execution | True (prevents task loss) |

• Pass serializable arguments (IDs, not model instances)
• Keep tasks idempotent — safe to retry without side effects
• Set reasonable timeouts with soft_time_limit and time_limit

Python Type Hints

| Type | Import | Use | |------|--------|-----| | Optional[T] | typing | Value may be None | | list[T] | Built-in (3.9+) | Typed list | | dict[K, V] | Built-in (3.9+) | Typed dictionary | | Union[A, B] | typing | Either type A or B | | Protocol | typing | Structural subtyping (duck typing) | | TypedDict | typing | Typed dictionary with fixed keys | | Literal["a", "b"] | typing | Restrict to specific values |

• Use from __future__ import annotations for postponed evaluation
• Type all function parameters, return values, and class attributes
• Use mypy or pyright for static type checking

Database Migrations

| Command | Purpose | |---------|---------| | python manage.py makemigrations | Generate migration files from model changes | | python manage.py migrate | Apply pending migrations to database | | python manage.py showmigrations | List all migrations and their status | | python manage.py sqlmigrate app_name 0001 | Show SQL for a specific migration | | python manage.py squashmigrations app_name 0001 0010 | Combine multiple migrations |

• Review generated migrations before applying to production
• Use RunPython for data migrations (separate from schema migrations)
• Test migrations on a copy of production data before deploying

Management Commands

| Aspect | Convention | |--------|-----------| | Location | app_name/management/commands/command_name.py | | Class | Subclass BaseCommand | | Entry point | handle(self, *args, options) method | | Arguments | Use add_arguments(self, parser) with argparse | | Output** | Use self.stdout.write() and self.style |

• Use management commands for one-off scripts, data fixes, and admin tasks
• Always add help text to commands for documentation

Async Django Views (ASGI)

| Feature | Sync | Async | |---------|------|-------| | View type | def view(request) | async def view(request) | | Server | WSGI (Gunicorn) | ASGI (Uvicorn, Daphne) | | ORM access | Direct | Wrap in sync_to_async | | HTTP calls | requests | httpx (async) |

• Saleor runs via ASGI with Uvicorn workers under Gunicorn
• Django ORM is synchronous — use sync_to_async or QuerySet.aiterator()
• Use httpx.AsyncClient for non-blocking HTTP requests

Django Settings and Virtual Environments

| Pattern | Description | |---------|-------------| | Environment variables | Load with os.environ or django-environ | | Split settings | settings/base.py, settings/dev.py, settings/prod.py | | Twelve-Factor | All configuration via environment variables |

| Tool | Command | Notes | |------|---------|-------| | venv | python -m venv .venv | Built-in, standard | | poetry | poetry install | Dependency resolution, lock file | | uv | uv venv && uv pip install -r requirements.txt | Fast Rust-based installer |

Best Practices

• Use select_related and prefetch_related to avoid N+1 query problems
• Keep Django signals lightweight — offload heavy work to Celery tasks
• Type all function signatures and use mypy for static analysis
• Make Celery tasks idempotent and pass only serializable arguments
• Review migration SQL before applying to production databases
• Use F() expressions for atomic field updates instead of read-modify-write
• Follow Saleor's existing code style when contributing to the core
• Use async views and httpx for I/O-heavy endpoints
• Store all configuration in environment variables following twelve-factor methodology

Fetch the Python and Django documentation for current ORM patterns, Celery task configuration, and async view setup before implementing.