Decision Tree

What are you building?
│
├── API-first / Microservices
│   └── FastAPI (async, modern, fast)
│
├── Full-stack web / CMS / Admin
│   └── Django (batteries-included)
│
├── Simple / Script / Learning
│   └── Flask (minimal, flexible)
│
├── AI/ML API serving
│   └── FastAPI (Pydantic, async, uvicorn)
│
└── Background workers
    └── Celery + any framework

Comparison Principles

Factor	FastAPI	Django	Flask
Best for	APIs, microservices	Full-stack, CMS	Simple, learning
Async	Native	Django 5.0+	Via extensions
Admin	Manual	Built-in	Via extensions
ORM	Choose your own	Django ORM	Choose your own
Learning curve	Low	Medium	Low

Selection Questions to Ask:

Is this API-only or full-stack?
Need admin interface?
Team familiar with async?
Existing infrastructure?

2. Async vs Sync Decision

When to Use Async

async def is better when:
├── I/O-bound operations (database, HTTP, file)
├── Many concurrent connections
├── Real-time features
├── Microservices communication
└── FastAPI/Starlette/Django ASGI

def (sync) is better when:
├── CPU-bound operations
├── Simple scripts
├── Legacy codebase
├── Team unfamiliar with async
└── Blocking libraries (no async version)

The Golden Rule

I/O-bound → async (waiting for external)
CPU-bound → sync + multiprocessing (computing)

Don't:
├── Mix sync and async carelessly
├── Use sync libraries in async code
└── Force async for CPU work

Async Library Selection

Need	Async Library
HTTP client	httpx
PostgreSQL	asyncpg
Redis	aioredis / redis-py async
File I/O	aiofiles
Database ORM	SQLAlchemy 2.0 async, Tortoise

3. Type Hints Strategy

When to Type

Always type:
├── Function parameters
├── Return types
├── Class attributes
├── Public APIs

Can skip:
├── Local variables (let inference work)
├── One-off scripts
├── Tests (usually)

Common Type Patterns

# These are patterns, understand them:

# Optional → might be None
from typing import Optional
def find_user(id: int) -> Optional[User]: ...

# Union → one of multiple types
def process(data: str | dict) -> None: ...

# Generic collections
def get_items() -> list[Item]: ...
def get_mapping() -> dict[str, int]: ...

# Callable
from typing import Callable
def apply(fn: Callable[[int], str]) -> str: ...

Pydantic for Validation

When to use Pydantic:
├── API request/response models
├── Configuration/settings
├── Data validation
├── Serialization

Benefits:
├── Runtime validation
├── Auto-generated JSON schema
├── Works with FastAPI natively
└── Clear error messages

4. Project Structure Principles

Structure Selection

Small project / Script:
├── main.py
├── utils.py
└── requirements.txt

Medium API:
├── app/
│   ├── __init__.py
│   ├── main.py
│   ├── models/
│   ├── routes/
│   ├── services/
│   └── schemas/
├── tests/
└── pyproject.toml

Large application:
├── src/
│   └── myapp/
│       ├── core/
│       ├── api/
│       ├── services/
│       ├── models/
│       └── ...
├── tests/
└── pyproject.toml

FastAPI Structure Principles

Organize by feature or layer:

By layer:
├── routes/ (API endpoints)
├── services/ (business logic)
├── models/ (database models)
├── schemas/ (Pydantic models)
└── dependencies/ (shared deps)

By feature:
├── users/
│   ├── routes.py
│   ├── service.py
│   └── schemas.py
└── products/
    └── ...

5. Django Principles (2025)

Django Async (Django 5.0+)

Django supports async:
├── Async views
├── Async middleware
├── Async ORM (limited)
└── ASGI deployment

When to use async in Django:
├── External API calls
├── WebSocket (Channels)
├── High-concurrency views
└── Background task triggering

Django Best Practices

Model design:
├── Fat models, thin views
├── Use managers for common queries
├── Abstract base classes for shared fields

Views:
├── Class-based for complex CRUD
├── Function-based for simple endpoints
├── Use viewsets with DRF

Queries:
├── select_related() for FKs
├── prefetch_related() for M2M
├── Avoid N+1 queries
└── Use .only() for specific fields

6. FastAPI Principles

async def vs def in FastAPI

Use async def when:
├── Using async database drivers
├── Making async HTTP calls
├── I/O-bound operations
└── Want to handle concurrency

Use def when:
├── Blocking operations
├── Sync database drivers
├── CPU-bound work
└── FastAPI runs in threadpool automatically

Dependency Injection

Use dependencies for:
├── Database sessions
├── Current user / Auth
├── Configuration
├── Shared resources

Benefits:
├── Testability (mock dependencies)
├── Clean separation
├── Automatic cleanup (yield)

Pydantic v2 Integration

# FastAPI + Pydantic are tightly integrated:

# Request validation
@app.post("/users")
async def create(user: UserCreate) -> UserResponse:
    # user is already validated
    ...

# Response serialization
# Return type becomes response schema

7. Background Tasks

Selection Guide

Solution	Best For
BackgroundTasks	Simple, in-process tasks
Celery	Distributed, complex workflows
ARQ	Async, Redis-based
RQ	Simple Redis queue
Dramatiq	Actor-based, simpler than Celery

When to Use Each

FastAPI BackgroundTasks:
├── Quick operations
├── No persistence needed
├── Fire-and-forget
└── Same process

Celery/ARQ:
├── Long-running tasks
├── Need retry logic
├── Distributed workers
├── Persistent queue
└── Complex workflows

8. Error Handling Principles

Exception Strategy

In FastAPI:
├── Create custom exception classes
├── Register exception handlers
├── Return consistent error format
└── Log without exposing internals

Pattern:
├── Raise domain exceptions in services
├── Catch and transform in handlers
└── Client gets clean error response

Error Response Philosophy

Include:
├── Error code (programmatic)
├── Message (human readable)
├── Details (field-level when applicable)
└── NOT stack traces (security)

9. Testing Principles

Testing Strategy

Type	Purpose	Tools
Unit	Business logic	pytest
Integration	API endpoints	pytest + httpx/TestClient
E2E	Full workflows	pytest + DB

Async Testing

# Use pytest-asyncio for async tests

import pytest
from httpx import AsyncClient

@pytest.mark.asyncio
async def test_endpoint():
    async with AsyncClient(app=app, base_url="http://test") as client:
        response = await client.get("/users")
        assert response.status_code == 200

Fixtures Strategy

Common fixtures:
├── db_session → Database connection
├── client → Test client
├── authenticated_user → User with token
└── sample_data → Test data setup

10. Decision Checklist

Before implementing:

Asked user about framework preference?
Chosen framework for THIS context? (not just default)
Decided async vs sync?
Planned type hint strategy?
Defined project structure?
Planned error handling?
Considered background tasks?

11. Anti-Patterns to Avoid

❌ DON'T:

Default to Django for simple APIs (FastAPI may be better)
Use sync libraries in async code
Skip type hints for public APIs
Put business logic in routes/views
Ignore N+1 queries
Mix async and sync carelessly

✅ DO:

Choose framework based on context
Ask about async requirements
Use Pydantic for validation
Separate concerns (routes → services → repos)
Test critical paths

Remember: Python patterns are about decision-making for YOUR specific context. Don't copy code—think about what serves your application best.

python-patterns

Python Patterns

⚠️ How to Use This Skill

1. Framework Selection (2025)