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Python development principles and decision-making for 2025.
Learn to THINK, not memorize patterns.
When to Use
Use this skill when making Python architecture decisions, choosing frameworks, designing async patterns, or structuring Python projects.
⚠️ How to Use This Skill
This skill teaches decision-making principles, not fixed code to copy.
ASK user for framework preference when unclear
Choose async vs sync based on CONTEXT
Don't default to same framework every time
1. Framework Selection (2025)
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 Nonefrom typing importOptionaldeffind_user(id: int) -> Optional[User]: ...
# Union → one of multiple typesdefprocess(data: str | dict) -> None: ...
# Generic collectionsdefget_items() -> list[Item]: ...
defget_mapping() -> dict[str, int]: ...
# Callablefrom typing importCallabledefapply(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
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")asyncdefcreate(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 testsimport pytest
from httpx import AsyncClient
@pytest.mark.asyncioasyncdeftest_endpoint():
asyncwith 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.
