Critical Patterns

Type Hints (REQUIRED)

✅ ALWAYS: Use type hints for all function signatures

def calculate_total(items: list[dict], tax_rate: float = 0.1) -> float: """Calculate total with tax.""" subtotal = sum(item["price"] for item in items) return subtotal * (1 + tax_rate)

❌ NEVER: Untyped functions

def calculate_total(items, tax_rate=0.1): return sum(i["price"] for i in items) * (1 + tax_rate)

Docstrings (REQUIRED)

✅ ALWAYS: Google-style docstrings

def process_order(order_id: str, validate: bool = True) -> Order: """Process an order by ID.

Args:
    order_id: Unique order identifier.
    validate: Whether to validate before processing.
    
Returns:
    Processed Order object.
    
Raises:
    OrderNotFoundError: If order doesn't exist.
"""

Custom Exceptions (REQUIRED)

✅ ALWAYS: Custom exceptions over generic

class OrderNotFoundError(Exception): """Raised when order is not found.""" pass

❌ NEVER: Generic exceptions

raise Exception("Order not found")

Decision Tree

Need formatting? → Use f-strings Need async? → Use asyncio with async/await Need data class? → Use @dataclass or Pydantic Need env management? → Use poetry or uv Need type checking? → Run mypy in CI

Code Examples

Modern Python Features

Pattern matching (3.10+)

match status: case "pending": process_pending() case "completed": finalize() case _: raise ValueError(f"Unknown status: {status}")

Dataclasses

from dataclasses import dataclass

@dataclass class User: id: str name: str email: str active: bool = True

Context Managers

✅ Good: Use context managers for resources

with open("file.txt", "r") as f: content = f.read()

Async context manager

async with aiohttp.ClientSession() as session: async with session.get(url) as response: data = await response.json()

Commands

python -m venv .venv # Create virtual environment source .venv/bin/activate # Activate (Unix) pip install -r requirements.txt mypy src/ # Type checking ruff check src/ # Linting pytest tests/ # Run tests

Resources

Additional specialized documentation:

• PEP 8 Standards: pep8-standards.md

• FastAPI: fastapi.md

• FastAPI Async: fastapi-async.md

• Django: django.md

• Deep Learning: deep-learning.md

• Web Scraping: web-scraping.md