python-patterns

Python Patterns Skill

Instructions for AI

Apply idiomatic Python patterns, modern best practices, and production-grade code standards. Use this skill when writing, reviewing, or refactoring Python code.

Reference: atstaeff/better-python for concrete before/after examples of all patterns below.

Core Patterns

1. Repository Pattern

Abstraction over data access, enabling testability and swappable backends.

from typing import Protocol
from uuid import UUID

class Repository[T](Protocol):
    async def get(self, id: UUID) -> T | None: ...
    async def save(self, entity: T) -> None: ...
    async def delete(self, id: UUID) -> None: ...
    async def list_all(self) -> list[T]: ...

2. Service Layer Pattern

Business logic encapsulated in services with injected dependencies.

class OrderService:
    def __init__(self, repo: OrderRepository, events: EventBus) -> None:
        self._repo = repo
        self._events = events

    async def place_order(self, command: PlaceOrderCommand) -> Order:
        order = Order.create(command)
        await self._repo.save(order)
        await self._events.publish(OrderPlaced(order_id=order.id))
        return order

3. Domain Events

Decouple side effects from business logic.

from dataclasses import dataclass
from datetime import datetime
from uuid import UUID

@dataclass(frozen=True)
class DomainEvent:
    occurred_at: datetime = field(default_factory=lambda: datetime.now(UTC))

@dataclass(frozen=True)
class OrderPlaced(DomainEvent):
    order_id: UUID
    customer_id: UUID
    total_amount: float

4. Result Pattern (Error Handling)

Explicit success/failure without exceptions for expected cases.

from dataclasses import dataclass
from typing import Generic, TypeVar

T = TypeVar("T")
E = TypeVar("E")

@dataclass(frozen=True)
class Ok(Generic[T]):
    value: T

@dataclass(frozen=True)
class Err(Generic[E]):
    error: E

type Result[T, E] = Ok[T] | Err[E]

# Usage
def divide(a: float, b: float) -> Result[float, str]:
    if b == 0:
        return Err("Division by zero")
    return Ok(a / b)