Python Patterns

Python Patterns

Write Python that is type-safe, testable, and a joy to maintain.

Core Principles

1. Explicit Over Implicit

Use type hints. Avoid *args and **kwargs when named parameters work. Favor clear interfaces over dynamic flexibility.

2. Composition Over Inheritance

Python's multiple inheritance is powerful but dangerous. Prefer composition and protocols over deep class hierarchies.

3. Async Done Right

Async is a tool for I/O-bound workloads, not a universal default. Use synchronous code for CPU-bound tasks, async for network calls and file I/O.

4. Test-First for Critical Paths

Your business logic should be testable without mocks. Use dependency injection. Keep I/O at the boundaries.

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Python Maturity Model

| Level | Typing | Async | Testing | Structure | |-------|--------|-------|---------|-----------| | 1: Script | No type hints | sync only | Manual testing | Single file | | 2: Module | Basic types (str, int) | Basic asyncio | pytest, some coverage | Package with __init__.py | | 3: Package | Full type hints with mypy | Async with proper patterns | pytest + fixtures + mocking | src-layout, entry points | | 4: Service | Generics, Protocols, TypedDict | Structured concurrency | Property-based, integration tests | Domain-driven structure | | 5: Library | Precise types, variance annotations | Trio / anyio | Fuzzing, benchmark tests | Public API surface explicit |

Target: Level 3+ for production services.

---

Actionable Guidance

Type Hints

Basic Patterns

from typing import Optional, Union, Sequence, TypeVar, Protocol, Any
from datetime import datetime

# Function signatures
def process_user(
    user_id: int,
    name: str,
    email: Optional[str] = None,
    tags: list[str] | None = None,  # Python 3.10+ union syntax
) -> dict[str, Any]:
    ...

# TypedDict for structured dicts
class UserData(TypedDict, total=False):
    id: int
    name: str
    email: str
    created_at: datetime

# Protocols (structural subtyping)
class Drawable(Protocol):
    def draw(self, context: Any) -> None: ...

def render(item: Drawable) -> None:
    item.draw(...)  # Any object with draw() method works

Generic Types

T = TypeVar('T')
U = TypeVar('U', bound=Comparable)

class Repository(Generic[T]):
    def get(self, id: int) -> T | None: ...
    def list(self) -> Sequence[T]: ...
    def save(self, item: T) -> T: ...

Async Patterns

Proper Async Context Managers

import asyncio
from contextlib import asynccontextmanager

@asynccontextmanager
async def db_session():
    session = await create_session()
    try:
        yield session
    finally:
        await session.close()

# Usage
async with db_session() as session:
    result = await session.query(...)

Structured Concurrency

async def fetch_all_data():
    # Run tasks concurrently with proper error propagation
    async with asyncio.TaskGroup() as tg:
        task1 = tg.create_task(fetch_users())
        task2 = tg.create_task(fetch_orders())
        task3 = tg.create_task(fetch_products())

    # All tasks completed (or TaskGroup raised on error)
    return task1.result(), task2.result(), task3.result()

Timeouts and Cancellation

async def fetch_with_timeout(url: str, timeout: float = 10.0) -> Response:
    try:
        async with asyncio.timeout(timeout):
            return await fetch(url)
    except TimeoutError:
        logger.warning(f"Request to {url} timed out after {timeout}s")
        raise ServiceUnavailableError(f"Timeout fetching {url}")

Project Structure

Recommended: src layout

project/
├── pyproject.toml
├── src/
│   └── mypackage/
│       ├── __init__.py
│       ├── domain/       # Business logic
│       │   ├── models.py
│       │   └── services.py
│       ├── infrastructure/  # External dependencies
│       │   ├── database.py
│       │   └── http_client.py
│       ├── api/           # Entry points
│       │   └── routes.py
│       └── config.py
├── tests/
│   ├── unit/
│   ├── integration/
│   └── conftest.py
└── README.md

pyproject.toml (modern Python packaging):

[build-system]
requires = ["setuptools>=68.0"]
build-backend = "setuptools.build_meta"

[project]
name = "mypackage"
version = "0.1.0"
dependencies = [
    "fastapi>=0.100",
    "pydantic>=2.0",
]

Testing Patterns

Fixtures for Clean Tests

import pytest
from datetime import datetime, timezone

@pytest.fixture
def sample_user() -> UserData:
    return UserData(
        id=1,
        name="Alice",
        email="alice@example.com",
        created_at=datetime.now(timezone.utc),
    )

@pytest.fixture
def repo(in_memory_db):
    return UserRepository(in_memory_db)

def test_create_user(repo, sample_user):
    saved = repo.save(sample_user)
    assert saved.id == 1
    assert saved.name == "Alice"

def test_get_nonexistent_user(repo):
    result = repo.get(999)
    assert result is None

Testing Async Code

@pytest.mark.asyncio
async def test_async_service():
    service = UserService(client=MockAsyncClient())
    result = await service.get_user(42)
    assert result.name == "Alice"

Property-Based Testing

from hypothesis import given, strategies as st

@given(st.integers(min_value=1, max_value=1000))
def test_user_id_is_positive(user_id: int):
    result = process_user(user_id)
    assert result["user_id"] > 0

@given(st.emails())
def test_valid_email_format(email: str):
    assert validate_email(email) is True

Error Handling

Custom Exception Hierarchy

class AppError(Exception):
    """Base exception for application errors."""
    def __init__(self, message: str, code: str | None = None):
        super().__init__(message)
        self.code = code or "UNKNOWN_ERROR"

class NotFoundError(AppError):
    def __init__(self, resource: str, id: int | str):
        super().__init__(f"{resource} not found: {id}", code="NOT_FOUND")

class ValidationError(AppError):
    def __init__(self, message: str, field: str | None = None):
        super().__init__(message, code="VALIDATION_ERROR")
        self.field = field

Result Pattern (Alternative to Exceptions)

from dataclasses import dataclass
from typing import Generic, TypeVar

T = TypeVar('T')
E = TypeVar('E')

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

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

Result = Ok[T] | Err[E]

def get_user(user_id: int) -> Result[User, AppError]:
    user = database.find(user_id)
    if user is None:
        return Err(NotFoundError("User", user_id))
    return Ok(user)

---

Common Mistakes

1. Mutable default arguments: def func(items=[]) — creates one list shared across calls. Use None and initialize inside.
2. Ignoring type hints in hot paths: Type hints have minimal runtime cost but catch bugs early. Use mypy/pyright in CI.
3. Blocking the event loop: Calling requests.get() inside async code blocks all coroutines. Use httpx.AsyncClient.
4. Overusing **kwargs: Pass-through kwargs obscure function signatures. Be explicit about parameters.
5. Not using __slots__: For classes with many instances, __slots__ reduces memory by ~50%.
6. Mixing sync and async carelessly: Calling async from sync (or vice versa) requires careful handling. Use asyncio.run() only at entry points.
7. Deep nested context managers: Chain too many async with blocks. Extract into helper methods or context manager composition.

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: cosmicstack-labs
• Source: cosmicstack-labs/mercury-agent-skills
• License: MIT
• Homepage: https://skills.mercuryagent.sh

Install and usage instructions live in the source repository linked above.