lang-python

name	lang-python
description	Python 3.13+ development specialist covering FastAPI, Django, async patterns, data science, testing with pytest, and modern Python features. Use when developing Python APIs, web applications, data pipelines, or writing tests.
version	0.0.1
updated	"2025-12-30T00:00:00.000Z"
status	active
credit	modu-ai/moai-adk

Quick Reference (30 seconds)

Python 3.13+ Development Specialist - FastAPI, Django, async patterns, pytest, and modern Python features.

Auto-Triggers: .py files, pyproject.toml, requirements.txt, pytest.ini, FastAPI/Django discussions

Core Capabilities:

• Python 3.13 Features: JIT compiler (PEP 744), GIL-free mode (PEP 703), pattern matching
• Web Frameworks: FastAPI 0.115+, Django 5.2 LTS
• Data Validation: Pydantic v2.9 with model_validate patterns
• ORM: SQLAlchemy 2.0 async patterns
• Testing: pytest with fixtures, async testing, parametrize
• Package Management: poetry, uv, pip with pyproject.toml
• Type Hints: Protocol, TypeVar, ParamSpec, modern typing patterns
• Async: asyncio, async generators, task groups
• Data Science: numpy, pandas, polars basics

Quick Patterns

FastAPI Endpoint:

from fastapi import FastAPI, Depends
from pydantic import BaseModel

app = FastAPI()

class UserCreate(BaseModel):
    name: str
    email: str

@app.post("/users/")
async def create_user(user: UserCreate) -> User:
    return await UserService.create(user)

Pydantic v2.9 Validation:

from pydantic import BaseModel, ConfigDict

class User(BaseModel):
    model_config = ConfigDict(from_attributes=True, str_strip_whitespace=True)

    id: int
    name: str
    email: str

user = User.model_validate(orm_obj)  # from ORM object
user = User.model_validate_json(json_data)  # from JSON

pytest Async Test:

import pytest

@pytest.mark.asyncio
async def test_create_user(async_client):
    response = await async_client.post("/users/", json={"name": "Test"})
    assert response.status_code == 201

---

Implementation Guide (5 minutes)

Python 3.13 New Features

JIT Compiler (PEP 744):

• Experimental feature, disabled by default
• Enable: PYTHON_JIT=1 environment variable
• Build option: --enable-experimental-jit
• Provides performance improvements for CPU-bound code
• Copy-and-patch JIT that translates specialized bytecode to machine code

GIL-Free Mode (PEP 703):

• Experimental free-threaded build (python3.13t)
• Allows true parallel thread execution
• Available in official Windows/macOS installers
• Best for: CPU-intensive multi-threaded applications
• Not recommended for production yet

Pattern Matching (match/case):

def process_response(response: dict) -> str:
    match response:
        case {"status": "ok", "data": data}:
            return f"Success: {data}"
        case {"status": "error", "message": msg}:
            return f"Error: {msg}"
        case {"status": status} if status in ("pending", "processing"):
            return "In progress..."
        case _:
            return "Unknown response"

FastAPI 0.115+ Patterns

Async Dependency Injection:

from fastapi import FastAPI, Depends
from sqlalchemy.ext.asyncio import AsyncSession
from contextlib import asynccontextmanager

@asynccontextmanager
async def lifespan(app: FastAPI):
    # Startup
    await init_db()
    yield
    # Shutdown
    await cleanup()

app = FastAPI(lifespan=lifespan)

async def get_db() -> AsyncGenerator[AsyncSession, None]:
    async with async_session() as session:
        yield session

@app.get("/users/{user_id}")
async def get_user(
    user_id: int,
    db: AsyncSession = Depends(get_db)
) -> UserResponse:
    user = await get_user_by_id(db, user_id)
    return UserResponse.model_validate(user)

Class-Based Dependencies:

from fastapi import Depends

class Paginator:
    def __init__(self, page: int = 1, size: int = 20):
        self.page = max(1, page)
        self.size = min(100, max(1, size))
        self.offset = (self.page - 1) * self.size

@app.get("/items/")
async def list_items(pagination: Paginator = Depends()) -> list[Item]:
    return await Item.get_page(pagination.offset, pagination.size)

Django 5.2 LTS Features

Composite Primary Keys:

from django.db import models

class OrderItem(models.Model):
    order = models.ForeignKey(Order, on_delete=models.CASCADE)
    product = models.ForeignKey(Product, on_delete=models.CASCADE)
    quantity = models.IntegerField()

    class Meta:
        pk = models.CompositePrimaryKey("order", "product")

URL Reverse with Query Parameters:

from django.urls import reverse

url = reverse("search", query={"q": "django", "page": 1}, fragment="results")
# /search/?q=django&page=1#results

Automatic Model Imports in Shell:

python manage.py shell
# Models from all installed apps are automatically imported

Pydantic v2.9 Deep Patterns

Reusable Validators with Annotated:

from typing import Annotated
from pydantic import AfterValidator, BaseModel

def validate_positive(v: int) -> int:
    if v <= 0:
        raise ValueError("Must be positive")
    return v

PositiveInt = Annotated[int, AfterValidator(validate_positive)]

class Product(BaseModel):
    price: PositiveInt
    quantity: PositiveInt

Model Validator for Cross-Field Validation:

from pydantic import BaseModel, model_validator
from typing import Self

class DateRange(BaseModel):
    start_date: date
    end_date: date

    @model_validator(mode="after")
    def validate_dates(self) -> Self:
        if self.end_date < self.start_date:
            raise ValueError("end_date must be after start_date")
        return self

ConfigDict Best Practices:

from pydantic import BaseModel, ConfigDict

class BaseSchema(BaseModel):
    model_config = ConfigDict(
        from_attributes=True,      # ORM object support
        populate_by_name=True,     # Allow aliases
        extra="forbid",            # Fail on unknown fields
        str_strip_whitespace=True, # Clean strings
    )

SQLAlchemy 2.0 Async Patterns

Engine and Session Setup:

from sqlalchemy.ext.asyncio import (
    create_async_engine,
    async_sessionmaker,
    AsyncSession,
)

engine = create_async_engine(
    "postgresql+asyncpg://user:pass@localhost/db",
    pool_pre_ping=True,
    echo=True,
)

async_session = async_sessionmaker(
    engine,
    class_=AsyncSession,
    expire_on_commit=False,  # Prevent detached instance errors
)

Repository Pattern:

from sqlalchemy import select
from sqlalchemy.ext.asyncio import AsyncSession

class UserRepository:
    def __init__(self, session: AsyncSession):
        self.session = session

    async def get_by_id(self, user_id: int) -> User | None:
        result = await self.session.execute(
            select(User).where(User.id == user_id)
        )
        return result.scalar_one_or_none()

    async def create(self, user: UserCreate) -> User:
        db_user = User(**user.model_dump())
        self.session.add(db_user)
        await self.session.commit()
        await self.session.refresh(db_user)
        return db_user

Streaming Large Results:

async def stream_users(db: AsyncSession):
    result = await db.stream(select(User))
    async for user in result.scalars():
        yield user

pytest Advanced Patterns

Async Fixtures with pytest-asyncio:

import pytest
import pytest_asyncio
from httpx import AsyncClient

@pytest_asyncio.fixture
async def async_client(app) -> AsyncGenerator[AsyncClient, None]:
    async with AsyncClient(app=app, base_url="http://test") as client:
        yield client

@pytest_asyncio.fixture
async def db_session() -> AsyncGenerator[AsyncSession, None]:
    async with async_session() as session:
        async with session.begin():
            yield session
            await session.rollback()

Parametrized Tests:

@pytest.mark.parametrize(
    "input_data,expected_status",
    [
        ({"name": "Valid"}, 201),
        ({"name": ""}, 422),
        ({}, 422),
    ],
    ids=["valid", "empty_name", "missing_name"],
)
async def test_create_user(async_client, input_data, expected_status):
    response = await async_client.post("/users/", json=input_data)
    assert response.status_code == expected_status

Fixture Factories:

@pytest.fixture
def user_factory():
    async def _create_user(db: AsyncSession, **kwargs) -> User:
        defaults = {"name": "Test User", "email": "[e-mail]"}
        user = User(**(defaults | kwargs))
        db.add(user)
        await db.commit()
        return user
    return _create_user

Type Hints Modern Patterns

Protocol for Structural Typing:

from typing import Protocol, runtime_checkable

@runtime_checkable
class Repository(Protocol[T]):
    async def get(self, id: int) -> T | None: ...
    async def create(self, data: dict) -> T: ...
    async def delete(self, id: int) -> bool: ...

ParamSpec for Decorators:

from typing import ParamSpec, TypeVar, Callable
from functools import wraps

P = ParamSpec("P")
R = TypeVar("R")

def retry(times: int = 3) -> Callable[[Callable[P, R]], Callable[P, R]]:
    def decorator(func: Callable[P, R]) -> Callable[P, R]:
        @wraps(func)
        async def wrapper(*args: P.args, **kwargs: P.kwargs) -> R:
            for attempt in range(times):
                try:
                    return await func(*args, **kwargs)
                except Exception:
                    if attempt == times - 1:
                        raise
        return wrapper
    return decorator

Package Management

pyproject.toml (Poetry):

[tool.poetry]
name = "my-project"
version = "1.0.0"
python = "^3.13"

[tool.poetry.dependencies]
fastapi = "^0.115.0"
pydantic = "^2.9.0"
sqlalchemy = {extras = ["asyncio"], version = "^2.0.0"}

[tool.poetry.group.dev.dependencies]
pytest = "^8.0"
pytest-asyncio = "^0.24"
ruff = "^0.8"

[tool.ruff]
line-length = 100
target-version = "py313"

[tool.pytest.ini_options]
asyncio_mode = "auto"

uv (Fast Package Manager):

# Install uv
curl -LsSf https://astral.sh/uv/install.sh | sh

# Create virtual environment
uv venv

# Install dependencies
uv pip install -r requirements.txt

# Add dependency
uv add fastapi

---

Advanced Implementation (10+ minutes)

For comprehensive coverage including:

• Production deployment patterns (Docker, Kubernetes)
• Advanced async patterns (task groups, semaphores)
• Data science integration (numpy, pandas, polars)
• Performance optimization techniques
• Security best practices (OWASP patterns)
• CI/CD integration patterns

See:

• reference.md - Complete reference documentation
• examples.md - Production-ready code examples

---

Context7 Library Mappings

/docs/astral.sh/uv
/tiangolo/fastapi - FastAPI async web framework
/django/django - Django web framework
/pydantic/pydantic - Data validation with type annotations
/sqlalchemy/sqlalchemy - SQL toolkit and ORM
/pytest-dev/pytest - Testing framework
/numpy/numpy - Numerical computing
/pandas-dev/pandas - Data analysis library
/pola-rs/polars - Fast DataFrame library

---

Troubleshooting

Common Issues:

Python Version Check:

python --version  # Should be 3.13+
python -c "import sys; print(sys.version_info)"

Async Session Detached Error:

• Solution: Set expire_on_commit=False in session config
• Or: Use await session.refresh(obj) after commit

pytest asyncio Mode Warning:

# pyproject.toml
[tool.pytest.ini_options]
asyncio_mode = "auto"
asyncio_default_fixture_loop_scope = "function"

Pydantic v2 Migration:

• parse_obj() is now model_validate()
• parse_raw() is now model_validate_json()
• from_orm() requires from_attributes=True in ConfigDict

---

Last Updated: 2025-12-30 Status: Active (v0.0.1)