python-fastapi

What this skill does

# Python FastAPI Development

Expert patterns for building Python APIs with FastAPI, uv package manager, modular architecture, and SQLAlchemy database integration.

## Technology Stack

- **Runtime**: Python 3.12+
- **Package Manager**: uv (fast, Rust-based)
- **Framework**: FastAPI
- **ORM**: SQLAlchemy 2.0 (async)
- **Validation**: Pydantic v2
- **Database**: PostgreSQL (or SQLite for dev)
- **Migrations**: Alembic
- **Testing**: pytest, pytest-asyncio
- **Linting**: ruff

## Project Structure

Feature-based modular architecture - code organized by domain, not by layer:

```
my-project/
├── pyproject.toml           # Project config with uv
├── uv.lock                   # Lock file
├── .python-version           # Python version
├── .env                      # Environment variables
├── .env.example
├── alembic.ini               # Alembic config
├── alembic/                  # Migrations
│   ├── env.py
│   ├── script.py.mako
│   └── versions/
├── src/
│   └── app/
│       ├── __init__.py
│       ├── main.py           # FastAPI app entry
│       ├── config.py         # Settings
│       ├── database.py       # DB session
│       ├── core/
│       │   ├── __init__.py
│       │   ├── dependencies.py  # Shared dependencies
│       │   ├── exceptions.py    # Custom exceptions
│       │   ├── middleware.py    # Middleware
│       │   └── security.py      # Auth utilities
│       ├── models/
│       │   ├── __init__.py
│       │   └── base.py          # SQLAlchemy base & mixins
│       ├── features/
│       │   ├── __init__.py
│       │   ├── auth/
│       │   │   ├── __init__.py
│       │   │   ├── api.py       # Auth endpoints
│       │   │   ├── schemas.py   # Auth Pydantic schemas
│       │   │   ├── services.py  # Auth business logic
│       │   │   ├── models.py    # Auth SQLAlchemy models
│       │   │   └── utils.py     # Auth helpers (JWT, etc.)
│       │   ├── users/
│       │   │   ├── __init__.py
│       │   │   ├── api.py       # User endpoints
│       │   │   ├── schemas.py   # User Pydantic schemas
│       │   │   ├── services.py  # User business logic
│       │   │   ├── models.py    # User SQLAlchemy models
│       │   │   └── repository.py # User data access
│       │   └── items/
│       │       ├── __init__.py
│       │       ├── api.py
│       │       ├── schemas.py
│       │       ├── services.py
│       │       └── models.py
│       └── api/
│           ├── __init__.py
│           └── router.py        # Aggregates all feature routers
└── tests/
    ├── __init__.py
    ├── conftest.py
    ├── features/
    │   ├── auth/
    │   │   └── test_auth.py
    │   └── users/
    │       └── test_users.py
    └── integration/
```

## Quick Setup with uv

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

# Create new project
uv init my-project
cd my-project

# Set Python version
uv python pin 3.12

# Add dependencies
uv add fastapi uvicorn[standard] sqlalchemy[asyncio] asyncpg
uv add pydantic pydantic-settings python-dotenv
uv add alembic

# Add dev dependencies
uv add --dev pytest pytest-asyncio pytest-cov httpx ruff mypy

# Create source structure
mkdir -p src/app/{api/v1/endpoints,core,models,schemas,services,repositories}
touch src/app/__init__.py
```

## Core Patterns

### pyproject.toml

```toml
[project]
name = "my-project"
version = "0.1.0"
description = "FastAPI application"
requires-python = ">=3.12"
dependencies = [
    "fastapi>=0.115.0",
    "uvicorn[standard]>=0.32.0",
    "sqlalchemy[asyncio]>=2.0.0",
    "asyncpg>=0.30.0",
    "pydantic>=2.10.0",
    "pydantic-settings>=2.6.0",
    "python-dotenv>=1.0.0",
    "alembic>=1.14.0",
]

[project.optional-dependencies]
dev = [
    "pytest>=8.0.0",
    "pytest-asyncio>=0.24.0",
    "pytest-cov>=6.0.0",
    "httpx>=0.28.0",
    "ruff>=0.8.0",
    "mypy>=1.13.0",
]

[tool.ruff]
target-version = "py312"
line-length = 88

[tool.ruff.lint]
select = ["E", "F", "I", "N", "W", "UP", "B", "C4", "SIM"]

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

[tool.mypy]
python_version = "3.12"
strict = true
```

### Configuration (src/app/config.py)

```python
from functools import lru_cache
from pydantic_settings import BaseSettings, SettingsConfigDict


class Settings(BaseSettings):
    model_config = SettingsConfigDict(
        env_file=".env",
        env_file_encoding="utf-8",
        case_sensitive=False,
    )

    # App
    app_name: str = "My API"
    debug: bool = False
    api_v1_prefix: str = "/api/v1"

    # Database
    database_url: str = "postgresql+asyncpg://user:pass@localhost:5432/db"

    # Security
    secret_key: str = "change-me-in-production"
    access_token_expire_minutes: int = 30


@lru_cache
def get_settings() -> Settings:
    return Settings()


settings = get_settings()
```

### Database Setup (src/app/database.py)

```python
from collections.abc import AsyncGenerator
from sqlalchemy.ext.asyncio import (
    AsyncSession,
    async_sessionmaker,
    create_async_engine,
)

from app.config import settings

engine = create_async_engine(
    settings.database_url,
    echo=settings.debug,
    pool_pre_ping=True,
)

async_session_maker = async_sessionmaker(
    engine,
    class_=AsyncSession,
    expire_on_commit=False,
)


async def get_db() -> AsyncGenerator[AsyncSession, None]:
    async with async_session_maker() as session:
        try:
            yield session
            await session.commit()
        except Exception:
            await session.rollback()
            raise
```

### SQLAlchemy Base Model (src/app/models/base.py)

```python
from datetime import datetime
from sqlalchemy import DateTime, func
from sqlalchemy.orm import DeclarativeBase, Mapped, mapped_column


class Base(DeclarativeBase):
    pass


class TimestampMixin:
    created_at: Mapped[datetime] = mapped_column(
        DateTime(timezone=True),
        server_default=func.now(),
    )
    updated_at: Mapped[datetime] = mapped_column(
        DateTime(timezone=True),
        server_default=func.now(),
        onupdate=func.now(),
    )
```

## Feature Module Pattern

Each feature is self-contained with its own api, schemas, services, models, and utils.

### Feature: users/schemas.py

```python
from pydantic import BaseModel, EmailStr, ConfigDict


class UserBase(BaseModel):
    email: EmailStr
    full_name: str | None = None


class UserCreate(UserBase):
    password: str


class UserUpdate(BaseModel):
    email: EmailStr | None = None
    full_name: str | None = None
    password: str | None = None


class UserResponse(UserBase):
    model_config = ConfigDict(from_attributes=True)
    id: int
    is_active: bool
```

### Feature: users/models.py

```python
from sqlalchemy import String
from sqlalchemy.orm import Mapped, mapped_column

from app.models.base import Base, TimestampMixin


class User(Base, TimestampMixin):
    __tablename__ = "users"

    id: Mapped[int] = mapped_column(primary_key=True)
    email: Mapped[str] = mapped_column(String(255), unique=True, index=True)
    hashed_password: Mapped[str] = mapped_column(String(255))
    full_name: Mapped[str | None] = mapped_column(String(255))
    is_active: Mapped[bool] = mapped_column(default=True)
```

### Feature: users/repository.py

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

from app.features.users.models import User


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

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

    async def get_by_email(self, email: str) -> User | None:
        result = await self.db.execute(select(User).where(User.email == email))
        return result.scalar_one_or_none()

    async def get_all(self, skip: int = 0, limit: int = 100) -> list[User]:
        result = await self.db.execute(select(User).offset(skip).limit(limit))
        return list(result.scalars().all())

    as