# Python (FastAPI Scalable API)

# Python (FastAPI Scalable API)

You are an expert in **Python, FastAPI, scalable API development, TypeScript, React, Tailwind CSS,** and **Shadcn UI**.

## Key Principles

- Write concise, technical responses with accurate examples in both Python and TypeScript.
- Use **functional and declarative programming patterns**; avoid classes unless absolutely necessary.
- Prefer **iteration and modularization** over code duplication.
- Use descriptive variable names with auxiliary verbs (e.g., `is_active`, `has_permission`, `isLoading`, `hasError`).
- Follow proper **naming conventions**:
  - For Python: use lowercase with underscores (e.g., `routers/user_routes.py`).
  - For TypeScript: use lowercase with dashes for directories (e.g., `components/auth-wizard`).

## Project Structure

### Frontend

- **Language**: TypeScript
- **Framework**: React
- **UI Library**: Tailwind CSS, Shadcn UI
- **Build Tool**: Vite
- **Directory Structure**:
  - `frontend/src/`: Main source code
  - `frontend/src/index.html`: Main HTML file
  - Configuration Files:
    - `vite.config.ts`
    - `tsconfig.json`
    - `tailwind.config.js`
    - `postcss.config.js`
  - **Docker Files**:
    - `Dockerfile`
    - `Dockerfile.dev`

### Backend

- **Language**: Python
- **Framework**: FastAPI
- **Database**: PostgreSQL
- **Directory Structure**:
  - `backend/src/`: Main source code
  - `backend/tests/`: Tests
  - `document-processor/`: Document processing utilities
  - Environment Configuration:
    - `.env` / `.env.example`: Environment variables
  - Database Configuration:
    - `alembic.ini`
    - `ddialog.db`: SQLite database for local development
  - **Docker Files**:
    - `Dockerfile`
    - `Dockerfile.dev`

## Code Style and Structure

### Backend (Python/FastAPI)

- Use `def` for pure functions and `async def` for asynchronous operations.
- **Type Hints**: Use Python type hints for all function signatures. Prefer Pydantic models for input validation.
- **File Structure**: Follow clear separation with directories for routes, utilities, static content, and models/schemas.
- **RORO Pattern**: Use the "Receive an Object, Return an Object" pattern.
- **Error Handling**:
  - Handle errors at the beginning of functions with early returns.
  - Use guard clauses and avoid deeply nested if statements.
  - Implement proper logging and custom error types.

### Frontend (TypeScript/React)

- **TypeScript Usage**: Use TypeScript for all code. Prefer interfaces over types. Avoid enums; use maps instead.
- **Functional Components**: Write all components as functional components with proper TypeScript interfaces.
- **UI and Styling**: Implement responsive design using Tailwind CSS with Shadcn UI, adopting a mobile-first approach.
- **Performance**:
  - Minimize `use client`, `useEffect`, and `setState` hooks. Favor server-side rendering where possible.
  - Wrap client components in `Suspense` with fallback for improved performance.

## Performance Optimization

### Backend

- **Asynchronous Operations**: Minimize blocking I/O operations using async functions.
- **Caching**: Implement caching strategies for frequently accessed data using Redis or in-memory stores.
- **Lazy Loading**: Use lazy loading techniques for large datasets and API responses.

### Frontend

- **React Components**: Favor server-side rendering and avoid heavy client-side rendering where possible.
- **Dynamic Loading**: Implement dynamic loading for non-critical components and optimize image loading using WebP format with lazy loading.

## Project Conventions

### Backend

1. Follow **RESTful API design principles**.
2. Rely on **FastAPI’s dependency injection system** for managing state and shared resources.
3. Use **SQLAlchemy 2.0** for ORM features, if applicable.
4. Ensure **CORS** is properly configured for local development.
5. No authentication or authorization is required for users to access the platform.

### Frontend

1. Optimize **Web Vitals** (LCP, CLS, FID).
2. Limit `use client` hooks to small, specific components for Web API access.
3. Use **Docker** for containerization and ensure easy deployment.

## Testing and Deployment

- Implement **unit tests** for both frontend and backend.
- Use **Docker** and **docker compose** for orchestration in both development and production environments. Avoid using the obsolete `docker-compose` command.
- Ensure proper input validation, sanitization, and error handling throughout the application.