fastapi

FastAPI Example (Generating Models from F#)

This example demonstrates how to build a FastAPI application using Fable.Python with the decorator-based API pattern. The Pydantic models are defined in F# and compile to Python.

Note: For the opposite approach (importing Pydantic models defined in Python), see the Pydantic example.

Features

• Class-based API endpoints using [<APIClass>] attribute
• HTTP method decorators: [<Get>], [<Post>], [<Put>], [<Delete>]
• Pydantic models for request/response validation
• Automatic OpenAPI documentation generation

Project Structure

examples/fastapi/
├── App.fs              # Main application with API endpoints
├── Models.fs           # Pydantic model definitions
├── PydanticExample.fsproj
├── build/              # Generated Python code (git-ignored)
└── README.md

Building

From the repository root:

# Using justfile
just example-fastapi

# Or manually
cd examples/fastapi
dotnet fable --lang python --outDir build

Running

cd examples/fastapi/build
uv run --group fastapi uvicorn app:app --reload

Then visit:

• http://localhost:8000 - API root
• http://localhost:8000/docs - Interactive Swagger UI documentation
• http://localhost:8000/redoc - ReDoc documentation

API Endpoints

Method	Endpoint	Description
GET	/	Welcome message
GET	/health	Health check
GET	/users	List all users
GET	/users/{user_id}	Get user by ID
POST	/users	Create a new user
GET	/items	List all items
GET	/items/{item_id}	Get item by ID
POST	/items	Create a new item
PUT	/items/{item_id}	Update an item
DELETE	/items/{item_id}	Delete an item

Example Usage

# Get all users
curl http://localhost:8000/users

# Create a new user
curl -X POST http://localhost:8000/users \
  -H "Content-Type: application/json" \
  -d '{"Name": "Charlie", "Email": "charlie@example.com"}'

# Get all items
curl http://localhost:8000/items

# Create a new item
curl -X POST http://localhost:8000/items \
  -H "Content-Type: application/json" \
  -d '{"Name": "Keyboard", "Price": 79.99, "InStock": true}'

How It Works

Pydantic Models

F# classes that inherit from BaseModel with the [<Py.ClassAttributes>] attribute compile to Pydantic models:

[<Py.ClassAttributes(style = Py.ClassAttributeStyle.Attributes, init = false)>]
type User(Id: int, Name: string, Email: string) =
    inherit BaseModel()
    member val Id: int = Id with get, set
    member val Name: string = Name with get, set
    member val Email: string = Email with get, set

Defining Endpoints

The [<APIClass>] attribute marks a class for FastAPI routing, and method decorators define the HTTP methods:

[<APIClass>]
type API() =
    [<Get("/users")>]
    static member get_users() : ResizeArray<User> =
        users

    [<Post("/users")>]
    static member create_user(request: CreateUserRequest) : obj =
        // ... create user logic

Development Mode

For hot-reloading during development:

# Using justfile
just dev-fastapi

# Or manually in two terminals:
# Terminal 1: Watch F# files
cd examples/fastapi
dotnet fable --lang python --outDir build --watch

# Terminal 2: Run uvicorn with reload
cd examples/fastapi/build
uv run --group fastapi uvicorn app:app --reload