# Python FastAPI Framework

FastAPI is a modern, fast (high-performance), web framework for building APIs with Python 3.6+ based on standard Python type hints.

## Installation

You need to install `fastapi` and an ASGI server, such as `uvicorn`.

```bash
pip install fastapi
pip install "uvicorn[standard]"
```

## Basic Application

Create a file `main.py`:

```python
from fastapi import FastAPI

app = FastAPI()

@app.get("/")
def read_root():
    return {"Hello": "World"}
```

### Running the Server

Run the server with:

```bash
uvicorn main:app --reload
```

*   `main`: the file `main.py` (the Python "module").
*   `app`: the object created inside of `main.py` with the line `app = FastAPI()`.
*   `--reload`: make the server restart after code changes.

## Path Parameters

You can declare path "variables" or "parameters" with the same syntax used by Python format strings.

```python
@app.get("/items/{item_id}")
def read_item(item_id: int):
    return {"item_id": item_id}
```

FastAPI uses type hints (`item_id: int`) to validate the data. If you visit `/items/foo`, you will get an HTTP error because "foo" is not an integer.

## Query Parameters

When you declare other function parameters that are not part of the path parameters, they are automatically interpreted as "query" parameters.

```python
@app.get("/items/")
def read_item(skip: int = 0, limit: int = 10):
    return {"fake_items_db": [{"item_name": "Foo"}, {"item_name": "Bar"}][skip : skip + limit]}
```

URL example: `http://127.0.0.1:8000/items/?skip=0&limit=10`

## Request Body

To send data to your API, you use Pydantic models.

```python
from typing import Optional
from fastapi import FastAPI
from pydantic import BaseModel

class Item(BaseModel):
    name: str
    price: float
    is_offer: Optional[bool] = None

app = FastAPI()

@app.post("/items/")
def create_item(item: Item):
    return item
```

## Automatic Documentation

FastAPI automatically generates interactive API documentation.

*   **Swagger UI**: Go to `http://127.0.0.1:8000/docs`.
*   **ReDoc**: Go to `http://127.0.0.1:8000/redoc`.

[[programming/python/python]]