Introduction

In my opinion, Python is the best language for building REST APIs for small portable systems like the Raspberry Pi. This is determined by several elements:

• Most of the IoT devices communicates have written libraries in Python
• Python allows you to create lightweight applications
• Python is a high-level language where we do not need to focus on implementation details
• We can use the FastAPI library to easily create web applications with a REST API with self-documentation by Swagger

Software

Any OS for the Rapsberry Pi should have Python installed by default. Let's verify this:

python -V

Install virtual environments venv:

sudo apt update
sudo apt install python3-venv -y

Create and run virtual environment:

python -m venv ~/.venv
source ~/.venv/bin/activate

Install FastAPI and Uvicorn:

pip install fastapi uvicorn

"Hello World" app example

Create first web app "Hello World". Write file app_fastapi_helloworld.py with body:

from fastapi import FastAPI

app = FastAPI()
@app.get("/")
def index():
    return "Hello World"

And run:

uvicorn app_fastapi_helloworld:app --host 0.0.0.0 --port 8000

Open in browser http://RASPBERRY_PI_IP:8000:

REST API method

Now let's write a real REST API method, create file app_fastapi_time.py:

from fastapi import APIRouter, FastAPI
from datetime import datetime

app = FastAPI()
router = APIRouter(prefix='/api')

@router.get("/get_time")
def get_time():
    return {"current_time": datetime.now().isoformat()}

app.include_router(router)

Run:

uvicorn app_fastapi_time:app --host 0.0.0.0 --port 8000

Open in browser http://RASPBERRY_PI_IP:8000/api/get_time:

POST request and defining request structure

We can define a JSON format in the request and in the response. Create new app app_fastapi_post.py with POST method:

from fastapi import APIRouter, FastAPI, Response
from pydantic import BaseModel, Field

app = FastAPI()
router = APIRouter(prefix='/api')

class MotorsDto(BaseModel):
    left: int = Field(ge=-100, le=100, description="The left motor can have values from -100 to 100.")
    right: int = Field(ge=-100, le=100, description="The right motor can have values from -100 to 100.")

@router.post("/motors")
def motors_set(data: MotorsDto):
    print("Left:", data.left)
    print("Right: ", data.right)
    return Response(status_code=200)

app.include_router(router)

Run:

uvicorn app_fastapi_post:app --host 0.0.0.0 --port 8000

Call POST method:

curl -X POST http://192.168.3.106:8000/api/motors -H "Content-Type: application/json" -d '{"left": 35, "right" : 90}'

Server side result:

We used the curl tool to make the call. However, the FastAPI framework has another feature that makes building REST APIs easier, it has a built-in Swagger library. When you type http://RASPBERRY_PI_IP:8000/docs into your browser, you will get a page with the HTTP methods available in our application. There are also structures defined to handle requests and responses, built-in and ours.

From this interface, we can call any available REST API method with parameters of our choice:

Uvicorn

We can simplify the way applications run on the Uvicorn server. Let's add a line at the end of our application:

if __name__ == "__main__":
    uvicorn.run(app, host="0.0.0.0", port=8000)

Now instead of a command:

uvicorn application_file_name:app --host 0.0.0.0 --port 8000

we can start the application by calling:

python application_file_name.py

Links

https://www.python.org/
https://docs.python.org/3/library/venv.html
https://fastapi.tiangolo.com/
https://pypi.org/project/uvicorn/
https://restfulapi.net/
https://developer.mozilla.org/en-US/docs/Web/HTTP/Methods
https://pypi.org/project/psutil/
https://swagger.io/