How to Use Dynamic Routes in FastAPI

Routing refers to how an API’s endpoints (URIs) respond to incoming requests. In FastAPI, a route path, in combination with a request method, define the endpoint at which requests can be made. You can define a dynamic route path that can contain one or multiple path parameters (variables). Each parameter will be wrapped in a pair of curly braces { }.

Let’s examine the most minimal example below for more clarity:

from fastapi import FastAPI

app = FastAPI()


@app.get('/users/{user_id}/{user_name}')
async def get_users(user_id: int, user_name: str):
    return {"user_data": {
        "id": user_id,
        "message": "Hello, " + user_name,
    }}

The value of the path params user_id and user_name will be passed to the handler function as the arguments user_id and user_name.

If you run the code above and go to http://localhost:8000/users/123/ABCDEF with your web browser, you will see this:

{"user_data":{"id":123,"message":"Hello, ABCDEF"}}

Screenshot:

If you look closer at the code, you would notice that the types of the parameters were declared in the function, where user_id is int and user_name is str. This helps validate the values passed to the URL. If you enter this URL http://localhost:8000/users/foo/bar into the address bar, you will get an error:

{"detail":
  [{
      "loc":["path","user_id"],
       "msg":"value is not a valid integer",
       "type":"type_error.integer"
  }]
}

The detailed information about the API was also automatically generated at http://localhost:8000/docs:

This will make it easier for developers using your API (for web frontend or mobile apps) to get the necessary insights.