We have got Router.url_path_for(...) method which is located inside the starlette package

Method-1: Using FastAPI instance

This method is useful when you are able to access the FastAPI instance in your current context. (Thanks to @Yagizcan Degirmenci)

from fastapi import FastAPI

app = FastAPI()


@app.get('/hello/')
def hello_world():
    return {"msg": "Hello World"}


@app.get('/hello/{number}/')
def hello_world_number(number: int):
    return {"msg": "Hello World Number", "number": number}


print(app.url_path_for('hello_world'))
print(app.url_path_for('hello_world_number', number=1))
print(app.url_path_for('hello_world_number', number=2))

# Results

"/hello/"
"/hello/1/"
"/hello/2/"

Drawback

• If we are using APIRouter, router.url_path_for('hello_world') may not work since router isn't an instance of FastAPI class. That is, we must have the FastAPI instance to resolve the URL

Method-2: Request instance

This method is useful when you are able to access the Request instance (the incoming request), usually, within a view.

from fastapi import FastAPI, Request

app = FastAPI()


@app.get('/hello/')
def hello_world():
    return {"msg": "Hello World"}


@app.get('/hello/{number}/')
def hello_world_number(number: int):
    return {"msg": "Hello World Number", "number": number}


@app.get('/')
def named_url_reveres(request: Request):
    return {
        "URL for 'hello_world'": request.url_for("hello_world"),
        "URL for 'hello_world_number' with number '1'": request.url_for("hello_world_number", number=1),
        "URL for 'hello_world_number' with number '2''": request.url_for("hello_world_number", number=2})
    }

# Result Response

{
    "URL for 'hello_world'": "http://0.0.0.0:6022/hello/",
    "URL for 'hello_world_number' with number '1'": "http://0.0.0.0:6022/hello/1/",
    "URL for 'hello_world_number' with number '2''": "http://0.0.0.0:6022/hello/2/"
}

Drawback

• We must include the request parameter in every (or required) view to resolve the URL, which might raise an ugly feel to developers.

Actually you don't need to reinvent the wheel. FastAPI supports this out-of-box (Actually Starlette), and it works pretty well.

app = FastAPI()

@app.get("/hello/{number}/")
def hello_world_number(number: int):
    return {"msg": "Hello World Number", "number": number}

If you have an endpoint like this you can simply use

In:  app.url_path_for("hello_world_number", number=3)
In:  app.url_path_for("hello_world_number", number=50)

Out: /hello/3/
Out: /hello/50/

In FastAPI, APIRouter and FastAPI(APIRoute) inherits from Router(Starlette's) so, if you have an APIRouter like this, you can keep using this feature

router = APIRouter()

@router.get("/hello")
def hello_world():
    return {"msg": "Hello World"}

In:  router.url_path_for("hello_world")
Out: /hello

url_for exists, but is provided by starlette, the server underpinning FastApi: https://www.starlette.io/routing/#reverse-url-lookups