Dinamically import routers. Do not do this! This is a bad recommendation.

haha.py

from fastapi import APIRouter

router = APIRouter(prefix="/haha")

@router.get("/haha")
def home():
    ...

main.py

import os
from importlib import import_module
from pathlib import Path
from typing import cast

import uvicorn
from fastapi import APIRouter, FastAPI

app = FastAPI()

ROUTERS_DIR = "routers"

for root, subdirs, files in os.walk(ROUTERS_DIR):
    for file in files:
        current_file = os.path.join(root, file)
        if current_file.endswith(".py"):
            module_name = current_file.replace(".py", "").replace("/", ".")
            module = import_module(module_name)
            for key, value in module.__dict__.items():
                if isinstance(value, APIRouter):
                    router = cast(APIRouter, getattr(module, key))
                    app.include_router(router)

if __name__ == "__main__":
    uvicorn.run(app)

potato.py

from fastapi import APIRouter

router = APIRouter(prefix="/potato")

@router.get("/")
def home():
    ...