pytest module not found as module cannot be imported

Question

I need to structure my fastapi project.

However, regarding testing, I cannot figure out how to propery import files.

This is the structure:

#test_api.py

from fastapi.testclient import TestClient
import os
import pytest
from unittest import mock

from api import app # this line is always red

client = TestClient(app)

Of course running pytest results in a 'module not found' error. What I am doing wrong here?