How to import file in Python with changed entry point (sys.path)?

I have this structure:

web/   
   api/
       app.py
       sets.json
   tests/
        test.py

app.py:

def func():
    with open('sets.json', 'r') as file:
        ...

test.py:

import sys
import os
sys.path.append(os.getcwd()+'/api/')

from app import func
...

I want to run tests from the root (web/). Import is successful. But when func are called, I get error: FileNotFoundError: [Errno 2] No such file or directory: 'sets.json'.

Why? Sys.path was changed and import is worked. Why I can't import sets.json?

As a result, I have got such solution: I create tests/conftest.py (for Pytest, it allows to apply my changes for all files) with code:

import sys
import os
os.chdir(os.getcwd()+'/api/')
sys.path.append(os.getcwd())

With this solution I can do imports and from web/api/ (in Docker) and from web/ (in PyTest) and get access to sets.json in web/api/.