Python unittest failing on function importing local file

I have a Python package with a directory that looks like this:

-root
 |
 |-src
 | |
 |  -app
 |   |
 |   |-__init__.py
 |   |-__main__.py
 |   |-file1.py
 |   |-file2.py
 |
 |-tests
   |
   |-__init__.py
   |-test_function.py

file1.py looks like this

from app import file2

def function():
    return file2.another_function()

test_function.py looks like this

import unittest

from src.app.file1 import function

class TestFunction(unittest.TestCase):
    def test_case(self):
        self.assertTrue(function())

However, whilst as a package this works fine, when running the test I get a ModuleNotFoundError: No module named 'app' on from app import file2.

It works when I put src. in front of app in file1.py but then the package breaks. Looking around, there isn't really a clear way to fix this so unsure what my path should be looking like.