You need to add that directory to the path:

import sys
sys.path.append('../src')

Maybe put this into a module if you are using it a lot.

There are three ways to do what you want.

1. Prefered way

root_dir
├── my_module
│   ├── __init__.py
│   ├── main_file.py
│   ├── file1.py
│   ├── file2.py 
└── tests
    ├── __init__.py
    └── file1_test.py

All files in my_module shoud import like

from my_module.file1 import some_function

Then you don't need to do any hacks

1. Keepin src and assuming statements like

from src.file1 import some_function

are not what you want you need to modify your tests like

import os
import sys
sys.path.insert(0, os.path.abspath('src')

1. Not modifying src, not modifying tests: Run tests with

PYTHONPATH=path/to/src pytest

Is the virtual environment where you installed 'src' same, where you are trying this.

Pls. run the

pip list

in that virtual environment to check if 'src' is installed.

Also, pls. check the import statement in the python interpreter of your virtual env.