Give access to a filesystem for a systemd service

I’m running API server with programmed endpoint to read a file content and return as a response.

File is placed in a /var directory, but when I trying to read this file, using Python standart library, I bump into “File not found” exception.

I assume that my python script is unable to access /var directory of a filesystem, as it’s running inside some sort of it’s own environment.

I already bumped into similar issue, but with NixOS packages. When my systemd service needed access to some sort of packages, I did this: = {
  path = with pkgs; [

This time I tried to add required path into this<serviceName>.path array: = {
  path = [

Although system was able to build with such approach, it also didn’t gave any positive results. cat command that endpoint uses to read file contents still returns non-zero exit status and server sends 500 response.

Here’s a python code, that describes the endpoint(just to show, how paths are declared there):

#!/usr/bin/env python3                                                                                                                                                                      
from flask import Flask, jsonify, request                                                                                                                                                   
from flask_restful import Resource, Api, reqparse                                                                                                                                           
import pandas as pd                                                                                                                                                                         
import ast                                                                                                                                                                                  
import subprocess                                                                                                                                                                           
import os

@app.route("/getDKIM", methods=["GET"])                                                                                                                                                     
def getDkimKey():                                                                                                                                                                           
    with open("/var/domain") as domainFile:                                                                                                                                                 
        domain = domainFile.readline()                                                                                                                                                      
        domain = domain.rstrip("\n")                                                                                                                                                        
    dkim = subprocess.check_output(["cat", "/var/dkim/" + domain + ".selector.txt"])                                                                                                        
    return jsonify(dkim)

if __name__ == '__main__':                                                                                                                                                             , debug=False)

Please advice, how to give access to the /var partition for the python script, packaged as a Nixpkgs package and running as systemd service