input: Write a function to create a basic REST API using Python and Flask to list all the countries in the world. The API should respond to GET requests at the '/countries' endpoint, returning a JSON list of countries read from a 'data.json' file.
  
  
  import flask
  from flask import Flask, request, jsonify
  import json
  
  app = Flask(__name__)
  
  @app.route('/countries', methods=['GET'])
  def get_countries():
      with open('data.json', 'r') as f:
          countries = json.load(f)
      return jsonify(countries)
  
  if __name__ == '__main__':
      app.run()
  
  Assert the API returns a JSON list of countries when accessed at '/countries' endpoint:
  
  Assert accessing '/countries' endpoint returns a JSON list of countries.
  
  Note: Use an appropriate test client or tool to verify the API response.
output: 
  def get_countries():
      with open('data.json', 'r') as f:
          countries = json.load(f)
      return jsonify(countries)
  