input: 
  def list_all_countries() -> str:
      """Create a basic REST API using Python and Flask to list all the countries in the world.
      The API should return a JSON list of countries when accessed at the '/countries' endpoint.
      >>> list_all_countries()
      '{\n    "countries": ["Country1", "Country2", "Country3"]\n}'
      """
  
output:     
      from flask import Flask, jsonify
  
      app = Flask(__name__)
  
      @app.route('/countries', methods=['GET'])
      def get_countries():
          countries = ["Country1", "Country2", "Country3"]
          return jsonify(countries=countries)
  
      if __name__ == '__main__':
          app.run(debug=True)
  