import subprocess

def open_file(file_path):
    try:
        subprocess.Popen(['open', file_path])  # macOS
    except OSError:
        try:
            subprocess.Popen(['xdg-open', file_path])  # Linux
        except OSError:
            try:
                subprocess.Popen(['start', file_path], shell=True)  # Windows
            except OSError:
                print('Unable to open file:', file_path)

open_file('path/to/file.txt')