py调用另一个py

import os 

#os.system('python hello1.py')


while 1: 
    print("****************
")
    print("****************
")
    num = input("please input a num: ")
    try:                                                                        
        print(num)
        if num == 'a':
            os.system('python hello1.py')
        if num == 2: 
            os.system('python ')
            print(successful)
        if num == 3: 
            os.system('python ')
        if num == 4: 
            print("****************
")
            print("****************
")
            break

    except ValueError:
        print("It's not a num, Please enter again
")