1、读取文件
语法： f = open(file = '/Users/crazy_heng/PycharmProjects/untitled/day1/usernam',mode = 'r',encoding='utf-8')
file = '/Users/crazy_heng/PycharmProjects/untitled/day1/usernam # 表示username这个文件的路径
mode = 'r' # 表示只读