首先在终端cd到相应路径，然后运行下面代码:

# -*- coding: utf-8 -*-

if __name__ == '__main__':
    fpath = './IOtest.txt'

    with open(fpath, 'r') as f:
        s = f.read()
        print(s)

    with open(fpath, 'a') as f:
        f.write('Add one line.')

    with open(fpath, 'r') as f:
        s = f.read()
        print(s)

在终端显示：

Add 1st line.
Add 2nd line.
Add 3rd line.

Add 1st line.
Add 2nd line.
Add 3rd line.
Add one line.