如下txt文件，读取Shipper和Booking No.这两行。

Dear customer, please note that changes to payment terms or prepaid payer after an invoice has been made available may be subject to a payer amendment fee starting from the 1st of July
2019. In case you would like to create, view or modify your Standing instructions for payer, please click here - https://sealandmaersk.com/payer-standing-instructions/#/

Shipper

LONG SAIL INTERNATIONAL LOGISTICS CO., LTD.
12F, WENHUA BUILDING WEST, SHENNAN EAST ROAD, SHENZHEN,CHINA
TEL:0755-25190767
FAX:0755-25129435

Booking No.

205465283

Export references

Svc Contract

首先我们以只读方式（r）打开原文件。然后使用循环对每一行进行遍历，并使用enumerate函数（将一个可遍历的数据对象(如列表、元组或字符串)组合为一个索引序列）将读取到的每一行打包到enumerate对象中。然后进行判断我们所要读取的某几行，注意这个索引也是从0开始的。然后以追加的方式（a）写入文件中即可。

#读取指定内容
with open('./1.txt', 'r') as f:
    for num, line in enumerate(f):
        if num == 3:
            print(line)
            with open('./z.txt', 'a') as fp:
                fp.write(line)
        if num == 10:
            print(line)
            with open('./z.txt', 'a') as fp:
                fp.write(line)
            break