readline&readlines

需求

txt文件很大，难以打开，需要读取n行看看数据格式。

开始

with open(f1_name, 'r') as f1:
    contents1 = f1.readlines()[0:n_line]
    with open(f2_name, 'w') as f2:
        for line in contents1:
            f2.write(line)

发现读个10行需要6s（1G文件为例，200G的话就慢死了）

后来

with open(f1_name, 'r') as f1:
    while n_line > 0:
        n_line = n_line - 1;
        contents.append(f1.readline())

    with open(f2_name, 'w') as f2:
        for line in contents:
            f2.write(line)

使用readline（）取代readlines

附