python - 如何将字符串添加到特定行

Question

我有一个包含 50 行的文件。如何使用 python/linux 将字符串“-----”添加到特定行，比如第 20 行？

Answer 1

你有没有尝试过这样的事情？：

exp = 20 # the line where text need to be added or exp that calculates it for ex %2

with open(filename, 'r') as f:
    lines = f.readlines()

with open(filename, 'w') as f:
    for i,line in enumerate(lines):
        if i == exp:
            f.write('------')
        f.write(line)

如果您需要编辑差异行数，您可以通过以下方式更新代码：

def update_file(filename, ln):
    with open(filename, 'r') as f:
        lines = f.readlines()

    with open(filename, 'w') as f:
        for idx,line in enumerate(lines):
            (idx in ln and f.write('------'))
            f.write(line)

Answer 2

$ head -n 20 input.txt > output.txt
$ echo "---" >> output.txt
$ tail -n 30 input.txt >> output.txt

Answer 3

如果要读取的文件很大，并且您不想一次读取内存中的整个文件：

from tempfile import mkstemp
from shutil import move
from os import remove, close

line_number = 20

file_path = "myfile.txt"

fh_r = open(file_path)
fh, abs_path = mkstemp()
fh_w = open(abs_path, 'w')

for i, line in enumerate(fh_r):
    if i == line_number - 1:
        fh_w.write('-----' + line)
    else:
        fh_w.write(line)

fh_r.close()
close(fh)
fh_w.close()

remove(file_path)
move(abs_path, file_path)

注意：我在这里使用了 Alok 的答案作为参考。