如何查找文件中的特定行？

Question

我有一个文本文件如下：

1
/run/media/dsankhla/Entertainment/English songs/Apologise (Feat. One Republic).mp3
3
/run/media/dsankhla/Entertainment/English songs/Bad Meets Evil.mp3
5
/run/media/dsankhla/Entertainment/English songs/Love Me Like You DO.mp3

我想在文件中搜索特定行假设该行是
song_path = "/run/media/dsankhla/Entertainment/English songs/Bad Meets Evil.mp3"
然后我想寻找 len(song_path)+2 BEHIND 以便我可以指向文件中的 3 。我该怎么做？
到目前为止，这是我的代码：

txt = open(".songslist.txt", "r+")
if song_path in txt.read():
    byte = len(song_path)
    txt.seek(-(byte), 1)
    freq = int(txt.readline())
    print freq     # 3
    freq = freq + 1
    txt.seek(-2,1)
    txt.write(str(freq))
    txt.close()

Answer 1

最好的方法是使用搜索，如本例所示：

fp = open('myfile')
last_pos = fp.tell()
line = fp.readline()
while line != '':
  if line == 'SPECIAL':
    fp.seek(last_pos)
    change_line()#whatever you must to change
    break
  last_pos = fp.tell()
  line = fp.readline()

您必须使用fp.tell 将位置值分配给变量。然后用 fp.seek 你可以倒退。

Answer 2

如果您的文件不是太大（太大而无法放入内存，read/write 相当慢）您可以绕过任何 "low level" 操作，例如搜索并完全读取您的文件，更改你想改变什么，然后写回所有内容。

# read everything in
with open(".songslist.txt", "r") as f:
    txt = f.readlines()

# modify
path_i = None
for i, line in enumerate(txt):
    if song_path in line:
        path_i = i
        break

if path_i is not None:
    txt[path_i] += 1 # or what ever you want to do

# write back
with open(".songslist.txt", "w") as f:
    f.writelines(txt)

使用seek时注意不要写"byte perfekt"，即：

f = open("test", "r+")
f.write("hello world!\n12345")
f.seek(6) # jump to the beginning of "world"
f.write("1234567") # try to overwrite "world!" with "1234567" 
# (note that the second is 1 larger then "world!")
f.seek(0)
f.read() # output is now "hello 123456712345" note the missing newline

如何查找文件中的特定行？

How to seek to a specific line in a file?

python

file

seek