解析文件，将每行添加到末尾，然后从顶部删除该行

Question

I am trying to move each line down at the bottom of the file; this is how the file look like:

daodaos  12391039
idiejda  94093420
jfijdsf  10903213
....
#completed

So at the end of the parsing, I am planning to get all the entry that are on the top, under the actual string that says # completed.

The problem is that I am not sure how can I do this in one pass; I know that I can read the whole file, every single line, close the file and then re-open the file in write mode; searching for that line, removing it from the file and adding it to the end; but it feels incredibly inefficient.

Is there a way in one pass, to process the current line; then in the same for loop, delete the line and append it at the end of the file?

file = open('myfile.txt', 'a')
for items in file:
    #process items line
    #append items line to the end of the file
    #remove items line from the file

Answer 1

Not really.

Your main problem here is that you're iterating on the file at the same time you want to change it. This will Do Bad Things (tm) to your processing, unless you plan to micro-manage the file position pointer.

You do have that power: the seek method lets you move to a given file location, expressed in bytes. seek(0) moves to the start of the file; seek(-1) to the end. The problem you face is that your for loop trusts that this pointer indicates the next line to read.

One distinct problem is that you can't just remove a line from the middle of the file; something exists in those bytes. Think of it as lines of text on a page, written in pencil. You can erase line 4, but this does not cause lines 5-end to magically float up half a centimeter; they're still in the same physical location.

How to Do It ... sort of

Read all of the lines into a list. You can easily change a list the way you want. When you hit the end, then write the list back to the file -- or use your magic seek and append powers to alter only a little of it.

Answer 2

我建议您以简单的方式进行操作：读取所有文件并将其存储在变量中，将完成的文件移至另一个变量，然后重写文件。

Answer 3

suggest to keep it simple read and writeback

with open('myfile.txt') as f:
    lines = f.readlines()
with open('myfile.txt', 'w') as f:
    newlines = []
    for line in lines:
        # do you stuff, check if completed, rearrange the list
        if line.startswith('#completed'):
            idx=i
            newlines = lines[idx:] + lines[:idx]
            break
    f.write(''.join(newlines)) # write back new lines

below is another version i could think of if insist wanna modify while reading

with open('myfile.txt', 'r+') as f:
    newlines = ''
    line = True
    while line:
        line = f.readline()
        if line.startswith('#completed'):
            # line += f.read() # uncomment this line if you interest on line after #completed
            f.truncate()
            f.seek(0)
            f.write(line + newlines)
            break
        else:
          newlines += line