Python在读取文件后删除文件中的行 [英] Python delete row in file after reading it

问题描述

我使用 python 2.7 我正在 while 循环中从文件中读取数据.当我成功读取行时，我想从文件中删除这一行，但我不知道该怎么做 - 有效的方法，所以我不会浪费太多 CPU.

I python 2.7 I am reading data from file in while loop. When I successfully read row, I would like to delete this row from a file, but I dont know how to do it - Efficient way so i dont waste to much of CPU.

    read = open("data.csv", 'r')
    for row in read:
        #code......
        if send == True:
            -->delete sent line from file, and continue to loop

推荐答案

在进行磁盘 IO 时，您不应该担心 CPU 使用率 -- 与几乎所有内存/CPU 操作相比，磁盘 IO 非常慢.

You shouldn't concern yourself about cpu usage when doing disk IO -- disk IO is very slow compared to almost any in-memory/cpu operation.

从文件中间删除有两种策略:

There are two strategies to deleting from the middle of a file:

1. 将所有要保留的行写入一个辅助文件，然后将辅助文件重命名为原始文件名.

1. writing all lines to keep to a secondary file, then renaming the secondary file to the original file name.

将文件的其余部分(尾部)复制到要删除的行的开头，然后从文件末尾截断 x 个字节(其中 x 等于要删除的行的长度.

copy the rest (tail) of the file to the beginning of the line you want to delete and then truncating x bytes off the end of the file (where x is equal to the length of the line you want to remove.

通常首选数字 1，因为它更容易并且不需要任何锁定.

Number 1 is usually preferred since it is easier and doesn't require any locking.

Mayank Porwal 为您提供了大部分策略 #1.以下是您将如何实施策略 #2:

Mayank Porwal has given you most of strategy #1. Here is how you would implement strategy #2:

# open the file for both reading and writing in binary mode ('rb+')
with open('rmline.txt', 'rb+') as fp:   
    while 1:
        pos = fp.tell()       # remember the starting position of the next line to read
        line = fp.readline()
        if not line:
            break  # we reached the end of the file

        if should_line_be_skipped(line):  # only you know what to skip :-)
            rest = fp.read()  # read the rest of the file
            fp.seek(pos)      # go to the start position of the line to remove
            fp.write(rest)    # write the rest of the file over the line to be removed
            fp.truncate()     # truncates at current position (end of file - len(line))
            fp.seek(pos)      # return to where the next line is after deletion so we can continue the while loop

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