optimization - Fastest Way to Delete a Line from Large File in Python

Question

I am working with a very large (~11GB) text file on a Linux system. I am running it through a program which is checking the file for errors. Once an error is found, I need to either fix the line or remove the line entirely. And then repeat...

Eventually once I'm comfortable with the process, I'll automate it entirely. For now however, let's assume I'm running this by hand.

What would be the fastest (in terms of execution time) way to remove a specific line from this large file? I thought of doing it in Python...but would be open to other examples. The line might be anywhere in the file.

If Python, assume the following interface:

def removeLine(filename, lineno):

Thanks,

-aj

Answer 1

You can have two file objects for the same file at the same time (one for reading, one for writing):

def removeLine(filename, lineno):
    fro = open(filename, "rb")

    current_line = 0
    while current_line < lineno:
        fro.readline()
        current_line += 1

    seekpoint = fro.tell()
    frw = open(filename, "r+b")
    frw.seek(seekpoint, 0)

    # read the line we want to discard
    fro.readline()

    # now move the rest of the lines in the file 
    # one line back 
    chars = fro.readline()
    while chars:
        frw.writelines(chars)
        chars = fro.readline()

    fro.close()
    frw.truncate()
    frw.close()