How to delete parts of a file in python?

Question

I have a file named a.txt which looks like this:

I'm the first line
I'm the second line.
There may be more lines here.

I'm below an empty line.
I'm a line.
More lines here.

Now, I want to remove the contents above the empty line(including the empty line itself). How could I do this in a Pythonic way?

Answer 1

You could do a little something like this:

with open('a.txt', 'r') as file:
    lines = file.readlines()

blank_line = lines.index('\n')
lines = lines[blank_line+1:] #\n is the index of the blank line

with open('a.txt', 'w') as file:
    file.write('\n'.join(lines))

and that makes the job much simpler.

Answer 2

The fileinput module (from the standard library) is convenient for this kind of thing. It sets things up so you can act as though your are editing the file "in-place":

import fileinput
import sys

fileobj=iter(fileinput.input(['a.txt'], inplace=True))
# iterate through the file until you find an empty line.
for line in fileobj:
    if not line.strip():
        break
# Iterators (like `fileobj`) pick up where they left off. 
# Starting a new for-loop saves you one `if` statement and boolean variable.
for line in fileobj:
    sys.stdout.write(line)

Answer 3

Any idea how big the file is going to be?

You could read the file into memory:

f = open('your_file', 'r')
lines = f.readlines()

which will read the file line by line and store those lines in a list (lines).

Then, close the file and reopen with 'w':

f.close()
f = open('your_file', 'w')
for line in lines:
    if your_if_here:
        f.write(line)

This will overwrite the current file. Then you can pick and choose which lines from the list you want to write back in. Probably not a very good idea if the file gets to large though, since the entire file has to reside in memory. But, it doesn't require that you create a second file to dump your output.

Answer 4

from itertools import dropwhile, islice

def content_after_emptyline(file_object):
    return islice(dropwhile(lambda line: line.strip(), file_object), 1, None)

with open("filename") as f:
    for line in content_after_emptyline(f):
        print line,

Answer 5

Basically you can't delete stuff from the beginning of a file, so you will have to write to a new file.

I think the pythonic way looks like this:

# get a iterator over the lines in the file:
with open("input.txt", 'rt') as lines:
    # while the line is not empty drop it
    for line in lines:
        if not line.strip():
            break

    # now lines is at the point after the first paragraph
    # so write out everything from here
    with open("output.txt", 'wt') as out:
        out.writelines(lines)

Here are some simpler versions of this, without with for older Python versions:

lines = open("input.txt", 'rt')
for line in lines:
    if not line.strip():
        break
open("output.txt", 'wt').writelines(lines)

and a very straight forward version that simply splits the file at the empty line:

# first, read everything from the old file
text = open("input.txt", 'rt').read()

# split it at the first empty line ("\n\n")
first, rest = text.split('\n\n',1)

# make a new file and write the rest
open("output.txt", 'wt').write(rest)

Note that this can be pretty fragile, for example windows often uses \r\n as a single linebreak, so a empty line would be \r\n\r\n instead. But often you know the format of the file uses one kind of linebreaks only, so this could be fine.

Answer 6

Naive approach by iterating over the lines in the file one by one top to bottom:

#!/usr/bin/env python

with open("4692065.txt", 'r') as src, open("4692065.cut.txt", "w") as dest:
    keep = False
    for line in src:
        if keep: dest.write(line)
        if line.strip() == '': keep = True