delete lines from file in python

Question

I have a file formated like this：

#1  0.13297254902 0.324803921569 0.434835294118 ...#many floats in one line
#2  0
#3  0.377305882353 0.595870588235 0.353215686275 ...
#4  1 #0/1 for the second line 
#5  ....

I want to process the file so that all blocks with second line is 0 can be deleted and left the file as

#1  0.377305882353 0.595870588235 0.353215686275 ...
#2  1
#3  0.403529411765 0.341654901961 0.379278431373 ... 
#4  1 #now they are all 1s
#5  .....

I tried the snippet below, but it can only see the 0/1 then remove the line, but I want to delete the line with floats above 0/1, not the line with floats below 0/1.

f = open(filePath, "r")
lines = f.readlines()
f.close()
f = open(filePath, "w")

for line in lines:
    if "1\n" in line:
        f.write(line)

Is there any other way I can choose which line to include and which not? Or maybe there's a way to process the file backwards?

TigerhawkT3 · Accepted Answer · 2015-07-02T00:01:27.603

2

We can use the next() function to get the next element in the file iterable. The shutil module allows us to move the new file, overwriting the original (thanks @JoranBeasley).

import shutil

with open(filePath, 'r') as f, open('new_' + filePath, 'w') as output:
    for line in f:
        n = next(f)
        if n != '0\n':
            output.write(line+n)

shutil.move("new_" + filePath, filePath)

Input:

0.13297254902 0.324803921569 0.434835294118 ...#many floats in one line
0
0.377305882353 0.595870588235 0.353215686275 ...
1 #0/1 for the second line

Output:

0.377305882353 0.595870588235 0.353215686275 ...
1 #0/1 for the second line

edited Jul 02 '15 at 00:01

answered Jul 01 '15 at 23:45

TigerhawkT3

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Yeah, it works! But is it possible to store them into the same file? – Mandary Jul 01 '15 at 23:48
1

`shutil.move("new_filepath.txt", filepath)`? – Joran Beasley Jul 01 '15 at 23:52
Mandary: You can do relatively easy in-place file modification using the [`fileinput`](https://docs.python.org/3/library/fileinput.html#module-fileinput) module. – martineau Jul 01 '15 at 23:57
@martineau ... but doesnt that require using its closure context(ie `with fileinput.open(asdasd)`?) in which case you cannot call next on its iterator? .. so that wouldnt work I dont think(at least not without some sledgehammering) ... – Joran Beasley Jul 02 '15 at 00:01
Thanks, @JoranBeasley - it'll now overwrite the original as desired. – TigerhawkT3 Jul 02 '15 at 00:03
1

@Joran: If you can do `for line in f` where `f` is a `fileinput.FileInput` instance, you should be able to do `next(f)`. – martineau Jul 02 '15 at 00:07
oh your right ... I had just never used it like that :P – Joran Beasley Jul 02 '15 at 16:09

delete lines from file in python

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