How to modify a line in a file using Python

Question

I am trying to do what for many will be a very straight forward thing but for me is just infuriatingly difficult. I am trying search for a line in a file that contains certain words or phrases and modify that line...that's it. I have been through the forum and suggested similar questions and have found many hints but none do just quite what I want or are beyond my current ability to grasp.

This is the test file:

# 1st_word  2nd_word

# 3rd_word  4th_word

And this is my script so far:

############################################################
file = 'C:\lpthw\\text'
f1 = open(file, "r+")
f2 = open(file, "r+")
############################################################

def wrline():
    lines = f1.readlines()
    for line in lines:
        if "1st_word" in line and "2nd_word" in line:
            #f2.write(line.replace('#\t', '\t'))
            f2.write((line.replace('#\t', '\t')).rstrip())
    f1.seek(0)

wrline()

My problem is that the below inserts a \n after the line every time and adds a blank line to the file.

f2.write(line.replace('#\t', '\t'))

The file becomes:

    1st_word    2nd_word


#   3rd_word    4th_word

An extra blank line between the lines of text.

If I use the following:

f2.write((line.replace('#\t', '\t')).rstrip())

I get this:

    1st_word    2nd_wordd

#   3rd_word    4th_word

No new blank line inserted but and extra "d" at the end instead.

What am I doing wrong?

Thanks

User 12692182 · Accepted Answer · 2020-05-01 14:15:46Z

1

Your blank line is coming from the original blank line in the file. Writing a line with nothing in it writes a newline to the file. Instead of not putting anything into the written line, you have to completely skip the iteration, so it does not write that newline. Here's what I suggest:

def wrline():
    lines = open('file.txt', 'r').readlines()
    f2 = open('file.txt', 'w')
    for line in lines:
        if '1st_word' in line and '2nd_word' in line:
            f2.write((line.replace('# ', ' ')).rstrip('\n'))
        else:
            if line != '\n':
                f2.write(line)
    f2.close()

answered May 1, 2020 at 14:15

User 12692182

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2 Comments

Gerard2012 Over a year ago

Thanks for the comment. But tried as you suggested and out put was this: 1st_word 2nd_word# 3rd_word 4th_word_word

User 12692182 Over a year ago

Then remove the .rstrip(), so that there is a newline between then

alec_djinn · Accepted Answer · 2020-05-01 16:14:41Z

1

I would keep read and write operations separate.

#read
with open(file, 'r') as f:
    lines = f.readlines()

#parse, change and write back
with open(file, 'w') as f:
    for line in lines:
        if line.startswith('#\t'):
            line = line[1:]
    f.write(line)

edited May 1, 2020 at 16:14

answered May 1, 2020 at 14:14

alec_djinn

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Comments

johnashu · Accepted Answer · 2020-05-01 14:29:22Z

0

You have not closed the files and there is no need for the \t

Also get rid of the rstrip()

Read in the file, replace the data and write it back.. open and close each time.

fn = 'example.txt'
new_data = []

# Read in the file
with open(fn, 'r+') as file:
    filedata = file.readlines()

# Replace the target string
for line in filedata:
    if "1st_word" in line and "2nd_word" in line:
        line = line.replace('#', '')
    new_data.append(line)

# Write the file out again
with open(fn, 'w+') as file:
    for line in new_data:
        file.write(line)

edited May 1, 2020 at 14:29

answered May 1, 2020 at 14:06

johnashu

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