如何使用 fileinput 替换文件中一行的一部分？

Question

import fileinput

if number_of_bags.is_integer():
    with fileinput.FileInput('CoinCount.txt',inplace=True) as fileobj:
        for line in fileobj:
            x = line.split(',')
            for w, word in enumerate(x):
                if w == 3 and word == 'N':
                    print(line.replace('N', 'Y'), end='')
                    i = i + 1
                else:
                    i = i + 1

else:
    with fileinput.FileInput('CoinCount.txt',inplace=True) as fileobj:
        for line in fileobj:
            x = line.split(',')
            for w, word in enumerate(x):
                if w == 3 and word == 'Y':
                    print(line.replace('Y', 'N'), end='')
                    i = i + 1
                else:
                    i = i + 1

When I run this program, the text file is completely overwritten and all of its contents removed, each line is structured like this:

Athena,£2,133.55,Y

I want this program to be able to go through each line (hence why the end shows i = i + 1 ) and replace the Y or N if it isn't supposed to be there (if the calculation is incorrect).

How do I edit this code so that it doesn't remove the txt files data?

Answer 1

There are several mistakes with how fileinput.FileInput is being used and the processing related to it is being done.

Something must be printed for each line of input if it is to be in the output. However, after the x = line.split(',') the fourth word will contain either a 'Y\n' or a 'N\n' , so the following if w == 3 and word == 'N': or if w == 3 and word == 'Y': conditional be never be True . This means nothing is ever printed. This is why the result is an empty file.

There's more than one way to fix things. In the code below, the newline at the end of each line is removed with rstrip() before splitting it up into separate words. Doing that will make the testing for 'Y' or 'N' work so they can be replaced. Then, regardless of whether the word was changed for not, the words are joined back together and a (possibly updated) line is print ed.

I also simplified the things so to make the whole thing shorter by not repeating so much of the code for the two possibilities.

import fileinput


if number_of_bags.is_integer():
    target, replacement = 'N', 'Y'
else:
    target, replacement = 'Y', 'N'

line_to_process = 4

with fileinput.FileInput('CoinCount.txt', inplace=True) as fileobj:
    for line in fileobj:
        words = line.rstrip().split(',')
        if fileobj.lineno() == line_to_process:  # Only process the Nth line.
            if words[3] == target:
                words[3] = replacement
        print(','.join(words))

    numlines = fileobj.lineno()  # Number of lines processed.

print(f'Done, {numlines} lines processed')