如何在不使用fileinput的情况下更改python中的一行？

Question

I want to replace a character on every line when the line match with my lookup_keyword. I managed to came up with below solutions. But the finaljoin only write the changes on the new file (only affected line is being written back)

How can I write unaffected line together in a new file?

I know that we can use fileinput inplace editing but that more suitable for keyword replacing.

Example input file,
123456789 [Thanos]
123456789 [Thanos]
123456789 [Thor]
123456789 [Loki]

Example output file,
1AA345KK789 [Thanos]
1AA345KK789 [Thanos]



Expected output file that I actually wanted.
1AA345KK789 [Thanos]
1AA345KK789 [Thanos]
123456789 [Thor]
123456789 [Loki]




lookup_keyword= "Thanos"

with open(filename) as myFile:
    # myFile,1 means start reading the line at line number one.
    # By default python will read start from 0.
    for num, line in enumerate(myFile, 1):
        if lookup_keyword in line:
            #print ('Found at line:', num)
            s = list(line)
            s[1] = 'AA'
            s[5] = 'KK'

            # join the list to a original string form
            finaljoin = "".join(s)

            #print (finaljoin)
            with open(new_filename, 'a+') as f:
                f.write(finaljoin)

Answer 1

The output file will only contain lines that you actually write to it.

If Thanos is in the line, you modify the line, open the output file, and append the changed line to it.

But if not, you do nothing, so nothing gets written.

You could fix it like this:

for num, line in enumerate(myFile, 1):
    #print ('Found at line:', num)
    s = list(line)
    if lookup_keyword in line:
        s[1] = 'AA'
        s[5] = 'KK'
    # join the list to a original string form
    finaljoin = "".join(s)

    #print (finaljoin)
    with open(new_filename, 'a+') as f:
        f.write(finaljoin)

That way, you're always creating a new line and appending it to the new file, whether you modified it or not.

Alternatively:

for num, line in enumerate(myFile, 1):
    if lookup_keyword in line:
        #print ('Found at line:', num)
        s = list(line)
        s[1] = 'AA'
        s[5] = 'KK'

        # join the list to a original string form
        finaljoin = "".join(s)
    else:
        finaljoin = line
    with open(new_filename, 'a+') as f:
        f.write(finaljoin)

That way you're not wasting effort splitting and rejoining the line when you don't want to modify it, and it's not that much more complicated.

Answer 2

input_file.txt

Example input file,
123456789 [Thanos]
123456789 [Thanos]
123456789 [Thor]
123456789 [Loki]    

Script.py

input_file_name = 'input_file.txt'
output_file_name = 'output_file.txt'

lookup_keyword= "Thanos"

with open(input_file_name) as input_file:
    next(input_file)        # skip first line
    with open(output_file_name, 'w') as output_file:
        for input_line in input_file:
            if lookup_keyword in input_line:
                input_list = list(input_line)
                input_list[1] = 'AA'
                input_list[5] = 'KK'

                output_line = ''.join(input_list)
            else:
                output_line = input_line

            output_file.write(output_line)