如何在python中替换文件中的特定值

Question

Im trying to replace the zero's with a value. So far this is my code, but what do i do next?

g = open("January.txt", "r+")
for i in range(3):
        dat_month = g.readline()

Month: January
Item: Lawn
Total square metres purchased:
0
monthly value = 0

Answer 1

dat_month = dat_month.replace("0", "45678")

To write to a file you do:

with open("Outfile.txt", "wt") as outfile:

And then

outfile.write(dat_month)

Answer 2

You could do that - but that is not the usual approach, and certainly is not the correct approach for text files.

The correct way to do it is to write another file, with the information you want updated in place, and then rename the new file to the old one. That is the only sane way of doing this with text files, since the information size in bytes for the fields is variable.

As for the impresion that you are "writing 200 bytes to the disk" instead of a single byte, changing your value, don't let that fool you: at the Operating system level, all file access has to be done in blocks, which are usually a couple of kilobytes long (in special cases, and tunned filesystems it could be a couple hundred bytes). Anyway, you will never, in a user-space program, much less in a high level language like Python, trigger a diskwrite of less than a few hundred bytes.

Now, for the code:

import os
my_number = <number you want to place in the line you want to rewrite>
with open("January.txt", "r") as in_file, open("newfile.txt", "w") as out_file:

    for line in in_file:
        if line.strip() == "0":
             out_file.write(str(my_number) + "\n")
        else:
             out_file.write(line)

os.unlink("January.txt")
os.rename("newfile.txt", "January.txt")

So - that is the general idea - of course you should not write code with all values hardcoded in that way (ie the values to be checked and written fixed in the program code, as are the filenames).

As for the with statement - it is a special construct of the language wich is very appropriate to oppening files and manipulating then in a block, like in this case - but it is not needed .

Programing apart, the concept you have to keep in mind is this: when you use an application that lets you edit a text file, a spreadsheet, an image, you, as user, may have the impression that after you are done and have saved your work, the updates are comitted to the same file . In the vast, vast majority of use cases, that is not what happens : the application uses internally a pattern like the one I presented above - a completly new file is written to disk and the old one is deleted, or renamed. The few exceptions could be simple database applications, which could replace fixed width fields inside the file itself on updates. Modern day databases certainly do not do that, resorting to appending the most recent, updated information, to the end of the file. PDF files are another kind that were not designed to be replaced entirely on each update, when being created: but also in that case, the updated information is written at the end of the file, even if the update is to take place in a page in the beginning of the rendered document.

Answer 3

Try this:

import fileinput
import itertools
import sys
with fileinput.input('January.txt', inplace=True) as file:
    beginning = tuple(itertools.islice(file, 3))
    sys.stdout.writelines(beginning)
    sys.stdout.write(next(file).replace('0', 'a value'))
    sys.stdout.write(next(file).replace('0', 'a value'))
    sys.stdout.writelines(file)