Replacing string in a file with a new string

Question

I have a list of strings in a text file. The strings are morning, night, sun, moon. What I am trying to do is to replace one of these strings with another string. For example, I would input morning to remove and replace it with afternoon. I am getting an error saying "builtins.ValueError: list.remove(x): x not in list" when the string clearly is in the list.

def main():
    x = input("Enter a file name: ")
    file = open(x , "r+")
    y = input("Enter the string you want to replace: ")
    z = input("Enter the string you to replace it with: ")
    list = file.readlines()
    list.remove(y)
    list.append(z)
    file.write(list)
    print(file.read())

main()

If there is a better way of achieving the same results doing it another way, let me know. Thanks for the help!

You mean editing the file in place without creating another one ? — Chiheb Nexus
– Chiheb Nexus, Commented Apr 9, 2017 at 2:47
First, please do not ever call your variables list because list() is a built-in function. Second, your strings in list have line breaks '\n' at the end. You should strip them off before attempting the remove. — DYZ
– DYZ, Commented Apr 9, 2017 at 2:48

Raymond Hettinger · Accepted Answer · 2017-04-09 03:04:48Z

5

Here are some ideas:

The str.replace() function is the simplest way to replace strings, s.replace(y, z).
The re.sub() function will let you search for patterns and replace with strings: re.sub(y, z, s).
The fileinput module will allow you to modify in-place.

Here's one way to do it:

import fileinput
import re

with fileinput.input(files=('file1.txt', 'file2.txt'), inplace=True) as f:
    for line in f:
        print( re.sub(y, z, line) )

Here another idea:

Instead of processing line-by-line, just read the whole file in as a single string, fix it up, and then write it back.

For example:

import re

with open(filename) as f:
    s = f.read()
with open(filename, 'w') as f:
    s = re.sub(y, z, s)
    f.write(s)

edited Apr 9, 2017 at 3:04

answered Apr 9, 2017 at 2:50

Raymond Hettinger

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John He · Accepted Answer · 2017-04-11 01:58:29Z

0

Assuming your txt save in src.txt:

morning
night
sun
moon

In windows,you can use this batch script,save in replace.bat:

@echo off
setlocal enabledelayedexpansion
set filename=%1
set oldstr=%2
set newstr=%3

for /f "usebackq" %%i in (%filename%) do (
    set str=%%i
    set replace=!str:%oldstr%=%newstr%!
    echo !replace!
)

Useage:

replace.bat src.txt morning afternoon > newsrc.txt

or grepWin.

Use sed or gawk may be more simple.

sed -i "s/morning/afternoon/g" src.txt

edited Apr 11, 2017 at 1:58

answered Apr 9, 2017 at 3:26

John He

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Comments

Eragon20 · Accepted Answer · 2017-04-09 03:23:09Z

-1

Perhaps you are looking for the Python replace() method?

str = file.readlines()
str = str.replace(y, z) #this will replace substring y with z within the parent String str

edited Apr 9, 2017 at 3:23

answered Apr 9, 2017 at 3:05

Eragon20

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