Replace string within file contents

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How can I open a file, Stud.txt, and then replace any occurences of “A” with “Orange”?

with open("Stud.txt", "rt") as fin:
    with open("out.txt", "wt") as fout:
        for line in fin:
            fout.write(line.replace('A', 'Orange'))

If you’d like to replace the strings in the same file, you probably have to read its contents into a local variable, close it, and re-open it for writing:

I am using the with statement in this example, which closes the file after the with block is terminated – either normally when the last command finishes executing, or by an exception.

def inplace_change(filename, old_string, new_string):
    # Safely read the input filename using 'with'
    with open(filename) as f:
        s =
        if old_string not in s:
            print('"{old_string}" not found in {filename}.'.format(**locals()))

    # Safely write the changed content, if found in the file
    with open(filename, 'w') as f:
        print('Changing "{old_string}" to "{new_string}" in {filename}'.format(**locals()))
        s = s.replace(old_string, new_string)

It is worth mentioning that if the filenames were different, we could have done this more elegantly with a single with statement.


with open(FileName) as f:'A', 'Orange')

with open(FileName, "w") as f:

Using pathlib (

from pathlib import Path
file = Path('Stud.txt')
file.write_text(file.read_text().replace('A', 'Orange'))

If input and output files were different you would use two different variables for read_text and write_text.

If you wanted a change more complex than a single replacement, you would assign the result of read_text to a variable, process it and save the new content to another variable, and then save the new content with write_text.

If your file was large you would prefer an approach that does not read the whole file in memory, but rather process it line by line as show by Gareth Davidson in another answer (, which of course requires to use two distinct files for input and output.

Something like

file = open('Stud.txt')
contents =
replaced_contents = contents.replace('A', 'Orange')

<do stuff with the result>

with open('Stud.txt','r') as f:
    newlines = []
    for line in f.readlines():
        newlines.append(line.replace('A', 'Orange'))
with open('Stud.txt', 'w') as f:
    for line in newlines:

If you are on linux and just want to replace the word dog with catyou can do:


Hi, i am a dog and dog's are awesome, i love dogs! dog dog dogs!

Linux Command:

sed -i 's/dog/cat/g' test.txt


Hi, i am a cat and cat's are awesome, i love cats! cat cat cats!

Original Post:

easiest way is to do it with regular expressions, assuming that you want to iterate over each line in the file (where ‘A’ would be stored) you do…

import re

input = file('C:\full_path\Stud.txt', 'r')
#when you try and write to a file with write permissions, it clears the file and writes only #what you tell it to the file.  So we have to save the file first.

for eachLine in input:

#now we change entries with 'A' to 'Orange'
for i in range(0, len(old):
    search = re.sub('A', 'Orange', saved_input[i])
    if search is not None:
        saved_input[i] = search
#now we open the file in write mode (clearing it) and writing saved_input back to it
input = file('C:\full_path\Stud.txt', 'w')
for each in saved_input:

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