Python: Inplace Editing using FileInput

Last Updated : 19 Feb, 2020

Python3's fileinput provides many useful features that can be used to do many things without lots of code. It comes handy in many places but in this article, we'll use the fileinput to do in-place editing in a text file. Basically we'll be changing the text in a text file without creating any other file or overheads. Syntax:

FileInput(filename, inplace=True, backup='.bak')

Note: The backup is extension for the backup file created before editing.

Example 1:Changing only the first line of file

Text file: fileinput-python-1

Python3 1==

# Python code to change only first line of file
import fileinput

filename = "GFG.txt"

with fileinput.FileInput(filename, 
                         inplace = True, backup ='.bak') as f:

    for line in f:
        if f.isfirstline():
            print("changing only first line", end ='\n')
        else:
            print(line, end ='')

Output: fileinput-python-2

Example 2:Search and replace line with other line in file Text file: fileinput-python-3

Python3 1==

# python3 code to search and 
# replace line with other line in file
import fileinput

filename = "GFG.txt"

with fileinput.FileInput(filename,
                         inplace = True, backup ='.bak') as f:
    
    for line in f:
        if "search this line and change it\n" == line:
            print("changing the matched line with this line",
                  end ='\n')
        else:
            print(line, end ='')

Output: fileinput-python-4

Example 3:Search text inline and replace that line with another line in the file. Text file: fileinput-python

Python3 1==

# python3 code to search text in 
# line and replace that line with 
# other line in file
import fileinput

filename = "GFG.txt"

with fileinput.FileInput(filename,
                         inplace = True, backup ='.bak') as f:
    for line in f:
        if "searchtext" in line:
            print("changing this line with line that contains searched text",
                  end ='\n')
        else:
            print(line, end ='')

Output: fileinput-python-6

Example 4:Search text and replace that text in file. Text file: fileinput-python1

Python3 1==

# python code to search
# text and replace that text
# in file

import fileinput

filename = "GFG.txt"

with fileinput.FileInput(filename, 
                         inplace = True, backup ='.bak') as f:
    
    for line in f:
        if "replace text" in line:
            print(line.replace("replace text",
                               "changed text"), end ='')
        else:
            print(line, end ='')