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In our previous articles, we have explained how to perform read, write and append operation on a file on disk. In this tutorial, we are going to explain how to modify the content of a file on disk.

There are three ways to modify the content of a file in Python.

• By opening a file in w+ mode.
• By opening a file in r+ mode
• By using fileinput and sys module.

But before we modify the content of an existing file present on the disk, we must open it. Let us read about the open() method and how we can use it.

• Calling the open() function to open a file to write


In order to open a file to perform the read operation, we could call the open() function of io module, which is a default module for performing file operations. Using the open() function, we could provide location of the file stored on the disk and in return(if the file is found), the open() function opens the file in a mode of your choice, to let you perform a specific file operation.

Let us take a look at a general syntax of open() function to open a file.



open('filename', mode);



• filename(a string value), is the name of file on which we are going to open and perform the file operation.
• mode(a string value), is a single or multiple file modes in which we are going to open a file.

Let's read about the required file mode value, to write the content of a file.






• Writing a file


Let's look at the mode required to write a file.



File Mode	Description
r+	Searches for the file and opens it in read plus update mode.(if the file is found).
w+	Searches for the file and opens it in write mode. If the file is found, its content is overwritten. If file is not found, a new file is created. Allows you to first write to the file and then read it and reading a file before writing to it, will return an empty string.






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• Using w+ mode to modify the file


Let's say that we already have an existing file on disk, named File1.txt, with content :

File1.txt

This is a beautiful day!


Using the w+ mode will erase the content of an existing file and if the file is not file, it creates a new file and writes data to it and then it will let you to read the file(something which w mode won't allow you to do).



# Python - Modifying a file
 
import os

# Calling the open() function to open a file named File3.txt in write mode
file = open('File1.txt', 'w+')


# Creating a string and appending the newline character \n at their end
str1 = 'Hello World. Keep Smiling!'



# Writing a string to the file using the write() method of file object
file.write(str1)


# Calling the seek method to set the cursor position to the zeroth bytes from the beginning of the file
# os.SEEK_SET = sets the cursor  to the beginning of the file
# os.SEEK_CUR = sets the cursor  to the current position in the file
# os.SEEK_END = sets the cursor  to the end position in the file
file.seek(0, os.SEEK_SET)


# Calling the read() method for file objects, to read all its content.
print(file.read())


# Closing the file after completing the write operation on it
file.close()




Output


Executing this program will modify(rather erases) the content of a file and now its content looks like like -



Hello World. Keep Smiling!








• Using r+ mode to modify the file


Let's say that we already have an existing file on disk, named File2.txt, with content :

File2.txt

Live and let live!


Using the a+ mode will let you not just read the content of the file but also modify the content of a file. Let us see how.



# Python - Appending lines to a file.
 
import os

# Calling the open() function to open a file named File3.txt in write mode
file = open('File2.txt', 'r+')


# Calling the read() method for file objects, to read all its content.
print('Reading the old content of the file :')
print(file.read())


# Creating a string and to write to the file
str1 = 'Freedom!'


# Calling the seek method to set the cursor position to 4 bytes forward from the beginning position of the file.
# os.SEEK_SET = sets the cursor  to the beginning of the file
# os.SEEK_CUR = sets the cursor  to the current position in the file
# os.SEEK_END = sets the cursor  to the end position in the file
file.seek(4, os.SEEK_SET)



# Writing a string to a file  file using the write() method of file object
file.write(str1)



# Calling the seek method to set the cursor position to zero bytes from the current position.
# Because we are going to read the file again, to read its updated content
file.seek(0, os.SEEK_SET)


# Calling the read() method for file objects, to read its updated content.
print('Reading the old content of the file :')
print(file.read())


# Closing the file after completing the write operation on it
file.close()




Output

Reading the old content of the file :
Live and let live!
Reading the old content of the file :
LiveFreedom! live!



Executing this program will modify the content of a file and now its content looks like like -



LiveFreedom! live!


But the biggest drawback of using the r+ and w+ mode is that we will really be able to insert the new content somewhere in the middle of an existing content, in such a way that it pushes the down the existing content. For this reason, we should use fileinput module with sys module.






• Using fileinput module to modify the file


Let's say that we already have an existing file on disk, named File3.txt, with content :

File3.txt

GendXr : Female
Age    : 23
City   : Istanbul
Weight : 56.6 Kg


Using the fileinput and sys mode will let you not just read the content of the file but also truly modify the content of a file by allowing you to not just overwriting its old content but by also inserting the new content somewhere in the middle of an existing content, in such a way that it pushes the down the existing content. Let us see how it is done.



# Python - Modifying a file using fileinput and sys module


import sys
import fileinput


# This for loop scans and searches each line in the file
# By using the input() method of fileinput module
for line in fileinput.input("File3.txt", inplace=True):
    
    # This will replace string "a" with "truck" in each line
    line = line.replace("e", "X")
    
    # write() method of sys module redirects the .stdout is redirected to the file
    sys.stdout.write(line)




Output

Reading the old content of the file :
Live and let live!
Reading the old content of the file :
LiveFreedom! live!



Executing this program will modify the content of a file named File3.txt in the current directory, by replacing the character 'e' in it with character 'X' and the updated content of the file looks like -



GXndXr : FXmalX
AgX    : 23
City   : Istanbul
WXight : 56.6 Kg








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