Replacing Strings in Files with Python

String manipulation is a common task in programming, and Python provides powerful tools to handle it efficiently. In this article, we will explore various methods to replace strings in any text file using the Python programming language. We will cover different scenarios and provide multiple examples to demonstrate each approach. By the end, you will clearly understand how to replace strings in files programmatically, whether you need to update a single file or process multiple files simultaneously. You can even replace special characters in files with the new string using these techniques. Let’s dive in and discover the techniques to accomplish this task effortlessly.

I. Reading and Writing Files in Python:

Before we delve into string replacement, it’s essential to understand how to read and write files in Python programming language. These fundamental operations will lay the foundation for manipulating strings within files using any Python program.

Python provides the built-in open() function to access files. Here’s an example that opens an input file in read mode and prints its contents:

# open the input file as a read only file.

with open('filename.txt', 'r') as file:

    contents =


To write data to a new file, we can open it in write mode using the ‘w’ parameter. The following example demonstrates how to replace the entire contents of a file with new data:

# old file contents are overwritten by the new string. This works even if you have a huge file of data.

with open('filename.txt', 'w') as file:

    file.write("New content goes here!") #the target file will get the replacement string.

Now that we have the basics covered, let’s explore different methods to replace specific strings within files. This is by far the best way or best method to replace any text with a new string, and create a new file for the output. Enough theory. Let’s look at a practical Python Code example. Here’s the sample code, in Python.

II. String Replacement Techniques for regular expression pattern:

Using the replace() Method:

The replace() method is a convenient way to replace occurrences of a specific string within another string. By combining it with file read and write operations, we can replace targeted strings in a file. Here’s an example in the following script in Python code:

with open('filename.txt', 'r') as file:
    contents =
    modified_contents = contents.replace('old_string', 'new_string')

    with open('filename.txt', 'w') as file:


Now you can open the original file, and see that all occurrences of the substring have been replaced with the specified string. The old string should be nowhere to be seen. With a small script in Python, we can do wonders.

Utilizing Regular Expression Pattern Matching Technique:

Python’s re module enables powerful pattern matching using regular expressions. We can leverage regular expressions to perform advanced string replacements. Consider the following example, where we replace multiple occurrences of a word with another replacement string. Create a new Python file. And write the following Python tutorial code.

import re

with open('filename.txt', 'r') as file:

    contents =

    modified_contents = re.sub(r'\bword\b', 'replacement', contents)

    with open('filename.txt', 'w') as file:


With this method, you can keep calling it to replace multiple text strings in the original file. Make sure to create a backup file before starting the string substitutions. The entire file is scanned, and simple replacements are done in no time. These Python coding techniques are best for complete beginner or even experienced users of Python script. It may be best to create a Python function to do this if you have to replace multiple occurrences of substrings in a list of files.

Replacing Strings in Multiple Files:

Often, we need to replace strings in multiple files simultaneously. We can employ a loop that iterates over a list of filenames to accomplish this. Here’s an example that replaces a string in multiple files:

import glob

files = glob.glob('path/to/files/*.txt')

target_string = 'old_string'

replacement_string = 'new_string'

for file in files:

with open(file, 'r') as f:

contents =

modified_contents = contents.replace(target_string, replacement_string)

with open(file, 'w') as f:

f.write(modified_contents) # creation of output file

Modifying Files in Place:

If the file size is small, we can read the entire contents into memory, perform the string replacement, and write it back to the same file. However, for larger files, it’s more efficient to read and write line by line. Here’s an example that replaces strings in a file line by line:

input_file = 'filename.txt'
output_file = 'filename_modified.txt'

target_string = 'old_string'
replacement_string = 'new_string'

with open(input_file, 'r') as file_in, open(output_file, 'w') as file_out:
for line in file_in:
    modified_line = line.replace(target_string, replacement_string)


Python programming language provides several techniques to replace strings in files efficiently. Whether you need to replace a single occurrence or process multiple files, the methods presented in this article will equip you with the necessary knowledge to achieve your goal. Simply put in the search string, and then replace it with the replacement string.

Remember to handle file operations carefully, considering factors such as file size, memory usage, and performance requirements. With the flexibility and power of Python, you can confidently manipulate strings in files and streamline your data processing tasks. Happy coding!

Vinai Prakash