Python Program to open a file in the read-write mode without truncating the file

In Python, we can open a file in read-write mode without truncating it by using the a+ mode. Truncating a file refers to deleting its existing content before opening. This article demonstrates how to preserve existing data while still being able to read and write to files.

What is a+ Mode?

The a+ mode opens a file for both reading and writing without truncating existing content. New data is appended to the end of the file, preserving all original content. The file pointer starts at the end for writing operations.

Syntax

open('filename', 'a+')

The open() function takes the filename and mode as parameters. The a+ mode indicates read-write access without truncation.

Example 1: Writing Data Using a+ Mode

Here we create a file with initial content, then read it back ?

# Create/write to file in a+ mode
with open("example.txt", "w") as f:
    f.write("Original content.\n")

# Open in a+ mode and append new content
with open("example.txt", "a+") as file:
    file.write("This is a test message.\n")
    file.seek(0)  # Move pointer to start
    content = file.read()
    print(content)

Original content.
This is a test message.

Example 2: Demonstrating No Truncation

When we write additional content, the original data remains intact ?

# Add more content without truncating
with open("example.txt", "a+") as file:
    file.write("Additional line without truncation.\n")
    file.seek(0)
    content = file.read()
    print(content)

Original content.
This is a test message.
Additional line without truncation.

Example 3: Reading and Writing in One Operation

We can read existing content, then append new data in the same file operation ?

with open("example.txt", "a+") as file:
    # Read existing content
    file.seek(0)
    existing_content = file.read()
    print("Existing content:")
    print(existing_content)
    
    # Append new content
    file.write("Final message added.\n")
    
    # Read complete file
    file.seek(0)
    complete_content = file.read()
    print("\nComplete file content:")
    print(complete_content)

Existing content:
Original content.
This is a test message.
Additional line without truncation.

Complete file content:
Original content.
This is a test message.
Additional line without truncation.
Final message added.

Key Points About a+ Mode

Comparison with Other Modes

Conclusion

The a+ mode is perfect for preserving existing file content while adding new data. Use seek(0) to read from the beginning, as the write pointer starts at the end of the file.