Python Program to Replace Characters in a File (a→b, z→a, A→B, Z→A)

Python Program for Character Replacement in a File

In this tutorial, we will learn how to create a Python program that:

• Takes input from the user
• Writes the input into a file
• Replaces characters as follows:

• a → b, b → c, ..., z → a
• A → B, B → C, ..., Z → A
• Other characters remain unchanged

The program also prints each character conversion on the screen and updates the same file.

Python Program Code

text = input("Enter content for file: ")

# Write original content to file
with open("input.txt", "w") as file:
    file.write(text)

print("\nCharacter Conversion:\n")

# Read, convert, print and overwrite the same file
with open("input.txt", "r+") as file:
    data = file.read()
    result = ""

    for ch in data:
        new_ch = ch

        if 'a' <= ch <= 'y':
            new_ch = chr(ord(ch) + 1)
        elif ch == 'z':
            new_ch = 'a'
        elif 'A' <= ch <= 'Y':
            new_ch = chr(ord(ch) + 1)
        elif ch == 'Z':
            new_ch = 'A'

        if ch != new_ch:
            print(f"{ch}  ->  {new_ch}")

        result += new_ch

    file.seek(0)
    file.write(result)
    file.truncate()

print("\nFile updated successfully.")

How the Program Works

1. User enters text using input()
2. The text is saved into a file
3. The file is reopened in r+ mode
4. Each character is checked and replaced if required
5. Conversions are printed on the screen
6. The same file is updated with modified content

Sample Output

Input:
aZz Hello

Output on Screen:
a -> b
Z -> A
z -> a
H -> I

Final File Content:
bAa Ifmmp

Conclusion

This program is very useful for understanding Python file handling, ASCII character manipulation, and real-time data conversion. It is especially important for O Level and Python practical examinations.