Python: Transfer and Delete specific lines from a Text File (Solved)

Practice Question 14

In this question, we will learn how to perform a “Cut and Paste” operation on specific file lines. We will move lines containing the word “error” to a log file and ensure they are removed from the original source.

The Logic (How it works)

Before writing the code, let us first understand the logic behind the solution.

Since we cannot easily delete a line inside a text file, we use a three-file strategy:

• The Source (f1): Read the original server.txt.

• The Log (f2): A file to store the “error” lines.

• The Temporary File (f3): A file to store everything that is NOT an error (the lines we want to keep).

• The Swap: After processing, we delete the original server.txt and rename our temporary file to server.txt. This makes it look like the lines were deleted!

Let’s Code as per the above logic :

import os

def transferErrorLines():
    f1 = open("server.txt", "r")
    f2 = open("error_log.txt", "w")
    f3 = open("temp.txt", "w")
    
    linelist = f1.readlines()
    
    for line in linelist:
        # Check if the word "error" exists in the line
        if "error" in line.lower(): 
            f2.write(line)
        else:
            # Keep the remaining lines in a temporary file
            f3.write(line)
            
    f1.close()
    f2.close()
    f3.close()
    
    # Cleaning up and updating the original file
    os.remove("server.txt")
    os.rename("temp.txt", "server.txt")

# To call the function:
transferErrorLines()

Important Notes for Students

• The os Module: You must import os at the top of your code. Without it, the remove() and rename() functions will not work.

• Order of Closing: Always close your files (f1.close(), etc.) before you try to remove or rename them. If a file is still “open” in Python, the operating system will block you from renaming it.

• Case Sensitivity: Using "error" in line.lower() is a “Pro Tip.” It ensures that “Error,” “ERROR,” and “error” are all caught and transferred.

Teacher’s Secret Tip for Exams:

This “Temporary File” logic is the standard way to Update or Delete records in Data File Handling (Text or Binary). Learn this pattern well; it applies to almost every “Record Modification” question!

Want to practice more? If you found this helpful, I have compiled a full list of 30 practice problems just like this one. Each question focuses on a different logic to help you become a pro at File Handling.

Click here to view the full 30-Question Practice Set

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