Python: Remove Lines Containing Digits from a File (Solved)

Practice Question 30

In this question, we will write a function that acts as a “Text Purifier.” It reads details.txt and removes every line that contains even a single numeric digit (0-9), saving the “clean” text-only lines into final.txt.

The Logic (How it works)

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

We need to verify if a line is “safe” to keep. Here is the step-by-step logic:

• Open Files: Open the source file for reading and the destination file for writing.

• Line-by-Line Scan: Use readlines() to look at each line individually.

• The “Flag” Logic: For each line, we assume it is “clean” (found_digit = False).

• Nested Loop: We look at every character in the line. If a character is a digit, we flip our flag to True and stop checking that line.

• Conditional Writing: If, after checking all characters, the flag is still False, it means the line is safe to write to final.txt.

Let’s Code as per the above logic :

def removeNumericLines():
    f1 = open("details.txt", "r")
    f2 = open("final.txt", "w")
    
    linelist = f1.readlines()
    
    for line in linelist:
        found_digit = False
        
        # Check every character in the current line
        for alpha in line:
            if alpha.isdigit():
                found_digit = True
                break  # No need to check other characters in this line
        
        # If no digit was found, write the line to the new file
        if found_digit == False:
            f2.write(line)
            
    f1.close()
    f2.close()
    print("Cleanup complete! Numeric lines removed and saved to final.txt")

# To call the function:
removeNumericLines()

Important Notes for Students

As a teacher, I want you to focus on these three key points so you don’t lose marks:

• Flag Variable: The found_digit variable is crucial. It helps the program “remember” if it saw a number anywhere in the line while it was looping through the characters.

• break for Efficiency: As soon as we find one digit (like in “Room 404”), we know the line has to go. The break saves time by skipping the rest of the characters in that line.

• Preserving Formatting: Unlike .split(), using readlines() and write(line) preserves the original spacing and newlines of the “clean” lines.

Teacher’s Secret Tip for Exams:

This logic is the foundation for all “Filtering” tasks. If the examiner asks to remove lines starting with a specific character, or lines shorter than 5 characters, you just change the if condition. The “Read -> Filter -> Write” structure remains the same!

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