Python: Merge Two Text Files Line by Line (Solved)

Practice Question 28

In this question, we will learn how to write a function that takes the content of two separate files and joins them into a third file. We will use the Parallel List Processing method, which is the most reliable way for students to handle this.

The Logic (How it works)

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

To merge files line by line, we treat each file as a list of lines:

• Open the Sources: Open file1.txt and file2.txt in read mode.

• Open the Destination: Open combined.txt in write mode.

• The “Transfer” Loop: 1. Read all lines from file1 and write them to combined. 2. Read all lines from file2 and write them to combined.

• Alternative “Interleave” logic: If you want Line 1 from File A then Line 1 from File B, you would use a single loop with an index. (The code below follows the simpler, standard “Concatenate” merge usually asked in exams).

Let’s Code as per the above logic :

def mergeTwoFiles():
    # Step 1: Open the destination file
    f_out = open("combined.txt", "w")
    
    # Step 2: Read and write content from the first file
    f1 = open("file1.txt", "r")
    f_out.write(f1.read())
    # Adding a newline to ensure the second file starts on a new line
    f_out.write("\n") 
    f1.close()
    
    # Step 3: Read and write content from the second file
    f2 = open("file2.txt", "r")
    f_out.write(f2.read())
    f2.close()
    
    f_out.close()
    print("Files merged successfully into combined.txt!")

# To call the function:
mergeTwoFiles()

Important Notes for Students

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

• Handling Newlines: I added f_out.write("\n") between the two files. Without this, if the first file doesn’t end with a newline, the first line of the second file will be joined to the end of the last line of the first file.

• Mode Matters: Notice that combined.txt is opened in "w" mode. This ensures that every time you run the function, it creates a fresh merged file. If you wanted to keep adding to it, you would use "a" (append) mode.

• Large Files: For very large files, we wouldn’t use .read() as it might consume too much RAM. We would use a for line in f1: loop to transfer data piece by piece.

Teacher’s Secret Tip for Exams:

If the examiner asks to merge them alternatively (Line 1 of A, then Line 1 of B, then Line 2 of A…), you would use readlines() for both and a loop like for i in range(len(list1)):.

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