Python Program to Merge Two Files

You can merge two files in Python by opening both files in read mode, reading their contents, and then writing them into a new file using with open('file1.txt') as f1, open('file2.txt') as f2, open('merged.txt', 'w') as out: and writing out.write(f1.read() + f2.read()).

Examples

Inputfile1.txt content: 'Hello\n' file2.txt content: 'World\n'

Outputmerged.txt content: 'Hello\nWorld\n'

Inputfile1.txt content: '123\n' file2.txt content: '456\n789\n'

Outputmerged.txt content: '123\n456\n789\n'

Inputfile1.txt content: '' file2.txt content: 'Only second file content\n'

Outputmerged.txt content: 'Only second file content\n'

How to Think About It

To merge two files, first open both files to read their contents. Then open a new file to write. Read all text from the first file, then read all text from the second file, and write both texts one after the other into the new file. This way, the new file contains all content from both files combined.

Algorithm

1

Open the first file in read mode.

2

Open the second file in read mode.

3

Open a new file in write mode to store merged content.

4

Read the entire content of the first file.

5

Read the entire content of the second file.

6

Write the content of the first file to the new file.

7

Write the content of the second file to the new file.

8

Close all files.

Code

python

with open('file1.txt', 'r') as f1, open('file2.txt', 'r') as f2, open('merged.txt', 'w') as out:
    out.write(f1.read())
    out.write(f2.read())

print('Files merged successfully into merged.txt')

Output

Files merged successfully into merged.txt

Dry Run

Let's trace merging file1.txt containing 'Hello\n' and file2.txt containing 'World\n'.

1

Open files

file1.txt opened for reading, file2.txt opened for reading, merged.txt opened for writing

2

Read file1.txt

Read content: 'Hello\n'

3

Read file2.txt

Read content: 'World\n'

4

Write to merged.txt

Write 'Hello\n' then 'World\n' into merged.txt

5

Close files

All files closed

Step	Action	Content Written to merged.txt
1	Open files
2	Read file1.txt
3	Read file2.txt
4	Write file1.txt content	Hello
5	Write file2.txt content	Hello World

Why This Works

Step 1: Open files safely

Using with open() ensures files are opened and closed properly without extra code.

Step 2: Read contents

Reading the entire content of each file with read() gets all text at once.

Step 3: Write merged content

Writing both contents one after another into the new file combines them in order.

Alternative Approaches

Line-by-line merging

python

with open('file1.txt') as f1, open('file2.txt') as f2, open('merged.txt', 'w') as out:
    for line in f1:
        out.write(line)
    for line in f2:
        out.write(line)

print('Files merged line by line')

This reads and writes line by line, useful for very large files to save memory.

Using fileinput module

python

import fileinput
with open('merged.txt', 'w') as out:
    for line in fileinput.input(['file1.txt', 'file2.txt']):
        out.write(line)

print('Files merged using fileinput')

This method merges files by iterating over all lines from both files seamlessly.

Complexity: O(n + m) time, O(n + m) space

Time Complexity

Reading and writing each file's content takes time proportional to the total size of both files, so O(n + m).

Space Complexity

Reading entire files at once uses memory proportional to their size, O(n + m). Line-by-line methods reduce memory use.

Which Approach is Fastest?

Reading all at once is simple and fast for small files; line-by-line or fileinput methods are better for large files to save memory.

Approach	Time	Space	Best For
Read all at once	O(n + m)	O(n + m)	Small to medium files, simple code
Line-by-line	O(n + m)	O(1)	Large files, memory efficient
fileinput module	O(n + m)	O(1)	Large files, easy iteration over multiple files