Compare two Files line by line in Python

Last Updated : 21 Mar, 2024

In Python, there are many methods available to this comparison. In this Article, We’ll find out how to Compare two different files line by line. Python supports many modules to do so and here we will discuss approaches using its various modules.

This article uses two sample files for implementation.

Files in use:

file.txt

compare fil python

file1.txt

compare fil python

Method 1: Using unified_diff()

Python has a Module which is specially used for comparing the differences between the files. To get differences using the difflib library, we have to call the unified_diff() function to this comparison.

Syntax:

unified_diff(file1, file2, fromfile, tofile, lineterm)

Parameter:

file1: List of String such as file_1_text

file2: List of String such as file_2_text

fromfile: first file name with extension

tofile: second file name with extension

lineterm: argument to “” so that the output will be automatically uniformly newline free

Approach

Import module
Open files
Compare using unified_diff() with appropriate attributes

Example:

Python3

# Importing difflib 
import difflib 
  
with open('file1.txt') as file_1: 
    file_1_text = file_1.readlines() 
  
with open('file2.txt') as file_2: 
    file_2_text = file_2.readlines() 
  
# Find and print the diff: 
for line in difflib.unified_diff( 
        file_1_text, file_2_text, fromfile='file1.txt',  
        tofile='file2.txt', lineterm=''): 
    print(line) 

Output:

— file1.txt

+++ file2.txt

@@ -1,5 +1,5 @@

Learning

Python

is

-too

-simple.

+so

+easy.

Method 2: Using differ

There is one Class available for comparing the differences between the files which named as Differ inside the difflib library. This class is used for comparing sequences of lines of text, and producing human-readable differences or deltas.

Code	Meaning
‘-‘	line unique to sequence 1
‘+’	line unique to sequence 2
‘ ‘	line common to both sequences
‘?’	line not present in either input sequence

Approach

Import module
Open files
Read contents line bt line
Call compare function with the use of differ class object

Example:

Python3

from difflib import Differ 
  
with open('file1.txt') as file_1, open('file2.txt') as file_2: 
    differ = Differ() 
  
    for line in differ.compare(file_1.readlines(), file_2.readlines()): 
        print(line) 

Output:

Learning

Python

is

– too

– simple.

+ so

+ easy.

Method 3: Using while loop and Intersection Method

Approach

Open both files in read mode
Store list of strings
Start comparing both files with the help of intersection() method for common strings
Compare both files for differences using while loop
Close both files

Example:

Python3

# Open File in Read Mode 
file_1 = open('file1.txt', 'r') 
file_2 = open('file2.txt', 'r') 
  
print("Comparing files ", " @ " + 'file1.txt', " # " + 'file2.txt', sep='\n') 
  
file_1_line = file_1.readline() 
file_2_line = file_2.readline() 
  
# Use as a COunter 
line_no = 1
  
print() 
  
with open('file1.txt') as file1: 
    with open('file2.txt') as file2: 
        same = set(file1).intersection(file2) 
  
print("Common Lines in Both Files") 
  
for line in same: 
    print(line, end='') 
  
print('\n') 
print("Difference Lines in Both Files") 
while file_1_line != '' or file_2_line != '': 
  
    # Removing whitespaces 
    file_1_line = file_1_line.rstrip() 
    file_2_line = file_2_line.rstrip() 
  
    # Compare the lines from both file 
    if file_1_line != file_2_line: 
        
        # otherwise output the line on file1 and use @ sign 
        if file_1_line == '': 
            print("@", "Line-%d" % line_no, file_1_line) 
        else: 
            print("@-", "Line-%d" % line_no, file_1_line) 
              
        # otherwise output the line on file2 and use # sign 
        if file_2_line == '': 
            print("#", "Line-%d" % line_no, file_2_line) 
        else: 
            print("#+", "Line-%d" % line_no, file_2_line) 
  
        # Print a empty line 
        print() 
  
    # Read the next line from the file 
    file_1_line = file_1.readline() 
    file_2_line = file_2.readline() 
  
    line_no += 1
  
file_1.close() 
file_2.close() 

Output:

Comparing files

@ file1.txt

# file2.txt

Common Lines in Both Files

Learning

Python

is

Difference Lines in Both Files

@- Line-4 too

#+ Line-4 so

@- Line-5 simple.

#+ Line-5 easy.

shubhanshuarya007

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Compare two Files line by line in Python

Method 1: Using unified_diff()

Python3

Method 2: Using differ

Python3

Method 3: Using while loop and Intersection Method

Python3

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