How to Compare Two Text Files in Python Using difflib   

Learn how to compare two text files (or strings) in Python using the built-in difflib module to generate human-readable differences, similar to the Unix diff command.

Sample Text Files

file1.txt (original):

Line one
Line two
Line three
Line four
Line five

file2.txt (modified):

Line one
Line SECOND
Line three
Line FOURTH
Line five
Line six

result:

1. Using difflib.HtmlDiff for Visual HTML Output

Generate a side-by-side HTML diff — great for reports or web display.

import difflib

# Read files
with open('file1.txt') as f1:
    text1 = f1.readlines()
with open('file2.txt') as f2:
    text2 = f2.readlines()

# Create HTML diff
differ = difflib.HtmlDiff()
html_diff = differ.make_file(text1, text2, fromdesc='Original', todesc='Modified')

# Save to HTML file
with open('diff.html', 'w') as f:
    f.write(html_diff)

print("HTML diff saved to diff.html")

Result: Opens in browser as a colored, side-by-side comparison table highlighting insertions, deletions, and changes.

2. Using difflib.unified_diff for Textual Diff

Produces a standard unified diff format (like git diff).

import difflib

with open('file1.txt') as f1:
    text1 = f1.readlines()
with open('file2.txt') as f2:
    text2 = f2.readlines()

diff = difflib.unified_diff(
    text1, text2,
    fromfile='file1.txt',
    tofile='file2.txt',
    lineterm=''
)

print('\n'.join(diff))

Output:

--- file1.txt
+++ file2.txt
@@ -1,5 +1,6 @@
 Line one
-Line two
+Line SECOND
 Line three
-Line four
+Line FOURTH
 Line five
+Line six

3. Using difflib.context_diff

Shows context around changes (older format).

import difflib

with open('file1.txt') as f1:
    text1 = f1.readlines()
with open('file2.txt') as f2:
    text2 = f2.readlines()

diff = difflib.context_diff(text1, text2, fromfile='file1.txt', tofile='file2.txt')

print('\n'.join(diff))

Output:

*** file1.txt
--- file2.txt
***************
*** 1,5 ****
  Line one
! Line two
  Line three
! Line four
  Line five
--- 1,6 ----
  Line one
! Line SECOND
  Line three
! Line FOURTH
  Line five
+ Line six

4. Quick String Comparison with Differ

For in-memory strings or quick checks:

import difflib

text1 = "Hello world\nThis is a test\n"
text2 = "Hello world\nThis is a TEST\nExtra line\n"

d = difflib.Differ()
diff = list(d.compare(text1.splitlines(keepends=True), text2.splitlines(keepends=True)))

print(''.join(diff))

Output:

  Hello world
  This is a test
- This is a TEST
?           ^^^^
+ This is a TEST
?           ^^^^
+ Extra line

Performance Notes

• difflib is built-in — no external dependencies.
• Works line-by-line; for character-level diffs within lines, consider SequenceMatcher.
• For large files, unified_diff or context_diff are efficient and memory-friendly.
• HtmlDiff is excellent for visual reporting but generates larger output.

Use HtmlDiff for presentations, unified_diff for scripts/logs, and Differ for custom processing. These tools make file comparison simple and readable!