`difflib`

The Python difflib module provides classes and functions for comparing sequences and generating human-readable difference reports. It implements the Ratcliff-Obershelp “gestalt pattern matching” algorithm for measuring sequence similarity and offers multiple output formats including unified diff, context diff, and HTML side-by-side tables:

>>> import difflib
>>> difflib.get_close_matches("pythn", ["python", "perl", "ruby"])
['python']

Key Features

Computes similarity ratios between any two sequences with SequenceMatcher
Generates unified diff output suitable for patch files with unified_diff()
Produces human-readable line-by-line deltas with Differ and ndiff()
Creates HTML tables for side-by-side comparison with HtmlDiff
Finds the closest matches for a string from a list with get_close_matches()
SequenceMatcher works on any hashable sequence type, not only lines of text

Frequently Used Classes and Functions

Object	Type	Description
`difflib.SequenceMatcher`	Class	Compares pairs of sequences and computes similarity ratios
`difflib.Differ`	Class	Generates human-readable delta output between sequences of lines
`difflib.HtmlDiff`	Class	Creates HTML tables for side-by-side line comparison with change highlights
`difflib.unified_diff()`	Function	Generates unified diff output with configurable context lines
`difflib.context_diff()`	Function	Generates context diff output in before/after block format
`difflib.ndiff()`	Function	Generates a Differ-style delta comparing two sequences of lines
`difflib.get_close_matches()`	Function	Returns the best fuzzy matches for a string from a list of candidates

Examples

Computing the similarity ratio between two strings:

>>> import difflib
>>> sm = difflib.SequenceMatcher(None, "python", "pythoon")
>>> sm.ratio()
0.9230769230769231

Generating a unified diff between two lists of lines:

>>> import difflib
>>> before = ["one", "two", "three"]
>>> after = ["one", "TWO", "three", "four"]
>>> for line in difflib.unified_diff(before, after, lineterm=""):
...     print(line)
---
+++
@@ -1,3 +1,4 @@
 one
-two
+TWO
 three
+four

Common Use Cases

The most common tasks for difflib include:

Comparing two versions of a text file and reporting what changed
Implementing fuzzy search or autocomplete by finding closest string matches
Generating patch files or diff reports for version control workflows
Displaying test failure details by comparing expected and actual output
Building merge tools or change-tracking features in text editors

Real-World Example

A script can compare two versions of a configuration file and print a unified diff showing exactly what changed between them:

import difflib

v1 = """\
[server]
host = localhost
port = 8080
debug = true
""".splitlines(keepends=True)

v2 = """\
[server]
host = 0.0.0.0
port = 443
debug = false
workers = 4
""".splitlines(keepends=True)

diff = difflib.unified_diff(
    v1, v2, fromfile="config_v1.ini", tofile="config_v2.ini"
)
print("".join(diff), end="")

Run the script from your command line. You’ll get an ouput like the following:

--- config_v1.ini
+++ config_v2.ini
@@ -1,4 +1,5 @@
 [server]
-host = localhost
-port = 8080
-debug = true
+host = 0.0.0.0
+port = 443
+debug = false
+workers = 4

The output follows the standard unified diff format, making it directly usable as a patch file or for display in code review tools.

Tutorial

Reading and Writing Files in Python (Guide)

In this tutorial, you'll learn about reading and writing files in Python. You'll cover everything from what a file is made up of to which libraries can help you along that way. You'll also take a look at some basic scenarios of file usage as well as some advanced techniques.

intermediate python

For additional information on related topics, take a look at the following resources: