How to Compare Files and Show a Diff in Python

Compare two text files and print a unified diff using Python's difflib module to highlight differences.

Easy Python 3.9+ Aug 9, 2026 Testing & modern typing 11 views 0 copies

diff files difflib testing comparison

Python code

23 lines

Python 3.9+

import difflib
from pathlib import Path

def compare_files(expected_path: str, actual_path: str) -> str:
    """Compare two text files and return a unified diff."""
    expected = Path(expected_path).read_text()
    actual = Path(actual_path).read_text()

    diff = difflib.unified_diff(
        expected.splitlines(keepends=True),
        actual.splitlines(keepends=True),
        fromfile=f"expected/{expected_path}",
        tofile=f"actual/{actual_path}"
    )
    return "".join(diff)

if __name__ == "__main__":
    # Create sample files to demonstrate
    Path("expected.txt").write_text("hello world\nline two\nline three\n")
    Path("actual.txt").write_text("hello world\nline 2 changed\nline three\n")

    result = compare_files("expected.txt", "actual.txt")
    print(result if result else "Files are identical")

Output

stdout

--- expected/expected.txt
+++ actual/actual.txt
@@ -1,3 +1,3 @@
 hello world
-line two
+line 2 changed
 line three

How it works

The difflib.unified_diff function produces a textual diff similar to Unix diff -u. It compares the lines of two files and outputs a unified format with --- and +++ headers showing file names, and lines prefixed with - for removals and + for additions. Using splitlines(keepends=True) preserves newline characters so the diff respects line endings. The function returns a generator, so join combines it into a single string for printing or further processing.

Common mistakes

Forgetting to use `keepends=True` in `splitlines`, which can cause misleading diffs
Not handling file-not-found errors, leading to unhelpful tracebacks
Assuming the diff is non-empty and printing it without checking
Using `read()` on binary files and then comparing text, which may raise errors

Variations

Use `difflib.ndiff` for a simpler line-by-line diff without file headers
Use `filecmp.cmp` to only check if files are identical without showing differences

Real-world use cases

Automated tests that compare generated output files against stored golden files to detect regressions.
A code review tool that shows a unified diff of changes between two versions of a configuration file.
A deployment script that highlights differences between local and production environment files.

How to Compare Files and Show a Diff in Python

Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

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