bsdiff4

bsdiff4 is a Python package for producing and applying binary patches in the BSDIFF4 format. It computes a compact delta between two byte strings or files and later applies that delta to reconstruct the target, which is ideal for shipping small updates instead of full binaries.

The core diff and patch routines are implemented as a C extension for speed, wrapped by a small pure-Python interface that adds file-level helpers and a BSDIFF4-format reader/writer. The package also installs two command-line tools, bsdiff4 and bspatch4, so the same functionality is available from the shell. The code derives from cx_bsdiff and the original BSD bsdiff utility.

What You Get

• diff() and patch() functions operating on byte strings
• file_diff(), file_patch(), and file_patch_inplace() helpers for on-disk files
• An interface to read and write the BSDIFF4 patch format
• bsdiff4 and bspatch4 command-line tools
• A compiled C extension for fast diffing and patching

Common Use Cases

• Shipping small incremental updates to a large binary instead of the whole file
• Generating delta patches between two versions of a data file
• Applying a received patch to reconstruct an updated file
• Reducing bandwidth for software or asset update distribution

Under The Hood

Architecture — The package is a thin Python layer over a C core. core.c implements the BSDIFF4 diff and patch algorithm as a compiled extension; format.py defines the BSDIFF4 container format (control/diff/extra blocks with bz2 compression) and the read/write logic; __init__.py exposes the high-level diff/patch and file_diff/file_patch/file_patch_inplace functions; cli.py provides the bsdiff4 and bspatch4 entry points. The design cleanly separates the algorithm (C) from the format handling and file/CLI conveniences (Python).

Tech Stack — Pure Python API over a C extension, built with setuptools (setup.py) and a Makefile. Runtime relies only on the standard library (including bz2 for block compression). The code descends from cx_bsdiff, which in turn derives from Colin Percival’s original BSD bsdiff.

Code Quality — The project is compact and long-lived (maintained since 2011). Tests live in bsdiff4/test_all.py and cover the round-trip diff/patch behavior across bytes and files. The narrow surface area and stable format keep the maintenance burden low, though recent development activity is minimal because the library is essentially feature-complete.

API Design — The API is small and obvious: diff(src, dst) returns patch bytes and patch(src, patch) returns the reconstructed bytes, with parallel file_* variants for paths. There is almost no boilerplate to get started, and the same operations are one shell command away via bsdiff4/bspatch4.