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Changelog — python-mip 1.17

Changes since 1.15.0 (1.16 was an unreleased RC)

New Features

HiGHS Solver Support

python-mip now ships with full support for the HiGHS open-source solver as a first-class backend (alongside CBC and Gurobi). HiGHS is a high-performance solver for LP and MIP problems with a permissive MIT licence.

Key capabilities added:

  • Full LP and MIP solve via HiGHS C API (through highsbox)
  • Warm-start (basis handoff) for LP re-solves
  • relax=True support in optimize()
  • Variable and constraint inspection/modification
  • Correct handling of UNBOUNDED vs INFEASIBLE status
  • Reduced memory footprint and improved file read/write consistency

HiGHS is installed as an optional dependency: pip install mip[highs].

macOS Apple Silicon (M1/M2/M3) Native Support

CBC now runs natively on Apple Silicon via a pre-built ARM64 binary, replacing the previous Rosetta 2 x86_64 fallback.

Infrastructure & Distribution

CBC Binaries via cbcbox

The bundled CBC shared libraries (.so, .dylib, .dll) have been removed from the python-mip source tree. CBC binaries are now distributed through the cbcbox PyPI package, which provides pre-built wheels for:

  • Linux x86_64 and aarch64 (ARM64)
  • macOS x86_64 and arm64
  • Windows x64

cbcbox is a dedicated package whose sole job is to ship up-to-date CBC binaries for all major platforms. This decoupling means future CBC upgrades are released without touching python-mip itself. The minimum required version is cbcbox>=2.902.

Automated PyPI Publishing

A new GitHub Actions workflow (.github/workflows/publish.yml) automatically publishes to PyPI whenever a v* tag is pushed. It uses OIDC Trusted Publisher authentication — no API tokens to rotate.

Modernised CI Matrix

Platform OS
Linux x86_64 ubuntu-24.04
Linux aarch64 ubuntu-24.04-arm (new)
macOS ARM64 macos-15 (new)
Windows x64 windows-2025 (new)

Python versions tested: 3.10, 3.11, 3.12, 3.13, PyPy 3.11.

Bug Fixes

  • CBC re-solve correctness: A bug introduced by newer CBC versions caused stale solution data to be returned when optimize() was called multiple times on the same model. Fixed by calling Cbc_reset() before each Cbc_solve(), with objective sense saved and restored around the reset.
  • isfile import missing in SolverCbc.read(): os.path.isfile was used but not imported, causing a NameError when loading a model from a file.
  • Windows DLL loading: On Python 3.8+, Windows ignores PATH when resolving DLL dependencies. Fixed by calling os.add_dll_directory() on the cbcbox bin/ directory.
  • Empty LinExpr in constraints: Constraints containing an empty linear expression were not handled correctly. Fixed by Sebastian Heger (#237).

Breaking Changes / Compatibility

  • Minimum Python version raised to 3.10. Python 3.8 and 3.9 have reached end-of-life and are no longer tested or supported.
  • Bundled CBC libraries removed — cbcbox is now a required dependency (installed automatically via pip).
  • gurobipy version constraint relaxed to >=10 (no upper bound).
  • cffi version constraint relaxed to >=1.15 (no upper bound).
  • highsbox version constraint relaxed to >=1.10.0 (no upper bound).

Acknowledgements

This release was a team effort. Thank you to everyone who contributed:

  • Robert Schwarz — HiGHS interface: initial implementation (PR #332) and extensive improvements (PR #418), including objective setter fix, option types, test coverage and highsbox migration. Co-authored with Bernard Zweers and Miguel Hisojo.
  • Túlio Toffolo — macOS Apple Silicon support, HiGHS testing infrastructure, CI modernisation, and many quality-of-life fixes.
  • Sebastian Heger — Bug fix for constraints with empty linear expressions (#237).
  • Dominik Peters — Removed upper limit on supported Python versions (#408).
  • Adeel Khan — HiGHS _core library support.
  • Haroldo Santos — cbcbox integration, CBC bug fixes, CI/CD automation, and release management.