Setting up CI

Currently the setup-python GitHub Action does not support installing a free-threaded build. For now, here are some relatively easy ways:

Ubuntu Linux CI setup via deadsnakes-action

The easiest way to get a free-threaded Python build on a CI runner is with the deadsnakes Ubuntu PPA and the deadsnakes-action GitHub Action:

jobs:
  free-threaded:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@...
      - uses: deadsnakes/action@...
        with:
          python-version: 3.13-dev
          nogil: true

You should replace the ellipses with versions for the actions. If there is a newer CPython 3.13 release available since this document was written or updated, use that version instead.

Windows CI setup via custom PowerShell

For installing a free-threaded build of Python on a Windows CI runner (runs-on: windows-latest), you can download and install directly from https://www.python.org/ftp/python/ as shown in the following PowerShell snippet (noting that the free-threaded binary is named python{version}t.exe, where the "t" is for free-"t"hreaded). For more tips see the docs on silent installation and options on Windows.

jobs:
  free-threaded:
    runs-on: windows-latest
    steps:
      - uses: actions/checkout@...
      - name: custom python install script
        shell: pwsh
        run: |
          $pythonInstallerUrl = "https://www.python.org/ftp/python/3.13.0/python-3.13.0-amd64.exe"
          Invoke-WebRequest $pythonInstallerUrl -OutFile setup-python.exe
          Start-Process "setup-python.exe" -argumentlist "/quiet PrependPath=1 TargetDir=C:\Python313 Include_freethreaded=1" -wait
          C:\Python313\python3.13t.exe -m pip install -r requirements.txt
          C:\Python313\python3.13t.exe -c "import sys; print(sys._is_gil_enabled())"

Building free-threaded wheels with cibuildwheel

cibuildwheel has support for building free-threaded wheels on all platforms. If your project releases nightly wheels, we suggest configuring cibuildwheel to build nightly free-threaded wheels.

If your project depends on Cython or the NumPy C API, you will need to install a Cython nightly wheel in the build, as the newest stable release of Cython cannot generate code that will compile under the free-threaded build. Cython 3.1.0 and NumPy 2.1.0 will be or are the first stable releases to support free-threaded python. See the project tracker for more detailed information about projects you may depend on.

You can install nightly wheels for both Cython and NumPy using the following install command:

pip install -i https://pypi.anaconda.org/scientific-python-nightly-wheels/simple cython numpy

To ensure wheels are built correctly under cibuildwheel, you will need to specify the following variables in the environment for the cibuildwheel action:

  - name: Build wheels
    uses: pypa/cibuildwheel@...
    env:
      CIBW_PRERELEASE_PYTHONS: true
      CIBW_FREE_THREADED_SUPPORT: true
      CIBW_BUILD: cp313t-${{ matrix.buildplat }}
      # TODO:
      # remove when a released cython can build free-threaded extensions
      CIBW_BUILD_FRONTEND: 'pip; args: --no-build-isolation'

As above, replace the ellipses with a cibuildwheel version.

If for some reason disabling build isolation is unworkable, you can also tell pip about the nightly wheel index and it will use it in an isolated build. To do this, set:

CIBW_BUILD_FRONTEND: 'pip; args: --pre --extra-index-url "https://pypi.anaconda.org/scientific-python-nightly-wheels/simple"'

Many projects use build instead of pip for the build frontend. See the cibuildwheel docs for more information about how to pass arguments to build and pip. See this comment on the build issue tracker if you need to use build and cannot disable build isolation.

Note that nightly wheels may not be available on all platforms yet. Windows wheels, in particular, are not currently available for NumPy or projects that depend on NumPy (e.g., SciPy).

You will also likely need to manually pass -Xgil=0 or set PYTHON_GIL=0 in your shell environment while running tests to ensure the GIL is actually disabled during tests, at least until you can register that your extension modules support disabling the GIL via Py_mod_gil and all of your runtime test dependencies do the same. See the porting guide for more information about declaring support for free-threaded python in your extension.