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Hugo van Kemenade
Hugo van Kemenade

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Help us test free-threaded Python without the GIL

#python #freethreaded #testing #cpython

Python 3.13 is due out in October 2024 and work is underway to implement experimental support for PEP 703 - Making the Global Interpreter Lock Optional in CPython.

See also Free-threaded CPython in "What's New in Python 3.13?"

As the Steering Council noted in their acceptance of the PEP, to succeed it's important to have community support.

Projects will need to test their code with free-threaded (aka "nogil" but don't call it that!) Python builds to help us find bugs in CPython, and to check their code is compatible.

Here's some ways to test.

  • Official installers
  • Build it yourself
  • deadsnakes
  • GitHub Actions?
  • Fedora

Official installers

The official macOS (starting in beta 2) and Windows installers has an option to install the free-threading binaries, which also installs python3.13t alongside the regular python3.13:

Build it yourself

Check out the CPython Git repo and build yourself using the --disable-gil configuration flag.

For example, on macOS I run:

$ GDBM_CFLAGS="-I$(brew --prefix gdbm)/include" \
  GDBM_LIBS="-L$(brew --prefix gdbm)/lib -lgdbm" \
  ./configure --config-cache \
              --with-system-libmpdec \
              --with-openssl="$(brew --prefix openssl@3.0)" \
              --disable-gil && make -s -j10
$ ./python.exe -c "import sysconfig; print(sysconfig.get_config_var('Py_GIL_DISABLED'))"
1
$ ./python.exe -c "import sys; print(sys._is_gil_enabled())"
False
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More details in the devguide:

GitHub Actions: deadsnakes/action

We can use the deadsnakes/action to test Ubuntu.

For example:

on: [push, pull_request, workflow_dispatch]

jobs:
  build:
    runs-on: ubuntu-latest
    strategy:
      matrix:
        python-version: [3.12, 3.13-dev]
    name: main
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-python@v5
        if: "!endsWith(matrix.python-version, '-dev')"
        with:
          python-version: ${{ matrix.python-version }}
      - uses: deadsnakes/action@v3.1.0
        if: endsWith(matrix.python-version, '-dev')
        with:
          python-version: ${{ matrix.python-version }}
          nogil: true
      - run: |
          python --version --version
          which python
          python -c "import sysconfig; print(sysconfig.get_config_var('Py_GIL_DISABLED'))"
          python -c "import sys; print(sys._is_gil_enabled())"
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GitHub Actions: actions/setup-python

I've asked for support at actions/setup-python#771, they're looking into it 🤞

In the meantime, give it an upvote, and use deadsnakes/action ⤴️

deadsnakes: PPA

The deadsnakes project provides Personal Package Archives of Python packaged for Ubuntu, included free-threaded builds.

For example, python3.13-nogil under "python3.13 - 3.13.0~a2-1+focal1" and "python3.13 - 3.13.0~a2-1+jammy1" at https://launchpad.net/~deadsnakes/+archive/ubuntu/ppa/+packages

Fedora

Fedora uses the python3.13t executable name as decided by the Steering Council:

$ # Install
$ sudo dnf install python3.13-freethreading
$ # Run
$ /usr/bin/python3.13t -c "import sysconfig; print(sysconfig.get_config_var('Py_GIL_DISABLED'))"
1
$ /usr/bin/python3.13t -c "import sys; print(sys._is_gil_enabled())"
False
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How to check your build

To confirm if you're using a free-threaded build, check the double --version option (starting in beta 2):

$ # Regular build
$ python3.13 --version --version
Python 3.13.0b2 (v3.13.0b2:3a83b172af, Jun  5 2024, 12:50:24) [Clang 15.0.0 (clang-1500.3.9.4)]
$ # Free-threaded build
$ python3.13t --version --version
Python 3.13.0b2 experimental free-threading build (v3.13.0b2:3a83b172af, Jun  5 2024, 12:57:31) [Clang 15.0.0 (clang-1500.3.9.4)]
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Or the Py_GIL_DISABLED macro:

$ # Regular build
$ python3.13 -c "import sysconfig; print(sysconfig.get_config_var('Py_GIL_DISABLED'))"
0
$ # Free-threaded build
$ python3.13t -c "import sysconfig; print(sysconfig.get_config_var('Py_GIL_DISABLED'))"
1
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By default, the GIL is disabled for free-threaded builds, and can be re-enabled by setting the PYTHON_GIL environment variable to 1 or running Python with -X gil 1. You can check with sys._is_gil_enabled():

$ # Regular build: GIL is always enabled
$ python3.13 -c "import sys; print(sys._is_gil_enabled())"
True
$ # Free-threaded build: GIL is disabled by default
$ python3.13t -c "import sys; print(sys._is_gil_enabled())"
False
$ # Free-threaded build: re-enable with -X
$ python3.13t -X gil=1 -c "import sys; print(sys._is_gil_enabled())"
True
$ # Free-threaded build: re-enable with env var
$ PYTHON_GIL=1 python3.13t -c "import sys; print(sys._is_gil_enabled())"
True
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References:

See also

Header photo: "George Mayerle test chart" by US National Library of Medicine, with no known copyright restrictions.

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