Maintenance of Python implementations¶

Life cycle of a Python implementation¶

Every Python implementation (understood as a potential target) in Gentoo follows roughly the following life cycle:

The interpreter is added to ~arch for initial testing. At this point, packages can not declare support for the implementation yet.
The new Python target is added. It is initially stable-masked, so only ~arch users can use it. At this point, packages start being tested against the new target and its support starts being declared in PYTHON_COMPAT.
When ready, the new interpreter is stabilized. The target is not yet available for stable users, though.
The stable-mask for the target is removed. For this to happen, the inconsistencies in stable graph need to be addressed first via stabilizing newer versions of packages.
Over time, developers are repeatedly asked to push testing packages for the new target forward and stabilize new versions supporting it. Eventually, the final push for updates happens and packages not supporting the new target start being removed.
If applicable, the new target becomes the default. The developers are required to test new packages against it. The support for old target is slowly being discontinued.
Eventually, the target becomes replaced by the next one. When it nears end of life, the final packages requiring it are masked for removal and the target flags are disabled.
The compatibility declarations are cleaned up from PYTHON_COMPAT and obsolete ebuild and eclass code is cleaned up.
Finally, the interpreter is moved to python repository where it lives for as long as it builds.

For example, Python 3.9 is at stage 1 at the time of writing. It is still in alpha stage, and upstream has not finalized its feature set, therefore it is too early to declare package support for Python 3.9 and there are no target flags.

Python 3.8 is moving from stage 2 to stage 3 — it is being stabilized by arch teams at this very moment. When that’s done, we will work on unmasking the flag on stable systems and it will become our next default target.

Python 3.7 is moving from stage 5 to stage 6. The vast majority of packages have been ported to it, and we have already announced the switch date.

When the switch happens, Python 3.6 will move from stage 6 to stage 7. We are going to support it for quite some time still but as things progress, we will eventually decide to remove it.

Python 3.5 and 3.4 are at stage 9. They live in the Python repository but have no targets. You can still use them e.g. inside a virtualenv to test your own software.

Python 2.7 is currently somewhere between stages 6 and 7. It is still enabled by default for backwards compatibility but we are aggressively removing it.

PyPy3 has recently reached stage 3. It is not clear if we are going to pursue enabling the target on stable system though. PyPy2.7 is at stage 8, as the targets were removed already and it is kept as a dependency and testing target.

Notes specific to Python interpreters¶

CPython patchsets¶

Gentoo is maintaining patchsets for all CPython versions. These include some non-upstreamable Gentoo patches and upstream backports. While it is considered acceptable to add a new patch (e.g. a security bug fix) to files/ directory, it should be eventually moved into the respective patchset.

When adding a new version, it is fine to use an old patchset if it applies cleanly. If it does not, you should regenerate the patchset for new version.

The origin for Gentoo patches are the gentoo-* tags the Gentoo fork of CPython repository. The recommended workflow is to clone the upstream repository, then add Gentoo fork as a remote, e.g.:

git clone https://github.com/python/cpython
cd cpython
git remote add gentoo git@git.gentoo.org:fork/cpython.git
git fetch --tags gentoo

In order to rebase the patchset, check out the tag corresponding to the previous patchset version and rebase it against the upstream release tag:

git checkout gentoo-3.7.4
git rebase v3.7.6

You may also add additional changes via git cherry-pick. Once the new patches are ready, create the tarball and upload it, then create the tag and push it:

mkdir python-gentoo-patches-3.7.6
cd python-gentoo-patches-3.7.6
git format-patch v3.7.6
cd ..
tar -cf python-gentoo-patches-3.7.6.tar python-gentoo-patches-3.7.6
xz -9 python-gentoo-patches-3.7.6.tar
scp python-gentoo-patches-3.7.6.tar.xz ...
git tag gentoo-3.7.6
git push --tags gentoo

PyPy¶

Due to high resource requirements and long build time, PyPy on Gentoo is provided both in source and precompiled form. This creates a bit unusual ebuild structure:

dev-python/pypy-exe provides the PyPy executable and generated files built from source,
dev-python/pypy-exe-bin does the same in precompiled binary form,
dev-python/pypy combines the above with the common files. This is the package that runs tests and satisfies the PyPy target.

Matching dev-python/pypy3* exist for PyPy3.

When bumping PyPy, pypy-exe needs to be updated first. Then it should be used to build a binary package and bump pypy-exe-bin. Technically, pypy can be bumped after pypy-exe and used to test it but it should not be pushed before pypy-exe-bin is ready, as it would force all users to switch to source form implicitly.

The binary packages are built using Docker nowadays, using binpkg-docker scripts. To produce them, create a local.diff containing changes related to PyPy bump and run amd64-pypy (and/or amd64-pypy3) and x86-pypy (and/or x86-pypy3) make targets:

git clone https://github.com/mgorny/binpkg-docker
cd binpkg-docker
(cd ~/git/gentoo && git diff origin) > local.diff
make amd64-pypy amd64-pypy3 x86-pypy x86-pypy3

The resulting binary packages will be placed in your home directory, in ~/binpkg/${arch}/pypy. Upload them and use them to bump pypy-exe-bin.

Adding a new Python implementation¶

Eclass and profile changes¶

When adding a new Python target, please remember to perform all the following tasks:

add the new target flags to profiles/desc/python_targets.desc and python_single_target.desc.
force the new implementation on dev-lang/python-exec via profiles/base/package.use.force.
mask the new target flags on stable profiles via profiles/base/use.stable.mask.
add the new target to _PYTHON_ALL_IMPLS and update the patterns in _python_impl_supported() in python-utils-r1.eclass.
add the new implementation to the list in app-portage/gpyutils/files/implementations.txt.

Porting initial packages¶

The initial porting is quite hard due to a number of circular dependencies. To ease the process, some of the high profile packages are ported first with tests and their dependencies disabled for the new implementation, e.g.:

 BDEPEND="
     app-arch/unzip
     test? (
         $(python_gen_cond_dep '
             dev-python/mock[${PYTHON_USEDEP}]
             dev-python/pip[${PYTHON_USEDEP}]
             >=dev-python/pytest-3.7.0[${PYTHON_USEDEP}]
             dev-python/pytest-fixture-config[${PYTHON_USEDEP}]
             dev-python/pytest-virtualenv[${PYTHON_USEDEP}]
             dev-python/wheel[${PYTHON_USEDEP}]
         ' python2_7 python3_{6,7,8} pypy3)
         $(python_gen_cond_dep '
             dev-python/futures[${PYTHON_USEDEP}]
         ' -2)
     )
 "

 python_test() {
     if [[ ${EPYTHON} == python3.9 ]]; then
         einfo "Tests are skipped on py3.9 due to unported deps"
         return
     fi

     distutils_install_for_testing
     # test_easy_install raises a SandboxViolation due to ${HOME}/.pydistutils.cfg
     # It tries to sandbox the test in a tempdir
     HOME="${PWD}" epytest ${PN}
 }

The recommended process is to, in order:

Port dev-python/setuptools and dev-python/certifi with tests disabled. Test it via tox in a git checkout.
Port dev-python/nose with additional dependencies disabled (tests skip missing dependencies gracefully).
Port dev-python/pytest and its runtime dependencies with pytest’s tests disabled (but tests of the dependencies enabled). This should yield around 20 packages. Test it via tox in a git checkout.
Port dev-python/urllib3 and its runtime dependencies with urllib3’s tests disabled (but tests of the dependencies enabled). This should yield another 20 packages. Test it from a git checkout (it uses nox, so you may want to write tox.ini yourself).

Once these packages are done, you should be able to work towards reenabling tests in them via porting their (deep) dependencies in groups of around 10 packages without cyclic dependencies extending out of the group.